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Special Issue "Progress in Jet Engine Technology"

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A special issue of Aerospace (ISSN 2226-4310).

Deadline for manuscript submissions: closed (30 September 2019) | Viewed by 66128

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literature review of jet engine

Dear colleagues,

In the past decade, the need for more efficient, low-noise, and environmentally friendly propulsors has brought innovative engine concepts and configurations to the attention of researchers within both academia and industry. Noticeable examples include but not limited to ultra-high by-pass ratio turbofans in podded configurations, distributed propulsion, boundary layer ingestion engines, and hybrid turbo/electric engines. Although such configurations are characterized by non-negligible technology readiness levels, they still require huge research efforts, both theoretical and experimental, to be validated.

This Special Issue aims to provide an overview of recent advances in jet engine technology for the civil sector, with special emphasis on new design configurations and performance assessment. Authors are invited to submit full research articles and review manuscripts addressing (but not limited to) the following topics:

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Encyclopedia Britannica

General characteristics

The prime mover, the propulsor.

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jet engine

Read a brief summary of this topic

jet engine , any of a class of internal-combustion engines that propel aircraft by means of the rearward discharge of a jet of fluid, usually hot exhaust gases generated by burning fuel with air drawn in from the atmosphere.

The prime mover of virtually all jet engines is a gas turbine . Variously called the core, gas producer, gasifier, or gas generator, the gas turbine converts the energy derived from the combustion of a liquid hydrocarbon fuel to mechanical energy in the form of a high-pressure, high-temperature airstream. This energy is then harnessed by what is termed the propulsor (e.g., airplane propeller and helicopter rotor) to generate a thrust with which to propel the aircraft.

Principles of operation

Figure 1: Cross section of a turbojet and (below) graph of typical operating conditions for its working fluid.

The gas turbine operates on the Brayton cycle in which the working fluid is a continuous flow of air ingested into the engine’s inlet. The air is first compressed by a turbocompressor to a pressure ratio of typically 10 to 40 times the pressure of the inlet airstream (as shown in Figure 1 ). It then flows into a combustion chamber, where a steady stream of the hydrocarbon fuel, in the form of liquid spray droplets and vapour or both, is introduced and burned at approximately constant pressure. This gives rise to a continuous stream of high-pressure combustion products whose average temperature is typically from 980 to 1,540 °C or higher. This stream of gases flows through a turbine, which is linked by a torque shaft to the compressor and which extracts energy from the gas stream to drive the compressor. Because heat has been added to the working fluid at high pressure, the gas stream that exits from the gas generator after having been expanded through the turbine contains a considerable amount of surplus energy—i.e., gas horsepower—by virtue of its high pressure, high temperature, and high velocity, which may be harnessed for propulsion purposes.

The heat released by burning a typical jet fuel in air is approximately 43,370 kilojoules per kilogram (18,650 British thermal units per pound) of fuel. If this process were 100 percent efficient, it would then produce a gas power for every unit of fuel flow of 7.45 horsepower/(pounds per hour), or 12 kilowatts/(kg per hour). In actual fact, certain practical thermodynamic limitations, which are a function of the peak gas temperature achieved in the cycle, restrict the efficiency of the process to about 40 percent of this ideal value. The peak pressure achieved in the cycle also affects the efficiency of energy generation. This implies that the lower limit of specific fuel consumption ( SFC) for an engine producing gas horsepower is 0.336 (pound per hour)/horsepower, or 0.207 (kg per hour)/kilowatt. In actual practice, the SFC is even higher than this lower limit because of inefficiencies, losses, and leakages in the individual components of the prime mover.

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Because weight and volume are at a premium in the overall design of an aircraft and because the power plant represents a large fraction of any aircraft’s total weight and volume, these parameters must be minimized in the engine design. The airflow that passes through an engine is a representative measure of the engine’s cross-sectional area and hence its weight and volume. Therefore, an important figure of merit for the prime mover is its specific power—the amount of power that it generates per unit of airflow. This quantity is a very strong function of the peak gas temperature in the core at the discharge of the combustion chamber. Modern engines generate from 150 to 250 horsepower/(pound per second), or 247 to 411 kilowatts/(kg per second).

Equation.

One thereby infers that the components of a propulsor must exert a force F on the stream of air flowing through the propulsor if this device accelerates the airstream from the flight velocity V 0 to the discharge velocity V j . The reaction to that force F is ultimately transmitted by the mounts of the propulsor to the aircraft as propulsive thrust.

There are two general approaches to converting gas horsepower to propulsive thrust. In one, a second turbine (i.e., a low-pressure, or power, turbine) may be introduced into the engine flow path to extract additional mechanical power from the available gas horsepower. This mechanical power may then be used to drive an external propulsor, such as an airplane propeller or helicopter rotor. In this case, the thrust is developed in the propulsor as it energizes and accelerates the airflow through the propulsor—i.e., an airstream separate from that flowing through the prime mover.

In the second approach, the high-energy stream delivered by the prime mover may be fed directly to a jet nozzle, which accelerates the gas stream to a very high velocity as it leaves the engine, as is typified by the turbojet . In this case, the thrust is developed in the components of the prime mover as they energize the gas stream.

In other types of engines, such as the turbofan , thrust is generated by both approaches: A major part of the thrust is derived from the fan, which is powered by a low-pressure turbine and which energizes and accelerates the bypass stream ( see below ). The remaining part of the total thrust is derived from the core stream, which is exhausted through a jet nozzle.

Equation.

Although the jet velocity V j must be larger than the aircraft velocity V 0 to generate useful thrust, a large jet velocity that exceeds flight speed by a substantial margin can be very detrimental to propulsive efficiency. Maximum propulsive efficiency is approached when the jet velocity is almost equal to (but, of necessity, slightly higher than) the flight speed. This fundamental fact has given rise to a large variety of jet engines, each designed to generate a specific range of jet velocities that matches the range of flight speeds of the aircraft that it is supposed to power.

The net assessment of the efficiency of a jet engine is the measurement of its rate of fuel consumption per unit of thrust generated (e.g., in terms of pounds, or kilograms, per hour of fuel consumed per pounds, or kilograms, of thrust generated). There is no simple generalization of the value of specific fuel consumption of a thrust engine. It is not only a strong function of the prime mover’s efficiency (and hence its pressure ratio and peak-cycle temperature) but also of the propulsive efficiency of the propulsor (and hence of the engine type). It also is a strong function of the aircraft flight speed and the ambient temperature (which is in turn a strong function of altitude, season, and latitude).

A review of health effects associated with exposure to jet engine emissions in and around airports

Environmental Health volume  20 , Article number:  10 ( 2021 ) Cite this article

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A Correction to this article was published on 24 February 2021

This article has been updated

Airport personnel are at risk of occupational exposure to jet engine emissions, which similarly to diesel exhaust emissions include volatile organic compounds and particulate matter consisting of an inorganic carbon core with associated polycyclic aromatic hydrocarbons, and metals. Diesel exhaust is classified as carcinogenic and the particulate fraction has in itself been linked to several adverse health effects including cancer.

In this review, we summarize the available scientific literature covering human health effects of exposure to airport emissions, both in occupational settings and for residents living close to airports. We also report the findings from the limited scientific mechanistic studies of jet engine emissions in animal and cell models.

Jet engine emissions contain large amounts of nano-sized particles, which are particularly prone to reach the lower airways upon inhalation. Size of particles and emission levels depend on type of aircraft, engine conditions, and fuel type, as well as on operation modes. Exposure to jet engine emissions is reported to be associated with biomarkers of exposure as well as biomarkers of effect among airport personnel, especially in ground-support functions. Proximity to running jet engines or to the airport as such for residential areas is associated with increased exposure and with increased risk of disease, increased hospital admissions and self-reported lung symptoms.

We conclude that though the literature is scarce and with low consistency in methods and measured biomarkers, there is evidence that jet engine emissions have physicochemical properties similar to diesel exhaust particles, and that exposure to jet engine emissions is associated with similar adverse health effects as exposure to diesel exhaust particles and other traffic emissions.

Peer Review reports

Exposure to air pollution, including ultrafine particulate matter (UFP), from industry and traffic is associated with adverse health effects [ 1 , 2 , 3 , 4 ]. Airports are significant high-emission sources and human exposure to these emissions is a growing health concern. Importantly, airport personnel are at risk of occupational exposure to jet engine emissions [ 5 ]. More knowledge is needed on exposure risks, adverse health effects, biomarkers and risk management options related to the diverse factors influencing human exposure to airport emissions [ 6 ] (Fig.  1 ).

figure 1

Overview of contributing factors in exposure risks from airports (APU: auxiliary power unit; GAC: ground air-conditioning cart, ECS: environmental control system).

However, data collection seems challenging. Commercial airports are large, complex and diverse work places, where aircraft, ground-support equipment (GSE), and related vehicles all contribute to mixed emissions [ 7 , 8 ]. In turn, commercial airports as well as military air stations are year-round active high security areas with restricted access, which can reduce the options for external researchers to collect optimal or sufficient measurements. Consensus or formal guidelines for optimal measurement design, instrumentation and analysis methods for the different emission components are lacking, which further complicates comparison of data and risk assessment [ 5 , 9 ].

With this review, we seek to compile available studies in the open scientific literature on health effects of jet engine emissions in occupational settings and in residential areas around airports, along with mechanistic effects studied in animal and cell models. The studies were selected based on key papers and systematic searches (search terms, method and selection criteria are disclosed in the Additional file 1 ). We briefly summarize the characteristics of jet engine emissions and highlight the complexity of this field of research, but detailed research on emissions and physical-chemical studies is beyond the scope of this review.

Toxicity of jet fuel exposure

The toxicity of (unburned) jet fuel as such has been considered in many studies (reviewed in [ 10 ]) since the early 1950’s, where the specifications of the hydrocarbon-based jet fuel, JP-4 (jet propellent-4), was published by the US air force. Major toxic effects reported for JP-4 were skin irritation, neurotoxicity, nephrotoxicity, and renal carcinogenicity in rats [ 11 ]. Jet fuels are mixtures of gasoline and kerosene with performance additives [ 10 ]. In 1994, US Air Force converted to JP-8, developed to be less volatile and less explosive upon crash incidents compared to JP-4. JP-8 (NATO F-34) is equivalent to Jet A-1 fuel used in commercial aircraft. A range of other kerosene-based jet fuels are in use, depending on aircraft type and differing in kerosene ratio and requirements for additives [ 5 ]. Measurements of a range of the common aircraft pollutants such as benzene, toluene, and chlorinated compounds in breath samples from exposed personnel on an airbase before and after work tasks showed significant exposure for all subjects, ranging from minor elevations up to > 100 times the values of the control group for fuel workers [ 12 ]. The uptake of JP-8 components both occur via inhalation and dermal contact, and apart from benzene, naphthalene in air and in exhaled breath condensate (EBC) may be useful as a biomarker of exposure to and uptake of JP-8 fuel components in the body [ 13 ]. Although most studies report low acute toxicity for both JP-4 and JP-8, JP-8 was reported to show effects such as respiratory tract sensory irritation [ 11 ], inflammatory cytokine secretion in exposed alveolar type II epithelial cells and in pulmonary alveolar macrophages [ 14 ], increased pulmonary resistance and decreased weight gain in rats upon inhalation exposure for 7 or 28 days [ 15 , 16 ]. Subchronic 90-days studies with rats with various exposure levels of JP-4 and JP-8 showed little toxicity, apart from male rat hydrocarbon nephropathy [ 11 ]. However, JP-8 fuel exposure has been linked to noise-activated ototoxic hearing loss in animal studies [ 17 , 18 ] and in occupational exposure cases [ 19 , 20 ], and to immunotoxicity [ 21 , 22 ].

It is likely that fuel refinements will advance in the future and be an important factor in emission reductions. A newer synthetic jet fuel (Fischer-Tropsch Synthetic Paraffinic Kerosene) under development to replace JP-8 in the future, was evaluated for toxicity in the required range of tests used to develop occupational exposure limits (OELs). The highest exposure level of 2000 mg/m 3 (6 h per day, 5 days a week for 90 days) produced multifocal inflammatory cell infiltrations in rat lungs, whereas no genotoxicity or acute inhalation effects were observed, and the sensory irritation assay indicated that the refined synthetic fuel was less irritating than JP-8 [ 23 ]. Evidence of cancer risk is, however, normally evaluated in two-year inhalation studies in rats.

Characteristics of jet engine emissions

Like other combustion engines, jet engines produce volatile organic compounds (VOC) such as CO 2 , NO x , CO, SO x and low molecular weight polycyclic aromatic hydrocarbons (PAH), and particulate matter (PM) with associated PAH, and metals [ 24 ]. Incomplete combustion of fossil fuels, including kerosene, results in the formation of carbon-rich (> 60%), aromatic bi-products called char, and condensates, which are known as soot. Char and soot can either be measured as elemental carbon (EC, used in atmospheric sciences) or black carbon (BC, used in soil and sediment sciences) [ 25 ]. This terminology originates from their measurement methods (BC is light-absorbing, determined by optical methods and EC is refractory, determined by thermo-optical and oxidizing methods) [ 26 ]. BC is often used in physical/chemical aerosol studies of airport- and urban emissions, such as in Costabile et al. [ 27 ] and Keuken et al. [ 28 ]. However, there is no apparent consistent correlation between BC concentrations and particle number concentrations across exposure studies at airports, but data is limited as noted by Stacey [ 9 ].

In general, emission levels are high, but vary depending on engine conditions and fuel type, as well as on operation modes such as idling, taxi, take-off, climb-out and landing [ 29 ].

Particulate matter (PM)

PM is divided by size ranges according to the aerodynamic diameter of the particles, where UFP are in the nanoscale of < 100 nm. Several studies have shown that aircraft emissions are dominated or even characterized by high concentrations of very small particles. This was underlined in a recent study by Stacey, Harrison and Pope carried out at Heathrow London in comparison to traffic background [ 30 ]. Some report particles in the range of 5–40 nm [ 31 ], and others particle diameters of 20 nm as compared to larger particles of > 35 nm measured at surrounding freeways [ 32 ]. Campagna et al. studied the contributions of UFP from a military airport to the surrounding area, by sampling on the airport grounds during flight activities, nearby the airport, in an urban area and in a rural area. The smallest primary particles were found within the airport (~ 10 nm) and the largest in the urban area (~ 72 nm). The highest UFP levels inside the airport were measured during taxi and take-off activities (4.0 × 10 6 particles/cm 3 ) [ 33 ]. Westerdahl et al. reported very high particle number concentrations at take-off of a single jet aircraft, with a 10 s peak of 4.8 million particles/cm 3 together with elevated NO x and BC levels [ 34 ].

The small particles are emitted in large numbers and tend to form complex agglomerates in ambient air that can be detected in larger particle size modes [ 35 , 36 ] (see [ 5 ] for elaboration). In a recent study in Montreal-Pierre-Elliott-Trudeau International Airport, the total particle number concentration over all sizes at the airport apron reached 2.0 × 10 6 /cm 3 , which was significantly higher compared to downtown Montreal (1 × 10 4 /cm 3 ). The geometric mean of observed ultrafine particle number density of nanoparticles was 1 × 10 5 /cm 3 at the apron and 1.1 × 10 4 /cm 3 outside the Departure Level entrance [ 37 ]. We recently published exposure measurements conducted at a commercial airport and non-commercial airfield, where air concentrations were measured to 7.7 × 10 6 particle/cm 3 or 1086 μg/m 3 of total particles during take-off of one single jet plane [ 36 ]. The majority of these particles were below the size detection limit of 10 nm for the instruments [ 36 ], which was also shown, and highlighted as a general challenge, by others [ 38 ].

The nanostructure of carbon particles are influenced by fuel type and combustion processes. Low thrust settings are associated with the smallest particle sizes. In one of their studies, Vander Wal et al. characterized the aircraft particles as predominantly organic carbon at low thrust and EC at higher thrust settings [ 38 ]. In turn, it was reported that soot reactivity, characterized by an outer amorphous shell, of soot particles from a turbofan test engine was lower in particles from ground idle as compared to particles from climb-out engine mode for two fuel types. Biofuel blending slightly lowered this soot reactivity at ground idle, but had the opposite effect at the higher power condition of climb-out. The authors comment that for soot reactivity, measured by an outer amorphous shell in the study, biofuels may be beneficial in airports where ground idle engine conditions are often in use, but the effect on emissions in climb-out conditions is undetermined [ 31 ]. According Moore et al., a 50:50 biofuel blending reduces particle emissions from aircraft with 50–70%, compared to conventional Jet-A fuel [ 39 ]. Another study did extensive analyses of emissions from four on-wing commercial aircraft turbo engines (two newer CFM56–7 engines and two CFM56–3 engines), also demonstrating that the type of emissions were significantly dependent on power. PM emission indices (g/kg − 1 fuel) were reported to increase from 0.011 to 0.205 g/kg − 1 fuel with a power increase from idle to 85%. In turn, the data showed that hydrocarbons are mostly emitted at ground idle engine conditions, as opposed to PM emissions being more significant at higher power thrusts, such as take-off and landing. EC fraction of PM also increased with increase in power [ 40 ]. Targino et al. measured large EC (BC) concentrations during boarding and disembarking (mean 3.78 μg/m 3 ), at the airport concourse (mean 3.16 μg/m 3 ) and also inside an aircraft on the ground with open doors (mean 2.78 μg/m 3 ) [ 41 ].

Lubrication oil and organophosphate esters

A recent study found that intact forms of unburned jet engine lubrication oil was a major component of emissions from aircraft [ 42 ]. Organophosphate esters (OPEs) are a large group of chemicals with toxic properties used as stabilizing agents in numerous consumer – and industrial products, including in aircraft lubricating oil and hydraulic fluids. Airplane emissions are thought to be an important source of OPEs in the environment. Not only does these chemicals accumulate in ecosystems, but it is also a concern due to the location of airports near populated areas [ 5 ]. Li et al. recently studied the concentrations of 20 OPEs in ambient air, soil, pine needles, river water, and outdoor dust samples collected around an airport in Albany, New York, and reported elevated total OPE concentrations in all samples. The spatial distribution of OPEs in air, soil, and pine needles correlated with distance to the airport. The average daily intake of OPEs via air inhalation and outdoor dust ingestion in the vicinity of the airport was up to 1.53 ng/kg bw/day for children and 0.73 ng/kg bw/day for adults [ 43 ]. Another study examined organophosphates, such as tri-n-butyl phosphate, dibutyl phenyl phosphate, triphenyl phosphate and tricresyl phosphate from turbine and hydraulic oils, as well as oil aerosol/vapors and total volatile organic compounds (VOC) in air with potential for occupational exposure for airport ground personnel. The measured exposure levels were mainly below the limit of quantification during work tasks, but provoked exposure situations resulted in significantly higher exposure levels compared to normal conditions, illustrated by oil aerosol up to 240 mg/m − 3 and and tricresyl phosphate concentrations up to 31 mg/m − 3 . Highest exposure levels were measured during loading from jet engine aircraft [ 44 ].

Exposure to toxic compounds via contaminated bleed air (from engine compressors), including OPEs, has been widely studied among cabin crew and pilots, and has been associated with adverse neurological effects and respiratory illness [ 45 , 46 ].

Metals and other elements

Metals which might be specific to airport emissions, either by abundance or type, such as the heavy-metal vanadium [ 47 ], could be potential chemical fingerprints. Abegglen et al. applied single particle mass spectrometry to investigate metal content and sources in emissions from different jet engines at various combustion conditions, and Mo, Ca, Na, Fe, Cu, Ba, Cr, Al, Si, Mg, Co, Mn, V, Ni, Pb, Ti and Zr were found to be significant frequently occurring metals. Fuel, lubrication oil, grease and engine wear are potential sources, but several metals were allocated to multiple sources [ 48 ].

In the studies of He et al and Shirmohammadi et al, particles were collected at Los Angeles Airport (LAX) and central Los Angeles (LA) and among other analyses, allocated according to elements associated with different sources [ 49 , 50 ]. S was considered as aviation-related and particle-bound Na was viewed as ocean-related, due to sea salt from the ocean near by LAX. Al, Ca, Ti and K were considered as trace elements for road dust from LAX and central LA. Mn, Fe, Cu, Zn, Ba, Pb, Ni, and Mg were associated with traffic emissions, including fuel and lubricating oil combustions and brake abrasions, engine and tire wear. In LAX particles, S accounted for the largest fraction (49.5%), followed by road dust elements (21.8%) and traffic-related elements (15.9%). In particles from central LA, elements from traffic, road dust, and aviation were represented equally (28.5, 31.5, and 33.4%, respectively) [ 49 , 50 ]. In a study from Montreal-Pierre-Elliott-Trudeau International Airport, several metals were found to be abundant in the particle fraction, such as Fe, Zn, and Al, and the authors speculate, that airports in fact may be hotspots for nanoparticles containing emerging contaminants [ 37 ]. A recent study investigated the levels of 57 elements at five sampling sites within the vicinity of Eskisehir Hasan Polatkan Airport in Turkey, based on moss bag biomonitoring using Sphagnum sp. in combination with chemical analyses of lubrication oil and aviation gasoline fuel used by general aviation, piston-engine, and turboprop aircraft. Moss bag biomonitoring was a useful tool in identification of the elements that accumulated downwind of the airport emissions. Characterization of the metal contents in moss bags and oil and fuel were in agreement, showing that Pb, along with Cd, Cu, Mo, Cr, Ni, Fe, Si, Zn, Na, P, Ca, Mg, and Al were dominating elements in the general aviation aircraft emissions [ 51 ].

Polycyclic aromatic hydrocarbons/volatile organic compounds

Polycyclic aromatic hydrocarbons (PAH), including several known carcinogens, are also candidates for chemical airport emission tracers. PAH are semi-volatile compounds, in between the gaseous and particulate phases. Lighter-weight PAHs (< 4 rings) present almost exclusively in the vapour-phase and PAHs with higher molecular weights (> 4 rings) are almost completely particle-bound [ 5 ]. It was reported that the apron of the Fiumicino Airport in Rome had higher levels of measured PAH (27.2 μg/m 3 ) compared to PAH levels in the airport building and terminal [ 52 ]. Another study of PAH in airport emissions at the apron reported that the five most abundant species of particle bound-PAHs for all sampling days were naphthalene, phenanthrene, fluoranthene, acenaphthene, and pyrene, with total concentrations between 0.152 μg/m 3 - 0.189 μg/m 3 (152.21–188.94 ng/m 3 ) depending on season. The most abundant fractions of benzo(a)pyrene (BaP) equivalent concentration (BaPeq) in different molecular weights were high-weight PAHs (79.29%), followed by medium-weight PAHs (11.57%) and low-weight PAHs (9.14%). The percentages of total BaPeq in the very small particles < 0.032 μm were 52.4% (mean concentration 0.94 ng/m 3 ) and 70.15% in particles < 100 μm (mean concentration 1.25 ng/m 3 ) [ 53 ]. Studies of the emissions from a helicopter engine at different thrusts included analysis of 22 PAH compounds, where 97.5% of the total PAH emissions were two- and three-ringed PAHs, with a mean total PAH concentration of 843 μg/m 3 and a maximum of 1653 μg/m 3 during ground idle. This was 1.05–51.7 times higher compared to a heavy-duty diesel engine, a motor vehicle engine, and an F101 aircraft engine. In turn, total level of BaP during one landing and take-off cycle (LTO) (2.19 mg/LTO) [ 54 ] was higher than the European Commission emission factor of 1.24 mg/LTO, stated in their PAH position paper, where emission factors are used to calculate the degree to which a source contributes to the total emission of a specific pollutant [ 55 ]. The Danish occupational exposure limit for PAH is 200 μg/m 3 [ 56 ], and reported PAH concentrations in ambient air across studies were below this level.

Volatile organic compounds (VOC) comprise a diverse group of organic chemicals, with different physicochemical and toxicological properties. Scientific studies of these emission compounds were meticulously reviewed by Masiol et al. [ 5 ], and as noted by the authors there is insufficient knowledge in terms of the significance of these compounds for airport exhaust health impacts [ 5 ]. Some VOC have known toxicities and other are suspected to have adverse health effects, and among the hydrocarbons found in aircraft exhaust, 14 single or complex compounds are listed as hazardous by the Federal Aviation Administration, which in addition to PAH compounds comprise benzene, styrene, xylene, toluene, acetaldehyde, 1,3-butadiene, n-hexane, acrolein, propionaldehyde, ethylbenzene, formaldehyde, and lead compounds [ 57 ]. A recent study assessed 46 VOC in the indoor air of the control tower maintenance room, potentially affecting employees, where a correlation was found between aircraft number and concentrations of light aldehydes/ketones [ 58 ].

Summary and perspectives

Emission measurement studies are continuously conducted at international airports, such as Amsterdam Airport Schiphol (AMS) [ 28 , 59 ], Rome Ciampino (CIA) [ 60 ], London Heathrow (LHR) [ 61 , 62 ], Beirut-Rafic Hariri International Airport (RHIA) [ 63 ], Hartsfield-Jackson Atlanta International Airport [ 64 ], Los Angeles International Airport (LAX) [ 32 , 49 , 65 ], and other large airports in California [ 66 ] which besides measurements of the previously mentioned compounds, also often include analyses of emission patterns and weather conditions, and characterizations of particle size- and mass distributions [ 67 ]. The data from these emission studies and physical-chemical studies of emissions including particle matter (PM), from which we referenced some in the previous sections, were recently reviewed thoroughly [ 9 ]. To summarize the previous section, we repeat some selected important points regarding airport-sourced particles that were deducted from the available data by Stacey [ 9 ]:

Particle numbers near airports are significantly higher than away from airports and jet engines are a significant source of UFP . This means that urban areas in the vicinity of airports are at risk of increased exposure to UFP in addition to normal daily background and traffic-related emissions, but airport personnel working on the ground are in significant risk of exposure, simply due to proximity.

The highest concentrations of UFP are measured downwind of aircraft . Due to the occupational potential of exposure for airport ground workers there is a growing necessity of further studies of dispersion, size distributions and environmental factors affecting these emissions. Stacey [ 9 ] highlights that measurements at longer distances are highly influenced by physical and chemical processes affecting the emissions in the air, including volatile compounds. As such, there is a need for increased standardization of methods and instruments to facilitate valid comparisons between studies within this field, as has been established in general for environmental particulate matter (PM) measurements.

Aircraft emissions are dominated by very small particles of < 20 nm. This may be a way to separate these from other emission sources, such as road traffic, where the main particle fraction are of larger sizes. Smaller particle size means higher specific surface area. Smaller particles deposit in the deep end of the lung during inhalation and the total surface area of the deposited nanoparticles has been suggested to be predictive of toxicological potential in the lung [ 68 ].

The majority of non-volatile airport emission particles are carbonaceous (consisting of elemental and organic carbon compounds) .The emissions from aircraft consists of high numbers of soot particles with associated PAHs and metals, and thus, their physico-chemical composition is similar to diesel exhaust particles [ 36 ].

Diesel exhaust is classified as carcinogenic to humans by IARC [ 69 ], and cause lung cancer, systemic inflammation, and inflammatory responses in the airways [ 70 ]. Animal studies have shown that the particulate fraction of diesel exhaust is mutagenic and carcinogenic [ 71 ], whereas filtered diesel exhaust does not cause cancer [ 72 ]. Exposure to standard reference diesel particle SRM1650b and carbon black (CB) induce pulmonary acute phase response, neutrophil influx, and genotoxicity in mouse models [ 73 , 74 , 75 , 76 , 77 , 78 ]. Genotoxicity has been observed even at very low doses of CB [ 79 ]. In a meta-analysis of exposure to diesel exhaust and lung cancer occurrence in three occupational studies, the identified dose-response relationship showed that occupational exposure to 1 μg EC/m 3 during a 45 year work life would cause 17 excess lung cancers per 10,000 exposed using the EC content of diesel exhaust as metric [ 80 ]. Another recent analysis of 14 case-control studies estimated exposure to diesel exhaust particles using job-exposure matrices. In this study, occupational exposure to 1 μg EC/m 3 during a 45 year work life would cause 4 excess lung cancers per 10,000 exposed using the EC content of diesel exhaust as metric [ 81 ].

Carcinogenic substances are evaluated and listed by the International Agency of Research in Cancer (IARC) under WHO according to accumulated scientific findings in cellular, animal and human studies. Group 1 entails substances with sufficient evidence of carcinogenicity in humans and group 2 includes substances that IARC has classified as probably (2A) or possibly (2B) carcinogenic to humans [ 82 ]. As almost all current aviation fuel/jet fuels are extracted from the middle distillates of crude oil (kerosene fraction), which is between the fractions for gasoline and diesel [ 5 ] (whose combustion emissions are classified as group 2B and group 1 carcinogens, respectively [ 69 ]), there is cause for concern in terms of the potential carcinogenicity of exposure to jet fuel combustion products.

Exposure studies

Reported exposure levels for PAH, BC and UPF in the studies below are presented in Table  1 .

Childers et al. (2000): An extensive study of PAH concentrations at an airbase was carried out, using real-time monitors and air samplers on different locations and in different flight-related and ground-support activities. Airborne and particle-bound PAH were measured in a break room, downwind from an aircraft (C-130H) during engine tests, in a maintenance hangar, in an aircraft (C-130H) cargo bay during cargo-drop training and during engine running on/off loading and backup exercises, and downwind from aerospace ground equipment (diesel-powered electrical generator and a diesel-powered heater). Measurements were carried out with three different monitors. Total PAH concentrations followed a general trend of downwind from two diesel aerospace ground equipment units > engine on/off-loading exercise > engine tests > maintenance hangar during taxi and takeoff > background measurements in the maintenance hangar. Reported mean total PAH concentrations in integrated air samples (vapor phase) were 0.6011 μg/m 3 (hangar background), 1.0254 μg/m 3 (hangar taxiing), 2.8027 μg/m 3 (engine test), 6.7953 μg/m 3 (engine running on/off) and 9.8111 μg/m 3 (aerospace ground equipment). Dominating PAH in all exposure scenario was naphthalene, the alkyl-substituted naphthalenes, and other PAHs in the vapor phase. Particle-bound PAHs, such as fluoranthene, pyrene, and benzo[a]pyrene were also found. During flight-related exercises, PAH concentrations were 10–15 higher than in ambient air, and it was found that PAH contents fluctuated rapidly from < 0.02 to > 4 μg/m 3 during flight-related activities [ 83 ].

Iavicoli et al. (2006): In this study, occupational exposure risk to PAH and biphenyl was evaluated in an Italian airport during winter. Concentration and purification of 12 samples of 25 PAH by gas chromatography-ion trap mass spectrometry sampled for 24 h in three different locations of the airport showed general low levels, with highest levels of naphthalene (0.13–13.05 μg/m 3 ), 2-methylnaphthalene (0.064–28.5 μg/m 3 ), 1-methylnaphtalene (0.024–35.3 μg/m 3 ), and biphenyl (0.024–1.610 μg/m 3 ). Measured levels of the carcinogens benzo[b + j + k]fluoranthene and benzo[a]pyrene were 0.0542 μg/m 3 and 0.0086 μg/m 3 respectively [ 84 ].

Buonanno et al. (2012): Occupational exposure and particle number distributions were studied at an aviation base on a downwind site, close to the airstrip and by 10 daily UFP samples with personal monitors placed with a crew chief (assists the pilots during ground activities) and a hangar operator (aircraft maintenance). Particle number distribution averaged a total concentration of 6.5 × 10 3 particles/cm 3 at the downwind site. Short-term peaks during the working day mainly related to takeoff, landing and pre-flight operations of jet engines were measured in the proximity of the airstrip. Personal exposure concentrations were higher than stationary monitoring measurements. Personal exposure of workers were at a median number concentration of 2.5 × 10 4 particles/cm 3 for the crew chief and 1.7 × 10 4 particles/cm 3 for the hangar operator during the 2 months measurement period. The crew chief experienced the highest exposures, with maximum values at approximately 8 × 10 4 particles/cm 3 [ 86 ].

Møller et al. (2014): Personal exposure monitoring of particle number concentration was carried out in five different occupational groups, namely baggage handlers, catering drivers, cleaning staff, airside security and landside security in CPH, for 8 days distributed over 2 weeks. The study reported significant differences among the occupational groups. Highest exposures were found in baggage handlers (geometric mean: 37 × 10 3 UFP/cm 3 ), which was 7 times higher in average compared to landside security which are indoor employees (geometric mean: 5 × 10 3 UFP/cm 3 ). In between highest and lowest exposure groups, were catering drivers, cleaning staff and airside security with similar exposure levels (geometric mean: 12–20 × 10 3 UFP/cm 3 ) [ 87 ].

Targino et al. (2017): Black carbon (BC) particle concentrations were measured within different micro-environments of 12 airports and on 41 non-smoking commercial flights. Great variability was seen depending on environment measured. 70% of personal exposure during a journey occurred in the airport concourses and during transit to/from the aircraft. 18% was contributed to the waiting time onboard an aircraft with open doors waiting for loading. Largest BC exposure were found during boarding and disembarking (mean BC = 3.78 μg/cm 3 ; 25th, 50th, 75th percentiles: 1.29, 2.15, 4.68), at the airport concourse (mean BC = 3.16 μg/cm 3 ; 25th, 50th, 75th percentiles: 1.20, 2.15, 4.0) and inside parked aircraft with open doors (mean BC = 2.78 μg/cm 3 ; 25th, 50th, 75th percentiles: 0.35, 0.72, 2.33). BC levels were low in the aircraft on the ground with closed doors (mean BC = 0.81 μg/cm 3 ; 25th, 50th, 75th percentiles: 0.2, 0.35, 0.72, respectively). Lowest concentration was found during flights in the air [ 41 ].

Ren et al. (2018) a : The number concentrations and size distributions inside the cabin of an aircraft waiting for take-off were investigated and analyzed in comparison to outdoor UFP and the use of the ground air-conditioning cart (GAC) and environmental control system (ECS), which are used to provide conditioned air between boarding and doors closing to prepare for take-off. The study showed that environmental particle number concentration varied significantly, ranging from 10 to 40 × 10 3 particles/cm 3 depending on wind, and take-off and landing activities. When the GAC was on, the indoor particle numbers followed those outdoors, with the ECS providing protection factors for crew and passengers from 1 to 73% for 15–100 nm particles, and from 30 to 47% for 100–600 nm particles. A 40 min wait 100 m downwind of the runway was calculated to be equal to 4 h exposure in a clean urban background environment away from the airport [ 89 ].

Ren et al. (2018) b : In this study, the potential exposure to passengers as well as indoor airport staff was investigated by measurements in the terminal building of Tianjin Airport in Beijing of CO 2 , PM 2.5 , and UFP concentration and particle size distribution during three seasons. The effects on the indoor air quality of airliner-generated particles penetrating from the outdoor environment through open doors and by heating, ventilation and air-conditioning systems was studied.

PM 2.5 concentrations in the terminal building varied during the seasons of winter, spring and summer with 337–105-167 μg/m 3 in the arrival hall, 385–130-170 μg/m 3 in the departure hall, and 400–156-216 μg/m 3 in ambient airport air, respectively. These were significant higher levels compared to Chinese standard and WHO annual mean value of 10 μg/m 3 during all the tested seasons. The indoor environment was significantly affected by the outdoor air levels (Spearman: p  < 0.01). Particle number concentration in the terminal building displayed two size distribution, with one mode at 30 nm and a mode at 100 nm, which was significantly different from the size distribution measured in a normal urban environment, which had one peak at 100 nm. The study reports particle number concentrations of 1.9–5.9 times higher in the terminal buildings than the concentrations measured in a normal urban environment by different size bins. Measured total UFP exposure during an entire average waiting period (including in the terminal building and airliner cabin) of a passenger was estimated to be equivalent to 11 h of exposure to normal urban emissions [ 90 ].

Bendtsen et al. (2019): In this study, the occupational exposure levels to particles was evaluated by measurements at a non-commercial airfield and particles were collected and characterized at a non-commercial airfield and from the apron of a commercial airport.

Electron microscopy showed that the aerosol at the non-commercial airfield appeared to be mainly aggregates of soot, whereas the aerosol at the apron of the commercial airport appeared much more complex dominated by agglomerated soot particles, salt crystals and pollen. At the commercial airport, particles were mainly below 300 nm in diameter and distributed in two modes with geometric mean diameters of < 20 nm and approximately 140 nm. At the non-commercial airfield, two full cycles of a normal workflow of plane leaving, plane arriving and refueling by were recorded in a jet shelter using stationary and portable devices including in the breathing zone of personnel. Average particle number concentration for a full workflow cycle of 170 min were 1.22 × 10 6 particles/cm 3 . For take-off and landing of one jet plane, average particle number concentrations and mass were 7.7 particles/cm 3 and 1086 μg/m 3 and 2.67 particles/cm 3 and 410 μg/m 3 , respectively. During the main combustion events of plane leaving and arriving, the instruments reached their upper detection limits of 10 6 particles/cm 3 (DiSCmini, which measures particle number concentration, mean particle size and lung-deposited surface area) and 10 8 particles/cm 3 (ELPI, which monitors real-time particle levels), including in the breathing zone monitor of the personnel. Prevalent particle sizes suggested that the jet engine combustion particles were < 10 nm in aerodynamic diameter [ 36 ].

Mokalled et al. (2019): In this study, 48 volatile organic compounds (VOC) from approximately 100 commercial aircraft during real operations of different engine modes at Beirut Rafic Hariri International Airport were assessed to identify specific markers, together with measurements of Jet A-1 kerosene fuel vapors and gasoline exhaust.

Heavy alkanes (C8-C14, mainly n-nonane and n-decane) contributed to 51–64% of the total mass of heavy VOCs emitted by aircraft. Heavy aldehydes (nonanal and decanal) was reported as potential tracers for aircraft emissions due to their exclusive presence in aircraft-related emissions in combination with their absence from gasoline exhaust emissions. Total concentration of heavy alkanes in the ambient air was 47% of the total mass of heavy VOCs measured. No aircraft tracer was identified among the light VOCs (≤ C7). VOC compositions in jet exhaust varied with combustion power, and it was shown that light VOC emissions decrease as the engine power increases. Auxiliary power unit (APU) emissions were identified to be of the same order of magnitude as main engine emissions [ 93 ].

Marcias et al. (2019): In this study, occupational exposure to ultrafine particles and noise was investigated for 33 male employees working in an airport taxiway in a smaller Italian airport. Job categories represented were aircraft ground equipment personnel, firefighting officer, flight security agent, and aviation fuel administration staff. Both stationary sampling (ELPI) and personal particle measurements were included. The morphology and chemical composition was determined by EM and EDS, and showed small soot particles in aggregates with sodium, potassium, magnesium, calcium, aluminium, carbon, nitrogen, silicon, oxygen, fluorine, chlorine and sulphur. The maximal UFP number concentration (9.59 × 10 6 particles/cm 3 ) on stationary equipment was measured during support tasks in taxiing and taking off of the aircraft. Median UFP number concentration measured with personal monitors on the 33 operators was 2.44 × 10 3 particles/cm 3 and a maximum of 13 × 10 3 particles/cm 3 . Average size range was 35–103 nm. A significant difference in mean size and distributions was found between job tasks, where flight security officers were exposed to particles with lower mean sizes as compared to aircraft ground equipment operators [ 92 ].

Residential exposure

Westerdahl et al. (2008): Air measurements were carried out in the vicinity of LAX to assess the spread of airport emissions in downwind ambient air to the immediate neighborhood. Ultrafine particle numbers (UFP), size distributions, particle size, black carbon (BC), nitrogen oxides (NOx), and particle-bound PAH were measured. The lowest levels of pollutants were measured upwind of the airport, where UFP ranged from 580 to 3800 particles/cm 3 , black carbon from 0.2 to 0.6 μg/m 3 , and particle-bound PAH from 18 to 36 ng/m 3 . In contrast, at 500 m downwind of the airport, average UFP counts of 50,000 particles/cm 3 were observed, which were significantly influenced by aircraft operations where peaks were observed. Black carbon, particle-bound PAH, and NO x were also elevated, although not in the same extent, and the authors observed that BC, particle numbers, and NOx levels varied together in similar patterns indicating they were associated with similar sources. Black carbon concentrations varied across the measurement sites, with a mean of 0.3 μg/cm 3 upwind from the airport, 0.7 μg/cm 3 downwind from the airport, 1.8 μg/cm 3 at the taxiway, and 3.8 μg/cm 3 in the terminal region. Mean PM-PAH levels were 18.2, 24.6, 50.1 and 60.1 ng/m 3 at the measurement sites, respectively. PM-PAH mean levels measured on two freeways were 47.0 ng/m 3 and 169.4 ng/m 3 . The maximum UFP measured was 4.8 × 10 6 particles/m 3 downwind from a jet aircraft taking off. NOx levels before the take-off were around 8 ppb and increased to 1045 ppb, mostly due to NO. Black carbon rose from approximately 800 to 9550 ng/m 3 , and PM-PAH values increased from 37 to 124 ng/m 3 . Significant variations were observed in particle sizes, where upwind measurements were dominated by particles of 90 nm, and downwind particles were of 10–15 nm in size. The author noted that UFP levels from aircraft were measured to persist up to 900 m from the runways, indicating potential risks for the nearby communities [ 34 ].

Lopes et al. (2019): In this study, data is presented from UFP monitoring at several sampling sites in the vicinity of Lisbon Airport in 2017 and 2018, for 19 non-consecutive days. Measurements included sites further away from the airport, under the landing/take-off path. Correlation analysis between air traffic activity and UFP concentrations was conducted and show the occurrence of high UFP concentrations in the airport vicinity. The particle counts increased 18–26 fold at locations near the airport, downwind, and 4-fold at locations up to 1 km from the airport. Results show that particle number increased with the number of flights and decreased with the distance to the airport [ 91 ].

Pirhadi et al. (2020): In this study, the contributions of airport activities to particle number concentrations (PNCs) at Amsterdam Schiphol was quantified by use of the positive matrix factorization (PMF) source apportionment model. Various pollutants were measured, including NOx and CO, black carbon, PM2.5 mass, and the number of arrivals and departures were measured for 32 sampling days over 6 months. Airport activities accounted for 79.3% of PNCs divided in aircraft departures, aircraft arrivals, and ground service equipment (GSE) (with contributions of local road traffic, mostly from airport parking areas). Aircraft departures and aircraft arrivals contributed to 46.1 and 26.7% of PNCs, respectively, and were characterized by particle diameters < 20 nm. GSE and local road traffic accounted for 6.5% of the PNCs and were characterized by diameters of around 60–80 nm. Traffic from surrounding freeways was characterized by particles of 30–40 nm and contributed to 18% of PNCs. In comparison, the urban background emissions dominated the mass concentrations with 58.2%, but had the least contribution to PNCs with 2.7% [ 85 ].

Summary of exposure studies

Occupational exposure to increased levels of nanosized particles [ 36 , 85 , 86 , 87 , 88 , 89 , 90 , 92 ], increased levels of PAH including known human carcinogens [ 52 , 83 , 84 ], and black carbon [ 41 ] were reported in the literature. Levels of exposure reported in these studies are summarized in Table 1 . One study reported that personnel monitors measured higher levels compared to stationary equipment [ 87 ], and it was shown that ground support equipment, such as diesel-powered electrical generators and heaters [ 83 ] and auxiliary power units [ 93 ] contribute significantly to emissions.

Three important main factors were identified which significant influenced occupational exposure: proximity to emission sources , where levels were generally higher in close proximity and down-wind to aircraft, fluctuations in emission levels , characterized by exposure peak events such as landing- or take-off, and job type , where outdoor ground-affiliated work types are at highest risk of exposure. As such, airport personnel can likely be grouped in low (office staff/landside jobs with indoor work, far away from emission sources), medium (catering/cleaning/landside security staff with intermittent outdoor work) and high (baggage handlers/aircraft mechanics/ crew chief) exposure groups.

The majority of studies on the contribution of airport emissions to air pollution in the surrounding environment are physical/chemical studies of particle numbers, mass and related air pollutants, which are reviewed elsewhere as previously described.

More studies reported increased risk of exposure correlating with decreased distance to airports [ 94 , 95 , 96 ] and time spent downwind from an airport [ 97 ], hence a significant factor for potential health effects for neighboring residential areas based on these studies is distance to airports, which relating to wind and atmospheric conditions is an important determinant for pollution levels.

Health effects

Here we present studies in which direct health effects have been assessed in humans, including in biomonitoring and epidemiological studies, and biological mechanisms-of-action assessed in animal or cell studies. Our main focus is particle exposure, however, studies focusing more on VOC/PAH are also presented.

Occupational studies

Møller et al. (2017 and 2019): A prospective, occupational cohort study in CPH, encompassing 69,175 men in unskilled positions as baggage handlers or in other outdoor work used register information of socioeconomic, demographic and health data together with a job-exposure matrix was based on GPS measurements within the airport, detailed information on tasks from 1990 to 2012, exposure to air pollution at home, and lifestyle details. Occupational exposure groups were categorized according to work time at the apron, “apron-years” (non-exposed, 0.1–2.9, 3.0–6.9 and ≥ 7 years). The reference group comprised different low-exposure occupational groups [ 98 ]. A follow-up study was conducted on an exposed group of 6515 male airport workers at 24–35 years of age in unskilled positions with a reference group of 61,617 men from greater Copenhagen area in unskilled jobs. Exposure was assessed by recordings of time spent on the airport apron and diagnoses of ischemic heart disease and cerebrovascular disease was obtained from the National Patient Register. No associations between cumulative apron-years and the two disease outcomes were found. On the other hand, since the exposed group had a mean age of 24–35 years, a 22-year follow-up may have been too short to detect cardiovascular effects [ 99 ].

Lemasters et al. (1997): In this early study, mixed low-level exposure to fuel and solvent was studied in a repeated measures design with male aircraft workers at a military air station serving as their own controls from pre-exposure to 30 weeks post-exposure. The study group consisted of six aircraft sheet metal workers mainly exposed to solvents, adhesives and sealants, six aircraft painters exposed to solvents and paints, 15 jet fueling operations personnel ( n  = 15) responsible for fuel delivery, fueling and defueling aircraft and repairing fuel systems, and 23 workers in the flight line crew exposed to jet fuel, jet exhaust, solvents and paint, and included ground crew and jet engine mechanics. Expired breath analysis was carried out for different trace compounds, but was found to have low values (< 25 parts per billion). An increase in sister chromatid exchange (SCE) compared to pre-exposure was found after 30 weeks of exposure for sheet metal workers (mean SCE per cell increased from 6.5 (SD: 0.8, range: 5.5–7.7) to 7.8 (SD: 0.3, range: 7.4–8.2) and painters (mean SCE per cell increased from 5.9 (SD: 0.7, range: 5.0–6.8) to 6.7 (SD: 1.0, range 5.3–7.8)), indicating exposure to genotoxic substances for these subgroups [ 100 ].

Tunnicliffe et al. (1999): In Birmingham International Airport, occupational exposure to aircraft fuel and jet stream exhaust was evaluated in terms of respiratory symptoms and spirometry in 222 full-time employees according to job title. Data was collected by questionnaire and with on-site measurement of lung function, skin prick tests, and exhaled carbon monoxide concentrations. Occupational exposure was assessed by job title, where baggage handlers, airport hands, marshallers, operational engineers, fitters, and engineering technicians were considered as high exposure groups, security staff, fire fighters, and airfield operations managers as medium exposure group, and low exposure groups consisted of terminal and office workers. Upper and lower respiratory tract symptoms were commonly reported in the questionnaire and 51% had one or more positive allergen skin tests. Cough with phlegm and runny nose were found to be significantly associated with high exposure (adj. OR = 3.5, CI: 1.23–9.74; adj. OR = 2.9, CI: 1.32–6.4, respectively). Upper and lower respiratory symptoms were common among exposed workers, but no significant difference was found in lung function. The authors conclude that it is more likely that these symptoms reflect exposure to exhaust rather than fuel [ 101 ].

Yang et al. (2003): The aim of this study was to evaluate self-reported adverse chronic respiratory symptoms and acute irritative symptoms among 106 airport workers in risk of exposure to jet fuel or exhaust (jet fuel handlers, baggage handlers, engineers etc.) compared to 305 terminal or office workers (control group) at Kaohsiung International Airport (KIA) in Taiwan. The odds ratio analyses were adjusted for possible confounding factors, such as age, marital status, education, duration of employment, smoking status, and previous occupational exposure to dust or fumes. The prevalence of acute irritative symptoms was not significantly different, whereas chronic respiratory symptoms such as cough (adj. OR = 3.41, CI: 1.26–9.28) and dyspnea (adj. OR = 2.34, CI: 1.05–5.18) were significantly more common among airport workers. The study did not report exposure measurements, but the authors conclude that the expected higher exposure of aviation fuel or exhaust in the ground personnel is the likely explanation for the increased incidence of self-reported chronic respiratory health-effects compared to the office personnel [ 102 ].

Whelan et al. (2003): Prevalence of respiratory symptoms among female flight attendants along with teachers was investigated by self-reported questionnaire in comparison to database-derived data on blue collar workers with no known occupational exposures, and it was found that female flight attendants and teachers were significantly more likely to report work related eye (12.4 and 7.4%), nose (15.7 and 8.1%), and throat symptoms (7.5 and 5.7%), and more episodes of wheezing and flu, compared to other female workers (2.9% eye, 2.7% nose, and 1.3% throat symptoms). Female flight attendants were significantly more likely than teachers and controls to report chest illness 3 years in retrospective (flight attendants: 32.9%, teachers: 19.3%, female workers: 7.2%) [ 103 ].

Cavallo et al. (2006): In this study, 41 airport employees in jobs with very close proximity to aircraft in service (fitters, airport hands, marshallers, baggage handlers) or in jobs with some proximity to aircraft (security staff, maintenance service personnel, cleaning staff, air field operations managers, runway shuttle drivers) in Leonardo da Vinci airport in Rome were evaluated for exposure to aircraft emissions along with biomarkers of genotoxicity in comparison to a control group of 31 office workers at the same airport. Job tasks in very close proximity to aircraft in service were considered to be high exposure jobs. Urinary PAH metabolites were used as biomarker of endogenous PAH exposure in parallel with PAH analyses of air samples. Exfoliated buccal cells and blood were evaluated for DNA damage, e.g. micronuclei, chromosomal aberrations and sister chromatid exchange (SCE). PAH exposure was measured during 24 h of 5 work days at the airport apron, airport building and terminal/office area from January to February 2005. Total mean of 23 PAHs (particle and vapour) at the apron, airport building and terminal departure area were 27.7, 17.2, and 9.5 μg/m 3 , respectively, with a prevalence of 2–3 ring PAHs with highest levels in the airport apron particularly for 1- and 2-methylnaphthalene and acenaphthene. Urinary PAH metabolite levels were similar for high exposure job groups and controls. The exposed group showed increased SCE (mean number: 4.61 ± 0.80) compared to control group (3.84 ± 0.58) and increased levels of chromosomal aberrations and DNA strand breaks in the Comet assay in both buccal cells and lymphocytes, indicating genotoxic exposures [ 52 ].

Radican et al. (2008): A follow-up study of 14,455 workers from 1990 to 2000 evaluated the mortality risk from trichloroethylene and other chemical exposures in aircraft maintenance workers. Relative risk (RR) for exposed compared to unexposed workers were calculated, and positive associations with several cancers were observed, but mortality had not changed substantially since 1990, with increased risk of all-cause mortality (RR = 1.04, CI: 0.98–1.09) or death from all cancers (RR = 1.03, CI: 0.91–1.17) [ 104 ].

Erdem et al. (2012): A study group consisting of 43 aircraft fuel maintenance staff, fuel specialists, and mechanics occupationally exposed to JP-8 fuel directly or via engines of jet planes were evaluated for the metabolites 1- and 2-naphthol and creatinine in urine as biomarkers of exposure to jet fuel. In turn, sister chromatid exchange (SCE) and micronuclei were evaluated in blood-derived lymphocytes as biomarkers of genotoxic exposure. Urinary markers and SCE were significantly increased in exposed workers (1-naphthol: 99.01 μmol/mol creatinine; 2-naphthol: 77.29 μmol/mol creatinine), by 10-fold as compared to a control group of 38 employees working in the same area without any work-related exposure to JP-8 fuel [ 105 ].

Marie-Desvergne et al. (2016): In this study, exposure to airport nanoparticles and metals was evaluated in airport workers by exhaled breath condensate (EBC) as a non-invasive representative of the respiratory system. EBC was collected from 458 airport workers from Marseille Provence Airport and Roissy Charles de Gaulle Airport in Paris, working directly on the apron (exposed) or in the offices (less exposed). In addition, ambient nanoparticle exposure levels were characterized in terms of particle number concentration, size distribution and by electron microscopy.

The study showed that airport workers were exposed to significantly higher particle numbers (1.0 × 10 4 –2.1 × 10 7 particles/cm 3 ) compared to office staff (10 3 –10 4 range equivalent to background traffic emissions), although office workers were periodically exposed to peaks of 10 4 –10 5 when the building doors were open. Airport workers were exposed to significantly smaller particles (mean geometric size: 17.7) compared to office workers (mean geometric size: 23.7). EBC was characterized by volume, total protein content, and a multi-elemental analysis was used to.

measure Na, Al, Cd, and Cr. Particles in EBC were analyzed with dynamic light scattering and electron microscopy (SEM-EDS).

A significantly higher concentration of Cd was found in apron worker EBC (mean: 0.174 ± 0.326 μg/l) in comparison with office workers (mean: 0.108 ± 0.106 μg/l). Particulate content in EBC was confirmed by DLS and SEM-EDS, but no differences were found between the two study groups, and measured EBC particle contents did not correlate with ambient exposure levels [ 88 ].

Studies on effects of residential exposure to airport emissions

Visser et al. (2005): In this population-based study, it was investigated if the residents living around Amsterdam Schiphol Airport were at higher risk of developing cancer compared to the general Dutch population. The regional cancer registry was used, estimating the cancer incidence from 1988 to 2003 in the population residing near the airport compared to the national cancer incidence. The exposure was defined by aircraft noise and postal code areas, as historical data on ambient air pollution were unavailable. The study did not include information on lifestyle factors, and therefore, did not control for smoking and other potential confounders. A core zone closest to the airport and a remaining ring zone was studied. Thirteen thousand two hundred seven cancer cases were identified in the study area, and a significant increase in the incidence of hematological cancers (standardized incidence ratio, SIR = 1.12, CI: 1.05–1.19) was found, mainly due to non-Hodgkin lymphoma (SIR = 1.22, 95% CI: 1.12, 1.33) and acute lymphoblastic leukemia (SIR = 1.34, CI: 0.95, 1.83). Respiratory system cancer incidence was significantly decreased (SIR = 0.94, CI: 0.90, 0.99), due to the low rate in males (SIR = 0.89). The study concludes that the overall cancer incidence in the residential areas closest to Amsterdam Schiphol Airport was similar to the national incidence. The increase in the risk of hematological cancers could not be explained by higher levels of ambient air pollution in the area [ 106 ].

Lin et al. (2008): In this cross-sectional study, it was assessed whether residents living near commercial airports had increased rates of hospital admissions due to respiratory diseases compared to those living further away. The study included all residents living within 12 miles from the center of each of three airports (Rochester in Rochester, LaGuardia in New York City and MacArthur in Long Island). Hospital admission data were collected by the New York State Department of Health for all residents who were hospitalized for asthma, chronic bronchitis, emphysema, chronic obstructive pulmonary disease and, for children aged 0–4 years, bronchitis and bronchiolitis during 1995–2000. Exposure indicators were distance from the airport and dominant wind patterns from the airports.

The relative risks of hospital admissions due to respiratory conditions for residents living < 5 miles from the airport were 1.47 (CI: 1.41–1.52) for Rochester and 1.38 (CI: 1.37–1.39) for LaGuardia, as compared to those living > 5 miles from the airports. No differences were observed for MacArthur airport. When considering hospital admission rates by distance for 12–1 miles towards the airports, a significant trend of increasing hospital admissions with closer distance to the airport was observed for the Rochester airport. The authors reported a stronger effect for traditionally lower socio-economic groups [ 94 ], which may be of more relevance in the US, due to the medical insurance system.

Habre et al. (2018): In this study, 22 non-smoking volunteers with mild to moderate asthma were recruited to do scripted mild walking activity in parks inside or outside a zone of high airport-related ultrafine particle exposure downwind of LAX. Physiological parameters were measured before and after exposure, and the study was conducted as a cross-over study, such that the participants served as their own controls. Personal exposure to black carbon, PAH, ozone, and PM 2.5 were measured and combined with source appointment analysis and health models. A difference in PM exposure was found between the high (mean particle number concentration of 53,342 particles/cm 3 and mean particle size of 28.7 nm) and the low exposure zone (mean particle number concentration of 19,557 particles/cm 3 and mean particle size of 33.2 nm). It was reported that IL-6 levels in blood were increased after the walk in the high exposure zone compared to the low exposure zone. Airport-related PM was distinguished from roadway traffic emissions by principal component analysis, and increase of airport-related PM was significantly associated with increased IL-6 levels [ 107 ].

Amsterdam Schiphol report (2019): Based on three studies with 191 primary school children from residential areas near Schiphol Airport, 21 healthy adults living adjacent to the airport [ 108 ], and an in vitro study [ 109 ], respectively, this Dutch report (not subjected to peer review) describes the findings of reduced lung function in children and adults following higher short-term exposure to ultrafine particles near Schiphol Airport. On days with high exposure, children suffered more from respiratory complaints and used more medicine. In the adults, short-term reductions in heart function were also found. The authors note that these effects may be larger for individuals already suffering from medical conditions. The authors point out that the effects are results of ultrafine particles from both air and road traffic, and that there are no indications that health effects of air traffic emissions are different from those caused by road traffic [ 59 ].

Lammers et al. 2020: This study investigated the health effects of controlled short-term exposure of 21 healthy non-smoking volunteers aged 18–35 years to UFP near Shiphol Airport Amsterdam. The volunteers were exposed 2–5 times to ambient are for 5 h while cycling. Cardiopulmonary outcomes such as spirometry, forced exhaled nitric oxide, electrocardiography and blood pressure were measured before and after exposure, and compared to measured total- and size-specific particle number concentrations (PNC). Average PNC was 53,500 particles/cm 3 (range 10,500–173,200). Increase in exposure to UFP was associated with a decrease in FVC and a prolongation of the corrected QT interval, which were associated with particle sizes < 20 nm (UFP from aviation), but not with particles > 50 nm (UFP from road traffic). Although the effects were relatively small and measured after single exposures of 5 h in young healthy adults [ 108 ], such effects could be important in susceptible sub-populations.

Animal studies and in vitro studies

Ferry et al. (2011): Immature primary human monocyte-derived dendritic cells (DCs) from healthy donor blood were exposed for 18 h to different doses of experimental jet exhaust particles in absence or presence of E. coli lipopolysaccharides (LPS). Antigen-presenting and stimulatory molecules were measured along with tumor necrosis factor (TNFα) and IL-10. The effects were assessed on immature and mature DCs as well as on cells during the maturation process.

The primary particles collected from the jet exhaust by direct impaction were found to be spherical and carbonaceous primary particles of ~ 10 nm and aggregates up to ~ 93 nm. No toxic effects were observed for doses below of 100 μg/mL jet engine particles. Maturation of immature dendritic cells by LPS stimulation induced a significant 500-fold increase in TNFα and 30-fold increase in IL-10. Immature dendritic cells produced low amounts of TNFα (fold change from LPS: 0.006) and IL-10 (fold change from LPS: 0.11), which increased non-significantly upon stimulation with particles (fold change from LPS: TNFα: 0.11, IL-10: 0.19). However, simultaneous exposure to LPS and a high particle dose of 100 μg/ml induced a 2-fold increase in TNFα production compared to LPS-maturation ( p  = 3 × 10 − 5 ). Different activation patterns were seen for the expression of HLA DR and CD86, which are dendritic cell maturation markers. It was concluded that jet exhaust particles may act as adjuvants to endotoxin-induced dendritic cell maturation, which may influence potential effects on human health [ 110 ].

Shirmohammadi et al. (2018): PM 0.25 collected at the vicinity of Los Angeles Airport (LAX) and from central Los Angeles (LA) close to and downwind from major freeways, from stationary sampling stations used for air quality control, were investigated. The particles were subjected to source allocation analyses of elements and carbon contents (see Introduction), and ROS formation was compared in rat alveolar macrophage cells (NR8383).

ROS activity measured as units of Zymosan equivalents were normalized by total PM0.25 mass to represent the intrinsic toxicity of the particles, and this mass-normalized ROS activity was similar for LAX (4600.93 ± 1516.98 μg Zymosan/mg PM) and central LA (4391.22 ± 1902.54 μg Zymosan/mg PM). According to the authors, volume-normalization of the ROS activity can be used as a metric for comparison of inhalation exposures, as an indicator of exposure severity. A slightly higher PM0.25 mass concentration in central LA meant overall similar volume-normalized ROS activity levels with no significant difference between the observed averages (LAX: 24.75 ± 14.01 μg Zymosan/m 3 , central LA: 27.77 ± 20.32 μg Zymosan/m 3 ). Thus, there were similar levels of ROS activity and similar toxic potential of the PM in the vicinity of LAX and in the vicinity of freeways in central LA [ 49 ].

He et al. (2018): PM 0.25 collected at Los Angeles Airport (LAX) and from central Los Angeles (LA) close to and downwind from major freeways (similar collection sites as in [ 49 ]) were investigated and compared. Particles were source-allocated by analyzing elements (see Introduction). Particles collected at LAX were primarily associated with aircraft emissions, and particles from central LA with urban traffic, road and dust emissions. The reactive oxygen species (ROS) potential was evaluated intracellularly in human bronchial epithelial cells (16HBE) after 1, 2, and 4 h of exposure, and IL-6, IL-8 and TNF were measured as markers of inflammation.

Exposure of 16HBE cells to 10 μg/mL particles produced significantly elevated ROS levels for both samples compared to unexposed cells. Particles from central LA generated slightly more ROS than LAX samples per mass unit, and both were at negative control level after 20 h recovery. ROS potential in PM from both airport and central LA correlated with some of the measured traffic-related transition metals (Fe and Cu). Particles from LAX induced increased expression of IL-6, IL-8 and TNFα compared to the negative control (1.7, 1.8, and 1.4-fold, respectively), whereas central LA-particles induced slightly lower expressions (1.3, 1.3, and 1.1-fold, respectively). Hence, overall LAX particles had similar inflammatory potency as particles from central LA, showing that airport PM 0.25 contributions to urban emission PM pollution possess similar inflammatory properties [ 50 ].

Jonsdottir et al. (2019): In this study, aerosol was collected from the world’s most used aircraft turbine (CFM56–7B26, run-in and airworthy) in a test cell at Zurich Airport. The test cell is open to the ambient environment and the aerosol was collected from both standard Jet A-1 fuel and a HEFA fuel blend. The toxicity of the non-volatile PM emissions was studied by direct particle deposition onto air-liquid interface cultures of human bronchial epithelial cells (BEAS-2B).

Cytotoxicity was evaluated by the release of cytosolic LDH from damaged cells, expression of the oxidative stress marker HMOX-1 and inflammatory cytokines IL-6 and IL-8.

Single, short-term (1 h) exposure to PM increased cell membrane damage, lead to oxidative stress and increased pro-inflammatory cytokines in bronchial epithelial cells, depending on fuel type and combustion conditions from which the particles were produced. PM from conventional fuel at ground-idle conditions was most potent, and the authors comment that PM from aircraft turbine exhaust may be a risk to respiratory health, also by making airway epithelia vulnerable to secondary exposure of other air pollution compounds and pathogens [ 111 ].

Bendtsen et al. (2019): In this study, the toxicity of particles collected in a commercial and a non-commercial airport were evaluated in vivo by intratracheal instillation in mice (see section 2.3 for occupational exposure measurements). Adult female C57BL/6 mice were exposed to 6, 18, and 54 μg particles/mouse dispersed in Nanopure water by sonication. The exposure doses were calculated on the basis of worst case scenario: of the maximum exposure level measured at the non-commercial airport of 1086 μg/m 3 at the peak event of plane departure, 9.6% were estimated to deposit in the alveolar lung regions. This was adjusted to the volume of a mouse lung and to 8 h of work, estimating exposure of 4, 12, and 39 days of work, respectively. Control mice were exposed to Nanopure water, and positive controls were carbon black Printex90 nanoparticles and SRM2975 diesel particles. Exposed mice were euthanized on day 1, 28, and 90 post-exposure. Inflammation was measured as inflammatory cell influx in bronchoalveolar lavage fluid as well as by the acute-phase response marker serum amyloid A ( Saa ) in lung (mRNA), liver (mRNA) and blood (protein). Genotoxicity was assessed by the comet assay on lung and liver tissue and cells from the bronchoalveolar lavage fluid. Analysis of the particles by scanning and transmission electron microscopy showed small primary particles and agglomerates of soot, which appeared uniform for non-commercial airport particles (mainly from jet engine emissions) and more heterogenous for the commercial airport particles (emissions from aircraft, ocean, traffic and background). Pulmonary exposure to particles from both airports induced genotoxicity and dose-dependent acute phase response, and inflammation at same levels as standard diesel exhaust particles and carbon black nanoparticles [ 36 ].

He et al. 2020: In this study, UFPs from aviation or road traffic emissions were collected near the major international airport, Amsterdam-Schiphol airport (AMS), along with UFPs from an aircraft turbine engine at low and full thrust. The toxicity of the particles was tested in human bronchial epithelial cells (Calu-3) combined with an air-liquid interface (ALI) system with exposure to UPFs at low doses from 0.09 to 2.07 μg/cm 2 . Cell viability, cytotoxicity and IL-6 and -8 secretion were assessed after 24 h exposure. Cell viability was < 80% for all doses. LDH release as measure of cytotoxicity was observed at the highest exposure dose around 1.5 μg/cm 2 together with increased production of IL-6 and IL-8 compared to control exposure (blank filter extraction or re-suspension solution). It was concluded that airport and road traffic UFP as well as UFP samples from the turbine engine had similar inflammatory properties [ 109 ].

Summary of health effect studies

Increased levels of metabolites in urine as biomarkers of internal exposure to jet fuel [ 105 ] were reported in biomonitoring studies of occupational exposure to airport emissions. Exposure to airport emissions was associated with increased levels of biomarkers of genotoxicity, in terms of increased levels of SCE [ 52 , 100 , 105 ] and DNA strand breaks in the Comet assay [ 52 ], which indicates exposure to genotoxic and potential carcinogenic agents in the emissions. In turn, there were occupational studies reporting increased levels of self-reported respiratory complaints [ 101 , 102 , 103 ].

We identified a limited number of studies and one report reporting correlations between airport emission levels and health effects of residents in the vicinity of airports: Aircraft emission levels were associated with increased hospitalization for asthma, respiratory, and heart conditions especially in susceptible subgroups such as children below 5 years of age, elderly above 65 years of age [ 66 , 94 ] and lower socioeconomic groups [ 97 , 112 ]. A Dutch report on Schiphol similarly reported that school children and adults took more medication and had more respiratory complaint on days with increased exposure to aircraft emissions and concludes that health effects of air traffic emissions are similar to those caused by road traffic [ 59 ]. A biomonitoring study showed increased blood levels of the inflammatory marker IL-6 in volunteers with mild to moderate asthma after a walk in a zone with high levels of aircraft emissions [ 107 ]. It is well-known that other types of air pollution including diesel exhaust cause morbidity and mortality [ 113 ]. Taken together, these results suggest that the exposure to aircraft emissions induce pulmonary and systemic inflammation, which potentially contributes to cancer, asthma, respiratory and coronary heart disease.

Five mechanistic studies on the toxicity of airport particles were identified, one animal study in mice and four cell studies: Airport particles were reported to act as adjuvants in the activation of inflammatory cells or pathways [ 110 ] and induce pro-inflammatory cytokines [ 111 ]. Airport particles were shown to have similar inflammatory potency and similar ability to induce DNA damage as traffic emission particles [ 50 ], such as diesel exhaust particles [ 36 ]. In turn, airport particles induced significant levels of the biomarker Saa following intratracheal instillation in mice, associated with risk of cardiovascular disease [ 36 ], and they have the potential to generate ROS at similar levels as traffic emission particles [ 49 , 50 ]. Thus, the conclusions from these in vitro and in vivo studies support the overall concern addressed in previous sections that airport emission particles are capable of inducing toxic responses comparable to the responses observed for other air pollution particles such as diesel exhaust particles.

Although a range of kerosene-based aircraft fuel types are in use, they are overall similar in chemical composition [ 24 , 29 ]. Kerosene lies between the distillated crude oil fractions of gasoline (gasoline combustion exhaust, IARC group 2b) and diesel (diesel combustion exhaust, IARC group 1) and the carcinogenic potential of jet fuel combustion products could be anticipated given the reported similarities to diesel exhaust particles. We highlight two important reported characteristics of airport particles:

The majority of non-volatile airport emission particles are carbonaceous and aircraft engines emit large amounts of nanoparticles, which are dominated by very small particles of < 20 nm, which form aggregates/agglomerates in ambient air

Particle numbers near airports are significantly higher than away from airports and jet engines are a significant source of UFP in ambient air. The highest concentrations of UFP are measured downwind of aircraft

The reported PAH levels [ 52 , 83 , 84 ] were all below the current Danish occupational exposure limit of 200 μg/m 3 . One study reported BC levels at the apron of 3.78 μg/m 3 and particle levels was overall reported to be between ~ 10 3 and 10 8 particles/cm 3 for exposed airport personnel (Table 1 ). The new exposure limit for diesel exhaust particles in EU is defined by the elemental carbon (EC) level and is 50 μg EC/m 3 [ 114 ]. The Netherland recently endorsed an OEL for diesel exhaust particles at 0.01 mg/m 3 measured as respirable EC. This was based on socioeconomic considerations and the Dutch prohibition risk level (OEL) is at 1.03 μg EC/m 3 [ 115 ], a level corresponding to 4 extra death cases of lung cancer per 1000 exposed, for 40 years of occupational exposure. Thus, the reported BC level [ 41 ] are well below the new EU OEL for diesel exhaust as well as the Dutch OEL, but exceed the Dutch prohibition risk level. Recently published data on the dose-response relationship between exposure to diesel exhaust particles and lung cancer in epidemiological studies estimated that occupational exposure to 1 μg/m 3 EC would cause 4 to17 excess lung cancer cases per 10,000 exposed [ 80 , 81 ].

The particle exposure levels can be compared to nanoparticle reference values used in The Netherlands, Germany and Finland as a provisional substitute when nano-specific OELs or DNELs for engineered nanoparticles are not available [ 116 ]. For low density insoluble nanomaterials such as carbon-based nanoparticles, the reference value is 40,000 particles/cm 3 . Compared to this reference value for engineered nanoparticles, the reported occupational exposure levels are high for some job groups.

Significant variations in emission levels are observed between airports, depending on factors such as size, type, location, and wind direction. However, the closer to the source of emissions, the higher the exposure. Proximity to exposure peak events such as landing and take-off is also an important determinant of high exposure. This is evident from the combined literature of occupational exposure measurements and ambient air measurements in residential areas around airports. As such, the highest levels of occupational exposure is found for airport personnel working at the apron, in close proximity to running jet engines. Airport personnel can likely be grouped in low (office staff/landside jobs with indoor work, far away from emission sources), medium (catering/cleaning/landside security staff with intermittent outdoor work) and high (baggage handlers/aircraft mechanics, crew chiefs) exposure groups [ 52 , 86 , 87 , 88 , 92 , 98 , 100 , 101 , 102 ]. To reduce occupational exposure, emission sources can be moved, the distance to emission sources can be increased, time spent in proximity to emission sources can be reduced and personal protection equipment can be used during peak exposures. Personal exposure may be higher than measured by stationary monitors, and thus, routine monitoring of personal exposure levels could be suggested.

Workplace experts, airport leaders and personnel groups have the necessary intrinsic knowledge and experience to suggest feasible, realistic options for reducing the exposure for specific job functions at individual airports.

The similarity of airport emission particles with diesel exhaust particles and pure carbon nanoparticles, with respect to physico-chemical properties as well as specific toxicological parameters was demonstrated in the animal study from our laboratory [ 36 ], and a growing number of studies report similar toxicity and health effects of emissions from airports and traffic. Airport emission particles likely have similar physico-chemical properties as diesel exhaust particles even though the primary particle size of jet engine emissions is somewhat smaller than the primary size of diesel exhaust particles. Diesel exhaust is classified as carcinogenic to humans by IARC [ 69 ], cause lung cancer, systemic inflammation, and inflammatory responses in the airways [ 70 ].

Aircraft emissions are associated with biomarkers of exposure, biomarkers of disease and health outcomes both for exposed workers [ 36 , 41 , 52 , 83 , 84 , 86 , 87 , 88 , 89 , 90 , 92 , 100 , 101 , 102 , 103 , 105 ] and for the general population living down-wind of airports [ 59 , 66 , 94 , 95 , 96 , 97 , 107 , 112 ]. Occupational exposure to aircraft emissions were associated with:

Biomarkers of exposure to jet fuel emissions

Biomarkers of genotoxic exposure

Self-reported respiratory distress

The reported adverse effects correlate with effects demonstrated in animal studies and in in vitro studies, where aircraft emission particles caused inflammation [ 50 , 110 , 111 ], acute phase response [ 36 ], reactive oxygen species [ 49 , 50 ] and DNA damage [ 36 ], which are biomarkers of risk of cancer, cardiovascular disease and respiratory disease. This supports the notion of a causal relationship between exposure to airport emissions and the observed health effects. Although mechanistic studies on airport emissions are scarce, knowledge from other closely related scientific areas still applies, such as particle toxicity, carcinogenicity/toxicity of VOCs and OPEs and epidemiological studies of health effects caused by air pollution [ 117 ].

Another relevant concern to raise in this context is the adverse health effects of low-level chronic occupational exposure to these chemicals, which is difficult to study [ 118 ]. OPEs have been associated with adverse health effects reported from cabin crew and pilots after occupational exposure to bleed air and fume events during flights, with symptoms of respiratory illness and neurological effects [ 119 ]. The dominant OPE used in lubrication oil is tri-cresyl phosphate (TCP), which are among the highly neurotoxic OPEs [ 120 ]. It has been suggested that brain exposure may occur via inhalation of circulating small jet particles associated with OPEs, crossing the blood-brain barrier [ 121 ] – neurotoxic effects of OPEs may also be an understudied occupational risk of apron staff.

It has been shown that air pollutants worsen pre-existing diseases, such as allergy or other inflammatory (airway) or cardiovascular conditions [ 2 , 3 , 4 , 122 , 123 , 124 ]. One example is a study examining the relationship between personal exposure to traffic emissions and acute respiratory health in school children with asthma residing in the Bronx, New York, which have the highest asthma incidence in New York City and state [ 125 ]. Personal samples of PM 2.5 , including the EC fraction, were collected 24 h daily for 40 school children with asthma from four schools, with spirometry and symptoms assessed several times daily. The study found increased relative risks of different airway symptoms, such as wheeze (RR = 1.45, CI: 1.03–2.04), shortness of breath (RR = 1.41, CI: 1.01–1.99), with relative risk of total symptoms of 1.30 (CI: 1.04–1.62). Interestingly, the symptoms were associated with increase in average 2-day school site and personal EC levels, but not mass of PM 2.5 [ 125 ]. As such, as demonstrated in asthmatic volunteers, residents living near airports, and supported by inflammatory effects shown in available in vitro studies, airport UFP and associated pollutants are, in addition to their direct adverse effects, likely to have the ability of worsen pre-existing disease.

The reported adverse health effects of jet engine emissions are similar to those caused by exposure to diesel exhaust and air pollution. However, given the lack of consensus on optimal measurement methods, equipment and quality control for near- and far field airport emissions and human risk assessments markers, more studies of exposure and of toxicological mechanisms are necessary.

These drawbacks are summarized efficiently by Lighty et al. in their paper on combustion compounds and health: “ There is a need for better integration of the combustion, air pollution control, atmospheric chemistry, and inhalation health research communities. Epidemiology has demonstrated that susceptible individuals are being harmed by ambient PM. Particle surface area, number of ultrafine particles, bioavailable transition metals, polycyclic aromatic hydrocarbons (PAH), and other particle-bound organic compounds are suspected to be more important than particle mass in determining the effects of air pollution. Time- and size-resolved PM measurements are needed for testing mechanistic toxicological hypotheses, for characterizing the relationship between combustion operating conditions and transient emissions, and for source apportionment studies to develop air quality plans ” [ 24 ].

Based on the accumulated knowledge so far, measures to reduce occupational exposure and emission levels at airports should be increased.

Availability of data and materials

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

Change history

04 april 2021.

A Correction to this paper has been published: https://doi.org/10.1186/s12940-021-00705-2

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Bendtsen, K.M., Bengtsen, E., Saber, A.T. et al. A review of health effects associated with exposure to jet engine emissions in and around airports. Environ Health 20 , 10 (2021). https://doi.org/10.1186/s12940-020-00690-y

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A review of health effects associated with exposure to jet engine emissions in and around airports

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Background: Airport personnel are at risk of occupational exposure to jet engine emissions, which similarly to diesel exhaust emissions include volatile organic compounds and particulate matter consisting of an inorganic carbon core with associated polycyclic aromatic hydrocarbons, and metals. Diesel exhaust is classified as carcinogenic and the particulate fraction has in itself been linked to several adverse health effects including cancer.

Method: In this review, we summarize the available scientific literature covering human health effects of exposure to airport emissions, both in occupational settings and for residents living close to airports. We also report the findings from the limited scientific mechanistic studies of jet engine emissions in animal and cell models.

Results: Jet engine emissions contain large amounts of nano-sized particles, which are particularly prone to reach the lower airways upon inhalation. Size of particles and emission levels depend on type of aircraft, engine conditions, and fuel type, as well as on operation modes. Exposure to jet engine emissions is reported to be associated with biomarkers of exposure as well as biomarkers of effect among airport personnel, especially in ground-support functions. Proximity to running jet engines or to the airport as such for residential areas is associated with increased exposure and with increased risk of disease, increased hospital admissions and self-reported lung symptoms.

Conclusion: We conclude that though the literature is scarce and with low consistency in methods and measured biomarkers, there is evidence that jet engine emissions have physicochemical properties similar to diesel exhaust particles, and that exposure to jet engine emissions is associated with similar adverse health effects as exposure to diesel exhaust particles and other traffic emissions.

Keywords: Airports; Biomarkers; Jet engine emissions; Occupational exposure; Particulate matter; Polycyclic aromatic hydrocarbons.

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The authors declare that they have no competing interests.

Overview of contributing factors in…

Overview of contributing factors in exposure risks from airports (APU: auxiliary power unit;…

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Home > Books > Direct Numerical Simulations - An Introduction and Applications

A Theoretical Review of Rotating Detonation Engines

Submitted: October 23rd, 2018 Reviewed: November 12th, 2019 Published: December 18th, 2019

DOI: 10.5772/intechopen.90470

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Rotating detonation engines are a novel device for generating thrust from combustion, in a highly efficient, yet mechanically simple form. This chapter presents a detailed literature review of rotating detonation engines. Particular focus is placed on the theoretical aspects and the fundamental operating principles of these engines. The review covers both experimental and computational studies, in order to identify gaps in current understanding. This will allow the identification of future work that is required to further develop rotating detonation engines.

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Ian j. shaw.

Jordan A.C. Kildare

Michael j. evans, alfonso chinnici, ciaran a.m. sparks, shekh n.h. rubaiyat.

Rey C. Chin

Paul r. medwell *.

*Address all correspondence to: [email protected]

1. Introduction

1.1 background.

Detonative combustion is a potential propulsion method for aerospace systems, offering high efficiency and low mechanical complexity. In comparison, deflagration is generally considered easier to control and has therefore dominated both experimental and real world engine applications. Research into detonation engines has been limited due to the lack of the necessary tools required to design and analyse such systems [ 1 , 2 ]. As such, practical development of detonation engines, notably the pulsed detonation engine (PDE) and the rotating or rotational detonation engine (RDE), has been limited [ 3 ]. Nevertheless, the application of detonation engines for propulsion is very promising, already proving to be compact, whilst providing highly efficient thrust generation [ 3 , 4 , 5 , 6 , 7 ]. This supersonic thrust could be utilised independently as a rocket engine, or as part of a gas turbine system. Interest in the development of RDE technology has grown and the challenges of utilising a more thermodynamically-efficient cycle have become better understood [ 8 , 9 ].

Combustion can occur at both subsonic and supersonic velocities, known as deflagration and detonation, respectively. Deflagration is typified by a regular flame, which propagates at less than the speed of sound. The heat release may be used to expel the resulting products, generating thrust. Deflagration has been used in a broad range of applications to produce power. However, in theory, deflagration lacks the thermodynamic efficiency of a detonation system, which is a system where combustion is initiated suddenly and “propagates utilising most, if not all, of the heat from combustion in an incredibly rapid shock wave” [ 10 ]. The heat generated by the exothermic chemical reaction sustains the shock wave. The concept of using detonation as a propulsion source has been proposed since the 1840s [ 11 ], but no substantial work had been completed until the 1950s when the development of models and concepts for a more lightweight and compact engine began [ 12 ]. The mechanisms that drive the detonation engine were not well understood at that time, so much of the research over the following decades was centred on the theoretical development of the engine.

As the name implies, the pulse detonation engine (PDE) has been proposed for propulsion using detonations [ 12 , 13 ]. In a PDE, a detonation chamber is filled with a fuel/oxidiser mixture, which is subsequently detonated. The accelerating detonation propels the exhaust from the chamber, thereby generating thrust. The chamber is then re-primed with fresh reactants, and re-detonated. With sufficiently high cycle speeds, large amounts of thrust may be generated in a small engine [ 14 , 15 ]. This type of engine has been found to be particularly efficient [ 3 , 16 , 17 ].

Development of the concept of a rotating detonation engine (RDE) began as a result of further work into detonative propulsion. This engine type is characterised by one or more detonation waves contained within an open-ended annular chamber. A fuel/oxidiser mixture is fed into one end of the chamber, and the detonation wave consumes these reactants azimuthally, expelling reactants from the open end of the annulus. In some literature, this type of engine may also be referred to as a continuous detonation wave engine (CDWE) or a spin detonation engine [ 6 ].

Early research into rotating detonations was conducted in the 1950s [ 18 ], with attempts to document the structure of detonation shock waves, including those in spinning detonations, with further developments through the 1960s [ 1 ]. Subsequent research has been conducted into the effects of geometry, rotation characteristics, spiralling of the wave, and other variables [ 6 , 19 , 20 , 21 , 22 ]. Another advancement in general detonation research is improvements in deflagration to detonation transitions (DDTs), leading to a greater understanding of the consumption of fuel in the chamber [ 23 , 24 , 25 ]. Further work has developed prototype RDEs to measure the thrust of small-scale units as a baseline for larger model behaviour, utilising the results from experimental work to verify theoretical results, and to generate new results [ 26 , 27 , 28 , 29 , 30 ].

In this review, several aspects of RDEs will be examined, starting with a brief comparison of RDEs and PDEs. This will be followed by further exploration into RDE operation, and methods of analysing RDEs, both experimentally and with numerical modelling. Finally, there will be an overview of areas still requiring further work.

1.2 Thermodynamic cycles

The majority of gas turbines that operate with a deflagration follow the Brayton (B) cycle: an isobaric (constant pressure) process, as shown in Figure 1 [ 31 ]. In contrast, a detonation is almost isochoric (constant volume) and may be modelled with the Humphrey (H) cycle, or, preferably, with the Fickett-Jacobs (FJ) cycle, which models detonation [ 3 , 31 ]. The H cycle assumes that combustion occurs in a fixed volume, resulting in a pressure spike as the products expand. Differentiation between the H and FJ cycles in Figure 1 can be seen through the state changes of 2– 3 ′ for the H cycle and 2– 3 ″ for the FJ [ 31 ]. This pressure spike decreases the volume of combustion for FJ while remaining constant for H. The next phase (FJ 3 ″ – 4 ″ , H 3 ′ – 4 ′ ) is similar for the two cycles, with the FJ cycle expanding further before reaching atmospheric pressure. Both then undergo a constant pressure compression through cooling back to the initial state 1. As seen in Figure 1 , the FJ cycle is more volumetrically efficient than the B cycle, and involves a higher pressure gain than the H, indicating that for the same initial isochoric compression, the FJ cycle is the more efficient of the three. This is supported by the thermodynamic efficiency equations for each of the cycles [ 31 ]:

literature review of jet engine

Thermodynamic cycles: Humphrey, Brayton, and Fickett-Jacobs. Adapted from Wolański [ 31 ].

where η B , η H , and η F are the thermal efficiencies of the Brayton, Humphrey, and Fickett-Jacobs cycles, T is temperature, p is pressure, k is the ratio of specific heats, and the numerical subscripts denote the position on the plot in Figure 1 [ 31 ]. A substitution of the relevant temperatures, pressures, and specific heat ratios into the above equations indicate the higher thermal efficiency of the FJ cycle. Additionally, the thermal efficiencies of various fuels under each of these thermodynamic cycles have been calculated and reported in Table 1 , further supporting the use of the FJ cycle when exploring detonation cycles as a high efficiency combustion method.

Calculated thermodynamic efficiencies for various fuels under different thermodynamic cycles [ 26 ].

1.3 Pulsed detonation engines

In a PDE, such as that shown in Figure 2 , a detonation chamber is filled with a fuel/oxidiser mixture and then ignited. The deflagration of the reactants accelerates, and through a deflagration-to-detonation transition (DDT), generates a shock wave. The products are accelerated from the end of the chamber, carried by the detonation front, generating thrust [ 30 , 31 ]. For each cycle, the chamber must be purged and then refilled with fresh fuel/oxidiser mixture and then detonated again, limiting the maximum practical frequency of operation to an order of 100 Hz [ 32 ]. This results in poor efficiency when scaled to high thrust levels as the discontinuous thrust cycles may not be fast enough to approximate the continuity required for propulsion purposes [ 32 , 33 , 34 , 35 ]. In some designs, it is also necessary to purge the chamber with an inert gas due to some residual combustion products remaining stagnant in the detonation chamber that interfere with the next detonation cycle. This process further restricts the operating frequency to approximately 50 Hz [ 3 , 16 ].

literature review of jet engine

Labelled schematic of a PDE. Adapted from [ 15 ].

In order to provide a more compact device, obstacles may be placed in the chamber to accelerate the DDT, but these reduce the specific impulse ( I sp ) [ 31 , 33 ]. Specific impulse can be defined as the change in momentum per unit mass of propellant used. An alternative approach is to remove the requirement for repeated DDT transitions, and hence the efficiency loss, by sustaining the detonation reaction. This approach leads directly to the concept of an RDE, which should provide a method of utilising the H or FJ cycle, in a much more compact form.

1.4 Rotating detonation engines

An RDE, such as the one shown as a cutaway in Figure 3 , consists of an annular combustion chamber, into which fuel and oxidiser, either premixed or non-premixed, are fed through a series of orifices [ 3 , 26 , 36 ]. Each fuel/oxidiser mix requires a slightly different orifice geometry for optimal operation, so some devices have an adjustable injector plate [ 37 , 38 ].

literature review of jet engine

Cross-section of a typical rotating detonation engine [ 38 ].

A detonation wave is initiated in the chamber, most commonly utilising a high speed flame that undergoes DDT by the time it enters the chamber [ 39 , 40 ]. As this wave propagates around the chamber, it consumes the fuel, generating a high pressure zone behind it. This zone expands, and due to the geometric constraints, exits the chamber, generating thrust [ 35 , 41 ]. An example of a CFD representation of the propagating wave can be seen in Figure 4 [ 42 ]. Behind the wave, fresh fuel enters the chamber at a constant rate, priming that section of the chamber for the wave to continue on the next revolution, thus making a self-sustaining wave as long as fresh mixture is supplied [ 35 , 43 ]. The detonation waves generally propagate close to the Chapman-Jouguet velocity (discussed in Section 3.2) for each fuel type (typically 1500–2500 m s −1 ), so the effective operational frequency of current RDEs is approximately 1–10 kHz. Frequency is dependent on the chamber geometry, fuel, and thermal and frictional losses [ 31 , 44 ]. The result is quasi-continuous thrust that approximates a continuous thrust through high frequency rotations, suitable for both direct propulsion applications and in the combustor of a gas turbine [ 31 , 32 , 45 ].

literature review of jet engine

3D model of the detonation wave propagation in an RDE [ 42 ]. The short arrows indicate the flow of fuel/oxidiser into the engine, and the long arrow indicates the direction of detonation propagation.

Important areas of RDE research include determining the wave characteristics, geometric constraints, the effects of pressure on the injection characteristics, determining fuel flow properties, and examining the geometry and structure of the detonation wave [ 3 , 4 , 30 , 31 , 41 , 42 , 44 ]. Additionally, there has been research into potential applications of detonation engines in which an RDE may be applied, such as air-breathing vehicles and gas turbines [ 46 ]. Despite a growing body of work on RDEs, there are still large gaps in current understanding that restrict practical application. Notably, optimising the system for wave stability, ensuring reliable detonation initiation, and ensuring the RDE does not overheat, are significant challenges facing engine development prior to commercial applications. Further development in this area would allow an engine to operate reliably over extended durations, with well-designed chamber and fuel supply.

2. Existing RDE designs

Most experimental RDEs are geometrically similar in design, consisting of an annulus made up of coaxial cylinders [ 5 , 38 , 47 ]. The chamber width, characterised by Δ , sometimes referred to as channel width, varies across designs. Several modular RDEs have been produced for testing various geometric parameters [ 30 , 37 , 48 , 49 ]. As will be discussed in Section 4.4, the number of alternative designs to the annulus is limited. An exception is the hollow cylinder model to determine the effects of having no inner wall on the detonation wave as well as the practical feasibility [ 50 ].

There is reasonable consistency across published designs in the methods of initiating detonation waves in the RDE. Detonator tubes, in which a high-speed flame is encouraged to transition from deflagration to detonation, have been regularly and reliably used [ 26 , 31 , 32 , 39 , 49 , 51 ]. It has been shown that the success of the detonation tube makes it an excellent initiator, producing a self-sustaining rotating detonation 95% of the time [ 26 ].

Like all jet-thrust reaction-based engines, the exhaust from a RDE may be channelled through a nozzle to increase thrust. Outlet and nozzle designs have varied across different RDEs. Many have not attached any nozzle, whilst some have chosen to utilise an aerospike [ 30 , 31 , 52 ]. The use of an aerospike increases performance through higher expansion area ratios, although the increased surface area results in higher heat flux and thus a loss of efficiency from the additional heat transfer [ 53 ]. Aerospikes may be directly attached to the end of the reaction chamber [ 31 ]. A diverging nozzle was found to increase the specific impulse, although the thrust increase was small, and for angles greater than 10°, the increase with angle was negligible [ 53 ]. None have made use of converging or converging-diverging nozzles, because the exhaust is typically flowing at supersonic velocities and thus could be choked through the converging cross-section. This would result in a loss of energy that would decrease the overall efficiency of the system.

A typical RDE, 90.2 mm in diameter, has been tested on a thrust sled [ 54 ]. It produced a thrust of 680 N using 176 g s −1 of C 2 H 4 /O 2 propellant at an equivalence ratio of 1.48 [ 54 ]. As can be seen from Table 2 , this is well below that required for typical supersonic flight applications. The specific impulse ( I sp ) of small scale operational RDEs has ranged from 1000–1200s depending on the fuel/oxidiser source used, though it is often H 2 with air [ 30 , 31 , 39 , 41 , 42 ]. The measured values of I sp in these small scale RDEs are significantly below computationally predicted range: 3000–5500 s [ 31 , 32 ]. However, a large scale RDE, discussed in further detail in Section 4, does operate with an I sp of approximately 3000 s [ 5 ]. The experimental values for I sp are similar to that of hydrocarbon-powered scramjets, but less than turbojets and ramjets. These low values for small-scale RDEs are likely due to the use of unoptimised designs, and low chamber pressures [ 31 ].

Thrusts and applications of various engines.

This is the thrust to weight ratio calculated using a pre-weight load cell system.

RDEs have been found to be successfully operable with a range of gaseous fuels including hydrogen, acetylene and butane, as well as various jet fuels [ 30 , 31 ]. Air, pure oxygen, and oxygen-enriched air have all be used as oxidisers [ 31 ]. Each of these has a variety of advantages and disadvantages, in both performance characteristics, and ease of obtaining, transporting, and storing the oxidiser. Particular difficulty is noted in the transport of gases such as H 2 and O 2 due to the high risk regarding transportation and significant compression of these chemical species [ 59 ]. In the case of transporting liquid fuels such as LH 2 and LOx cryogenic units are also required, adding to the already challenging process. The performance characteristics for several of these fuel types will be discussed further in Section 4.4.

The detonation wave velocity in operational H 2 /air RDEs has been found to be on the order of 1000 m s −1 [ 30 , 39 ]. In these RDEs, the operational frequencies are on the order of 4000 Hz, which produces quasi-continuous thrust [ 3 , 32 ]. As wave speed is a key factor in the development of thrust, stable waves with high speeds are ideal for propulsion purposes. Stable detonation waves have reached maximum speeds in the range of 1500–2000 m s −1 in most designs using a H 2 /air or H 2 /O 2 fuel/oxidiser combination (more commonly the former), suggesting that there is open research into whether there is upper limit for detonation wave speed, and subsequently the thrust that may be produced [ 3 , 22 , 26 , 60 ]. However, at very high frequencies (19–20 kHz), there may be multiple waves rotating around the annulus [ 60 , 61 , 62 ]. Multiple wave modes of propagation appear to be affected by fuel/oxidant equivalence ratio as well as total mass flow rate through the system. The high frequencies are a result of multiple waves travelling at approximately the same speed as the normal single wave. This phenomenon has the potential to provide more continuous thrust, though the higher frequency may limit I sp due to insufficient refuelling of the detonation cell between waves. These wave modes have reliance on factors including fuel injection velocity, critical minimum fill height (discussed further in Section 4.3) as well as the detonation velocity [ 31 ]. Due to the inherent instabilities of rotating detonation waves, there are no specific relationships that can be determined between these factors and specific designs, only that they have an influence. Multiple wave fronts have been observed in several different RDE designs, where the general geometry has remained fairly similar [ 30 , 31 ].

There are several methods of recording data from an operating RDE. Thrust generated may be measured with a thrust plate, and the flow rates of fuel and oxidiser may be measured or controlled within the supply lines [ 30 ]. The details of the shock may be recorded with pressure sensors attached to the chamber head, and external cameras [ 30 ]. Pressure sensors record the increased pressure generated by the shock, and by using multiple sensors, the detonation wave propagation velocity may be determined. A high-speed camera may be set up to capture the operation of the engine, allowing various parameters to be recorded, including the detonation wave propagation velocity, although this method is limited by spatial resolution, as the channel width can be quite small [ 30 , 39 ]. A camera may also be used to image from the side, if the outer surface of the annulus is made of a transparent material [ 63 ]. Additionally, OH* chemiluminescence may be used to detect, record, and analyse the detonation waves in UV-transparent optically-accessible RDEs [ 64 , 65 ]. These radicals are indicative of the reaction zone, and so, by analysis of their chemiluminescence, the structure of the detonation can be inferred. Often this detection is done through a quartz side window integrated into the RDE [ 63 ]. Peak intensity of the OH* chemiluminescence indicates the location of the detonation front, and so the effects of varying factors such as equivalence ratio and chamber geometries can be documented. Images are often phase-averaged and can by “unwrapped” for comparison to equivalent two-dimensional, “linearised”, simulations and designs.

3. Detonation waves

The structure of shock waves in gases was examined in detail by Voitsekhovskii in 1969, including those of shock waves in spinning detonations [ 66 ]. These examinations resulted in the first diagram of the structure of a spinning shock wave, and the identification of a number of features, which are identified from the computational model of an RDE shown in Figure 5 [ 32 ]. This model used premixed hydrogen/air as the fuel/oxidiser mixture and has been “unwrapped” into two-dimensions (this approach is described in Section 5.1). Feature A is the primary detonation front; Feature B is an oblique shock wave that propagates from the top of the detonation wave; Feature C is a slip line between the freshly detonated products and older products from the previous cycle; Feature D is a secondary shock wave; Feature E is a mixing region between the fresh premixture and the product gases, where deflagration may occur [ 67 ]; Feature F is the region where the injector nozzles are blocked; and Feature G is the unreacted premixture.

literature review of jet engine

Pressure contour indicating the cell structure of a detonation wave in an RDE with a premixed supply, taken from a computational modelling study [ 32 ]. (a) Pressure contour indicating the full structure of detonation in an RDE, “unwrapped” into two dimensions. Feature A is the detonation wave, Feature B is the oblique shock wave, Feature C is the slip line between the freshly detonated products and products, Feature D is a secondary shock wave, Feature E is a mixing region between the fresh premixture and the product gases, Feature F is the region with blocked injector nozzles, and Feature G is the unreacted premixture. The arrow denotes the direction of travel of the detonation wave. (b) A close-up image of the detonation front.

In both Figure 5b and Figure 8c (Section 4.3) the detonation cell structure can be seen, with high pressure zones outlining each cell. These lines of high pressure contain triple points, where the transverse and oblique shocks meet the Mach stem of the detonation wave [ 68 , 69 ]. The concentrated pressure at these triple points is the point of maximum energy release, and the subsequent pressure spike when two triple points collide generates new detonation cells [ 68 , 70 ]. While this generation is the main reason behind the propagation of detonation waves, the triple points still require further investigation as to the effects they have on the overall characteristics of a detonation wave [ 70 ]. The direction of these triple points can be seen as the white lines in Figure 8c with trailing high pressure zones forming the walls of the detonation cells. As the detonation cell width is defined by the geometry of the system and the chemical composition of the detonating fuel, it seems that the triple point velocity and direction must also directly relate to these factors, although limited research has been done to formally connect these points.

In an RDE, the detonation wave remains attached to the base of the annulus, as illustrated in Figure 5b and in Figure 6 [ 3 , 6 , 71 ]. This is due to the continuous fuel/oxidant supply [ 3 , 71 ], as a premixture or allowed to mix in the chamber ahead of the detonation wave [ 32 , 39 ]. There is also some evidence that stable, lifted waves may also be possible if there is insufficient mixing between the fuel and oxidant [ 27 , 44 ]. The propagating detonation wave combusts the reactants [ 32 , 39 ] which generates a region of extremely high pressure immediately behind the wave. This pressure is on the order of 15–30 times higher than the pressure ahead of the detonation, preventing flow through the injectors [ 3 ]. The high pressure zone expands in a Prandtl–Meyer fan, allowing fresh fuel and oxidiser to enter the chamber [ 35 ]. This expansion propels the mixed products axially along the engine, generating thrust. In addition to the primary shock, an oblique shock and a secondary attached shock are also generated (Features B and D in Figure 5a ).

literature review of jet engine

Diagram showing the general structure of the detonation in an unwrapped RDE [ 3 ].

At the interface between the premixed reactants and the combustion products, there is a significant difference between the conditions of the unburnt fuel/oxidiser mixture and the products. This causes some deflagration along the slip line, as shown in Figure 6 , generating Kelvin-Helmholz instabilities, which vary the detonation propagation velocity [ 3 , 22 , 72 , 73 ]. This decrease in the propagation velocity results in an increase in the pressure, disturbing the oncoming shock wave and forcing the sonic flow directly behind the shock wave to undergo supersonic flow acceleration [ 74 ]. As shown in Figure 6 there is a section of injector flow blockage that occurs as the wave passes the fuel array. The high pressure front from the shock wave causes stagnation of the injector flow, or even back-flow which, if not handled, could cause catastrophic failure of the system [ 3 , 6 , 36 ]. This back-flow is a strong reason as to why the fuel and oxidants should not be premixed in practical systems or experimental investigations as it can result in flashback.

3.2 Shock initiation

The Chapman-Jouguet (CJ) condition can be defined as the requirements for the leading shock of a detonation to not be weakened by the rarefactions of the upstream detonation products [ 75 ]. This sonic plane then acts to allow the supersonic expansion of the detonated gases to occur without disturbance by rarefactions downstream of the flow [ 75 ]. The CJ condition can be used to approximate the detonation velocities in three-dimensional models but is better suited to a one dimensional analysis with an infinitesimally thin detonation front [ 76 ]. Despite this, it is used in most instances of numerical modelling as a guide as to whether the wave is performing as expected for the given parameters of the RDE [ 4 , 6 , 27 , 31 , 32 , 42 , 75 , 77 ]. Chapman and Jouguet’s theory only applies to kinetic energy, disregarding the chemical energy of the reacting species, and hence, the Zel’Dovich-von Neumann-Doring (ZND) model is used as a more complete representation of the shock, taking into account the finite chemical reaction area directly upstream of the leading shock [ 3 , 21 , 45 , 75 , 78 , 79 , 80 ].

There are two methods which may be used to initiate the detonative shock in an RDE—directly in the chamber, or indirectly via a high speed flame in a deflagration to detonation transition (DDT) tube [ 26 , 31 , 39 , 49 , 51 ]. These tubes are very similar in structure to a PDE. Directly initiating the detonation in the chamber via commercial spark plugs has been found to be generally unreliable, with only a 40% success rate for shock initiation when using CH 4 in O 2 [ 26 ]. Particular difficulty is noted in ensuring the detonation travels in the desired direction [ 26 , 32 ]. In contrast, indirect initiation via a DDT tube has had a 95% success rate for the same fuel/oxidant combination [ 26 , 31 ]. The indirect method involves using a detonator tube that can be set up in any orientation relative to the chamber, although tangential is favoured for initiating the detonation direction. Initiation is then caused by a small volume of a highly detonative mixture being ignited by spark plugs before DDT occurs, thus initiating the RDE. Perpendicular initiation can also be used, but this often results in the development of two detonation waves that rotate around the chamber in opposite directions [ 31 ]. Collision of these opposing waves usually destabilises the system as the waves weaken and reflect back in the direction of origin [ 31 ]. Desired direction also appears to be affected by initial total pressure and ignition distribution around the fuel plenum [ 27 , 81 ]. For a desired single wave direction and propagation, tangential initiation is the most suitable method. Although slightly less compact due to the initiator tube, this may be reduced by placing obstacles in the tube to accelerate the DDT, or by using a more detonative fuel than that used in the primary process [ 31 , 48 , 62 , 82 , 83 ]. Using an initiator tube, however, may produce small wavelets ahead of the main detonation front, which, if present, reduce the detonation propagation velocity by up to 60% [ 84 ]. Once the main detonation is running, the interface between the initiator tube and main chamber must be closed off prior to the shock completing a revolution of the chamber [ 84 ]. Additionally, there may be a slight delay, on the order of milliseconds, between the detonation exiting the DDT tube and the commencement of full RDE operation in order to purge the spent reactants from the DDT process [ 85 ]. This delay seems to only be transient with no large effects on shock structure or stability, and the excess products are expelled along with the rest of the exhaust [ 85 ].

3.3 Instabilities

Three-dimensional modelling has shown that increasing the width of the channel—whilst maintaining the equivalence ratio, injection pressure, chamber length, and injector configuration—increases the detonation velocity, but the transverse shock wave ceases to be aligned with the radial direction [ 22 , 27 , 86 ]. As can be seen in Figure 7 , the point of contact with the inner wall begins to lead the detonation wave as the channel width increases [ 22 ]. This phenomenon generates reflected shocks from the outer annulus wall, which may produce instabilities in the primary shock. It has been suggested through qualitative observation, however, that the effect of upstream reflected shocks on the shock structure may only be minimal [ 39 , 87 ]. Once the channel becomes sufficiently wide, as shown in Figure 7c , the shock wave detaches from the inner wall, briefly forming a horseshoe shape against the outer wall [ 22 ]. This allows significant amounts of fuel to pass through the engine without combusting, and produces large instabilities and fragmentation in the detonation wave, which causes the structure to collapse [ 22 ]. These lead to a significant loss of performance, and secondary detonations in the exhaust [ 22 ]. It has been noted that increasing the channel width also results in increased variance of I sp , and that, combined with high fuel flow rates, leads to the formation of secondary waves, which in turn leads to hotspots and choking the fuel supply [ 42 , 62 ]. This is likely due to the increase in size of the interface area producing greater Kelvin-Helmholz instabilities, resulting in larger variances in the detonation velocity [ 42 ].

literature review of jet engine

Schematic of three different RDE designs showing the effect of varying the channel width on detonation structure. Arrows show detonation wave propagation direction. The red line is detonation wave, indicative only. Based on research from [22]. (a) Narrow channel, (b) mid-sized channel, and (c) wide channel.

It has been found that using a fuel-rich mixture produces stable waves with high detonation velocity and efficiency [ 80 , 88 ]. Higher mass flow rates have also been attributed to increasing the chance of a stable wave being formed [ 6 , 89 ]. Additionally, it has been shown that the equivalence ratio has a strong influence on the effectiveness of detonation and the stability of the system [ 80 ]. Detailed investigation has shown that the stability of the system is improved with increased equivalence ratio, but indicated a maximum equivalence ratio of 1.27, before the detonation wave became short-lived and transient, which is unsuitable for practical purposes [ 60 ]. Whether this is a universal limit, or a limit of that particular investigation is unclear, and requires further research. Furthermore, the findings indicated that lower equivalence ratio influences the number of wave fronts produced, with stoichiometric seeming to be a transition point to a stable one wave propagation mode [ 60 , 86 , 90 ]. It is interesting to note that for lean mixtures, the initial channel pressure needs to be higher for a stable detonation to propagate [ 88 ].

4. Factors influencing the design of RDEs

The wave propagation velocity varies with the fuel/oxidiser combination. A variety of mixtures have been tested in a detonation tube of an RDE, with their wave propagation velocities and wavefront pressures shown in Table 3 , which is indicative of their varying performance in an RDE. It should be noted that the pressure, energy and specific impulse in Table 3 are determined with a detonation tube, and provide a numerical comparison between each fuel/oxidiser combination. Hydrogen/oxygen mixes have been ideal for modelling purposes due to the simple chemistry involved, and are often used in experimental work due to the predictable behaviour. Additionally, the high detonation propagation velocity and wavefront pressure of hydrogen makes it a suitable fuel for real applications. Another common fuel choice is methane, due to the satisfactory propagation velocity and specific impulse in testing [ 31 ]. As mentioned in Section 2, the theoretical I sp is still greater than that of a standard turbojet propulsion system, irrespective of fuel selection [ 91 ].

Fuels, wave propagation velocities and pressures, heat of combustion ( Δ H r ), and specific impulse I sp [ 36 ].

Transportability of fuel, and maintenance of fuel lines, are deciding factors in determining which fuels can be used. These issues are especially important for aerospace applications. Gases such as H 2 and O 2 are particularly volatile and reactive, hence can be difficult to transport in the large quantities needed for use in an RDE. Therefore, gaseous fuels and non-air oxidisers are challenging and largely unsuitable for real world applications [ 5 ]. However, H 2 does have a high heat of combustion that is not matched by liquid hydrocarbon fuels. Jet fuel, kerosene, octane and other long-chain hydrocarbons provide a practical alternative to the H 2 /O 2 mixture though. High volumetric energy density as a result of liquid state, as well as greater ease of transportability makes these hydrocarbons a more feasible fuel choice.

There are several issues regarding fuel choice that deserve further discussion. In particular, the use of cryogenic fuels for cooling the engine is a beneficial approach, increasing thermal efficiency, as well as reducing the thermal load on other components such as mounting systems [ 3 ]. Another advantage is a higher volumetric energy density that comes from the compression of normally gaseous fuel sources. Testing of liquid oxygen (LOx) and gaseous or liquid hydrogen (GH 2 /LH 2 ) fuel/oxidant systems for viability has been performed, but implementation in real world scenarios is challenging [ 92 , 93 ]. Liquid hydrocarbons require further investigation to demonstrate their effectiveness in producing thrust through detonation [ 30 ], particularly because of the need for flash vapourisation to avoid multiphase effects in the mixing process [ 30 , 51 ].

4.2 Injection

An axial fuel injection process through a circumferential orifice plate was consistent across most simulations and real world models as an injection scheme [ 5 , 6 , 22 , 26 , 30 , 32 , 36 , 38 , 39 , 41 , 42 , 42, 52 , 61 , 62 , 82 , 86 , 88 , 92 , 94 , 95 , 96 , 97 , 98 , 99 ]. Further research is required into fuel blockage effects due to the high pressure of the shock wave, with particular emphasis on the effects of increasing fuel pressure to alleviate blockage and increase overall engine performance [ 100 ]. In the majority of numerical and physical models, such as Figure 3 , fuel and oxidiser are injected through an orifice place around the annulus, allowing them to continually feed the propagating detonation wave. Typically, the fuel and oxidiser are fed in separately, and allowed to mix in the chamber [ 26 ]. This design is also used in most numerical models, although some have used premixed fuel/oxidiser as a simplified boundary condition. Almost all physical designs have been built without a premixed fuel/oxidant injection scheme due to concerns with flashback [ 99 ]. In a premixed design, the shock wave may propagate into the injection plenum, carrying with it the reaction front. With sufficient pressure though, typically 2.3–3 times the chamber pressure, this can be avoided [ 32 ].

Investigation into flow characteristics of a turbulent inflow have shown that there are specific zones within the chamber which favour different forms of combustion: some zones favour deflagration, and others favour detonation [ 101 ]. The larger deflagration zones created reduce the thermodynamic efficiency of the engine, indicating that fuel flowrate influences the reliability of an RDE [ 101 ]. It has been suggested that high inlet velocities generate incomplete combustion and hot spots, reducing detonation wave stability and reducing system efficiency, although further research is required [ 102 ]. As indicated in Section 3.3, the introduction of instabilities in the flow profile can decrease the efficiency of the engine as well as disrupt the detonation wave itself. Further findings indicate that increasing the fuel injection area, particularly by increasing the number of orifices, results in more efficient pressure gain [ 86 , 97 , 99 , 103 ]. This produces a larger expansion wave of the previous combustion reactants, generating higher thrust, without disrupting the flow-field characteristics [ 98 ]. However, with lower fuel injection velocities comes an increased risk of flashback. There is, therefore, some optimal fuel injection area for operation which requires further work to verify [ 98 ]. Finally, the pressure ratio between the inlets and the engine outlet also has an effect on the I sp of the engine, with pressure ratios of less than 10 showing notable reductions in impulse [ 32 , 72 ]. Thus, because of these conflicting requirements, injector design is complex and more research is required such that fuel consumption and thrust output are optimised.

4.3 Scalability

Existing RDEs tend to be relatively small, and therefore may need to be scaled up, or arranged in parallel, to produce thrust required for practical applications, such as those listed in Table 2 . One method of scaling RDEs is to run multiple identical devices in parallel, in a similar manner to that used to run multiple PDEs [ 34 , 104 ]. However, this would require more complex plumbing, increasing the weight of the overall system, and thus decreasing the thrust-to-weight ratio. However, this solution has not been explored in any depth and its viability is unknown.

In order to make larger RDEs, in-depth research into the geometry of the combustion chamber is required. A number of relationships between the critical detonation wave height and the various dimensions have been identified [ 27 , 30 ]. Detonation structure, as described in Section 3.1 is composed of small diamond shaped detonation cells that make up the front. The widths of these cells are dependent on the energy of the detonation (related to the fuel in use) as well as the available geometry for detonation. In this way, the equivalence ratio can be a large determining factor [ 30 , 105 , 106 ]. Critical minimum fill height is the minimum mixture height required for a detonation wave to propagate through a given fuel/oxidiser mixture. It has been found that the critical minimum fill height, h ∗ , and the minimum outer wall diameter, d c min , are related to the detonation cell width, λ , by

and the minimum channel width, Δ min is related to the h ∗ by

Finally, the minimum axial length of an RDE, L min is related to the actual fill height, h , by

although lengths under 2–3 times the minimum result in reduced efficiency due to incomplete combustion [ 27 ]. However, in simulations, it has been suggested that for low inlet-nozzle pressure ratios the wave the wave height grew with the chamber length, reducing the I sp , of the engine [ 42 ]. For high pressure ratios, no such reduction was indicated [ 42 ]. Figure 8 indicates the physical representations of the above variables.

literature review of jet engine

Geometric parameters of an RDE. The red area is the area filled by the fuel/oxidiser mix in which the detonation propagates. (a) Top view, (b) side view, and (c) detonation cell width adapted from [79].

There is not yet any theoretical data for λ , but there are multiple models which may be used to predict the value under various conditions [ 78 ]. It is known that more highly reactive mixtures, such as H 2 /O 2 , have lower λ values, and so have minimum chamber diameters on the order of 40–50 mm. Liquid hydrocarbons, such as kerosene and jet fuel, combusting in air, have reactions with higher λ , so, when Eq. (5) is applied, the minimum chamber diameter is calculated to be 500 mm [ 3 ].

Modelling a large-scale RDE presents a challenge due to increasing computational requirements with increasing size, so limited work has been done in this area. Nevertheless, a larger scale experimental RDE has been demonstrated [ 5 ]. This RDE had an outer chamber diameter of 406 mm, and a channel width of 25 mm, and an air inlet slit that could be varied across the range 2–15 mm [ 5 ]. It produced a consistent thrust of 6 kN with a combined fuel/oxidiser flow rate of 7.5 kg s −1 , whilst also producing an I sp at 3000 s, consistent with the computational models noted in Section 2 [ 5 , 31 ]. This is approximately four times the physical size, ∼ 40 times the consumption of combined fuel/oxidiser, and ∼ 12 times the thrust of other RDEs noted in Section 2 [ 46 , 54 ]. Although still producing low thrust compared with conventional jet engines, such as those listed in Table 2 , it is also half the diameter of the modern engines [ 57 , 58 ]. Furthermore, 6 kN would be more than sufficient thrust for use in a Harpoon missile [ 56 ], and this RDE shows that they are capable of being scaled beyond small sizes.

4.4 Alternative designs

The design used in most simulations and experimental work is a coaxial cylinder structure [ 3 , 27 , 31 , 35 ]. This simple geometry is advantageous for both modelling and manufacturing. Design variations including using nozzles, aerospikes such as that shown in Figure 9 , or an entirely hollow cylinder, have been utilised in several RDE designs [ 5 , 52 ].

literature review of jet engine

Example of an aerospike nozzle configuration [ 52 ].

Alternative chamber geometries have been largely limited to adjustments in the diameters of the chamber [ 4 , 42 ], including with different sized engines [ 15 , 31 , 39 , 54 ]. Other work has been conducted on a single RDE with interchangeable outer wall sections [ 22 , 30 ]. As noted in Section 2 and Section 3, both of these factors influence the stability and the performance of RDEs. The effect of varying the length of the chamber on the detonation propagation has been investigated, which led to the previously mentioned requirement that the chamber be at least twice, and preferably four to six times, the fuel fill height [ 4 , 96 ].

Hollow RDEs, dubbed “centrebodiless” designs, have been tested with two different designs [ 50 , 61 ]. One design was identical to a conventional RDE 100 mm across, but the inner cylinder terminated parallel to the fuel/oxidiser injectors [ 61 ]. In this design, tested with 169.7 g s −1 of CH 4 /O 2 at an equivalence ratio of 1.154, it was found that the detonation was unstable [ 61 ]. The fuel and oxidiser were free to move into the space usually occupied by the centre body, and thus insufficiently mixed to sustain a stable detonation [ 61 ]. However, when the same geometry was tested with 253.3 g s −1 of CH 4 /O 2 at an equivalence ratio of 0.665, the mixture became sufficiently mixed to sustain a stable four-wave detonation structure [ 61 ]. Another design was completely hollow, allowing oxygen-enriched air to be pumped through the centre of the chamber, and fuel was supplied around the edge [ 50 ]. In this design, stable detonations, operating at ∼ 8000 Hz were achieved at an equivalence ratio of ∼ 0.4 [5-]. However, this design required that the molecular ratio of nitrogen-to-oxygen in the oxidiser be approximately two for detonation. Nitrogen-to-oxygen ratios of ∼ 2.5 produced deflagration, and a ratio of 3.75—approximately standard air—led to the RDE self-extinguishing [ 50 ]. Nevertheless, the need for oxygen enrichment introduces additional cost and challenges for practical RDEs in propulsion applications. It was also noted that the oxidant flow provides an outward pressure that acts like a wall but carries no extra weight, and even adds a small amount of thrust as the air is expelled [ 50 ]. Both designs can be looked at as successful proofs of concepts, and potential first steps in simplifying the geometry of an RDE, with the latter being potentially useful in applications such as afterburners [ 50 , 61 ]. However, this concept has not been explored with pre-heated reactants, such as those which would be present in an afterburner.

The attachment of turbines to RDEs has been proposed [ 8 , 9 , 31 , 32 , 45 ]. It has also been noted that there is a secondary shock propagating from the detonation, which exits the outlet of the chamber [ 32 ]. However, turbine blades are sensitive to shocks. As such, the effect of the secondary shocks on the blades of potential turbines must be investigated. It is worth noting that an experimental PDE array has been tested with an attached turbine, in the form of an automotive turbocharger [ 31 ]. In that case, a buffer chamber was inserted between the PDE and the turbine [ 31 ], and such a technology may be suitable for RDEs.

5. Modelling and development tools

5.1 planar and three-dimensional modelling approaches.

Computational fluid dynamics (CFD) modelling is a powerful tool for the analysis of rotating detonations prior to, or in tandem with, experimental systems. The majority of numerical studies have aimed to provide in-depth understanding and details of the detonation structure [ 22 , 41 , 62 , 67 , 72 , 94 , 107 , 108 ] or assess the physical and modelling factors influencing performance [ 32 , 67 , 73 , 109 ].

Computational models of the azimuthal detonations in RDEs may use full three-dimensional geometries [ 20 , 22 , 67 , 94 , 95 , 107 , 110 ] or simplified, two-dimensional geometries [ 6 , 32 , 41 , 43 , 62 , 72 , 73 , 108 , 109 , 111 , 112 , 113 , 114 ]. The former, higher-fidelity, approach can incorporate complex geometric and flow features, although require ∼ 10,100 million numerical cells for high fidelity large-eddy simulations (LES) or direct numerical simulations (DNS) [ 22 , 94 , 95 , 112 ]. These may subsequently result in considerable computational expense in conjunction with detailed turbulence and combustion chemistry. In contrast, by assuming that the channel width is much smaller than the diameter, the annulus geometry may be “unwrapped” [ 108 ] and treated as a planar flow [ 41 ]. The azimuthal detonation repeatedly travels through the domain using periodic boundaries (i.e. the outflow from one side feeds into the other side). Such a model was shown previously in Figure 5a [ 32 ], where the detonation is travelling left-to-right and the two vertical edges of the image are the periodic boundaries. This can be seen by noting the height of the unreacted premixture region (Feature G) at each side of the figure. The stationary geometry shown in Figure 5a [ 32 ] shows a full, two-dimensional, unwrapped RDE geometry, and allows the detonation to freely—and repeatedly—propagate through the domain. It may, in some cases, be beneficial to examine the detonation in its own frame, by matching the domain velocity to the negative of the detonation speed; however, this requires significant trial-and-error as the detonation speed cannot be accurately approximated as the CJ velocity for this purpose [ 108 ].

Two-dimensional modelling of RDEs assumes that the flowfield along the centre of the channel is representative of shock and deflagration structure across the entire width. Consequently, this inherently assumes slip-wall conditions and that the detonation-front is normal to the two-dimensional geometry. In the unwrapped two-dimensional geometry, all fuel is injected axially from one edge (the bottom edge in Figure 5a [ 32 ]) and is exhausted through the opposite edge (the top edge in Figure 5a ) [ 6 , 32 , 72 , 111 ]. It therefore follows that all exhaust products must leave the domain axially, due to conversation of angular momentum. This was confirmed in early two-dimensional modelling, which found that the density-averaged azimuthal velocity was less than 3% of the axial velocity [ 41 ]. Such a criterion could be extended to assessing whether a three-dimensional model, at some fixed radius within the channel, could be treated as an unwrapped planar domain.

Detonation wave curvature, imperfect mixing, three-dimensional turbulent structures and transverse shocks are features reported in three-dimensional computational modelling [ 22 , 67 , 79 , 94 , 107 ] and experimental studies [ 62 ]. These features arise from the effects of channel size [ 22 ], discrete injectors [ 79 ] and interactions between transverse waves and walls [ 62 , 79 ]. These features are inherently three-dimensional and cannot be captured using planar, periodic models, and require more complex computational geometries.

5.2 Boundary conditions in computational models of RDEs

Fuel/oxidiser inlets may be modelled as simple points, lines, surfaces or complex, discrete injectors. The latter may be treated as a series of inlets in two-dimensional models, assuming upstream micro-mixing [ 109 , 112 ]. Differences in the injector configuration can lead to differences in detonation pressure [ 112 ], or lifted flame behaviour in the event of poor mixing in a partially premixed system [ 109 ]. The study which observed the latter phenomenon, however, was undertaken using the Euler equations, which may affect the fidelity of modelled mixing (discussed later in this section), and a simplified induction parameter model (described in Section 5.4) [ 109 ], although this has also been observed experimentally in C 2 H 2 -fuelled RDEs [ 115 ].

Inlet boundary conditions in premixed models, are often defined by inlet throat-to-nozzle-exit ratios. These, and the set upstream pressure, control whether the inlets are blocked, subsonic or choked and are chosen to range from 0.1–0.2 [ 6 , 109 , 110 , 112 ], although ranges as large as 0.07–0.3 have shown little effect on I sp [ 73 ]. More complex fuel injector geometries have been assessed through three-dimensional modelling [ 94 ], demonstrating the effects of the complex detonation/deflagration interactions on imperfect mixing, however, neither instantaneous (fuel or air) plenum pressures nor detonation wave-speeds could be correctly predicted.

5.3 Turbulence modelling in RDE simulations

Rotating detonation engines have often been numerically modelled using the compressible Euler Equations [ 6 , 20 , 32 , 41 , 43 , 62 , 72 , 95 , 108 , 110 , 111 , 112 ]. The Euler equations conserve momentum, mass and energy, but do not account for viscosity, following the assumption that the detonation structure dominates viscous dissipation. Viscous effects may, however, be incorporated into numerical studies of RDEs through the use Reynolds-averaged Navier Stokes (RANS) modelling [ 107 , 113 ], LES, LES-RANS hybrids such as [improved] delayed detached eddy simulations (IDDES) [ 67 , 94 ], or DNS [ 22 ]. Of these approaches, Euler, IDDES and DNS studies [ 22 , 41 , 67 ] have all been able to capture Kelvin-Helmholtz instabilities in the unreacted/reacted and the post-shock mixing layers (see Figure 5a as an example), using sufficiently small element sizing in both two- and three-dimensional models.

The grid required to resolve large structures in RDE mixing layers is dependent on the size of the geometry. Elements of 200 μm have been shown to predict shear layer instabilities using either Euler equations or IDDES in an RDE with a mid-channel diameter of 90 mm [ 67 ] and an ∼ 140 mm inner diameter RDE required axial and azimuthal elements smaller than 200–300 μm to capture the structures in a DNS study [ 22 ]. In contrast, Kelvin-Helmholtz structures were not observable in models of a 1 mm outer diameter RDE with computational elements larger than 1.25 μm [ 73 ]. In all cases, these minimum azimuthal element sizes are ≲ 0.21% of their respective mid-channel diameters, suggesting a minimum relative element size relative to geometry. These element sizes are not, however, proportional to the CJ induction lengths which are ∼ 200–300 μm for stoichiometric H 2 /air mixtures near 300 K [ 116 , 117 ], compared to ∼ 50 μm H 2 /O 2 [ 117 ].

Both viscosity and species diffusion have been stated as critical features in non-premixed models of RDEs, promoting the use of IDDES or LES in modelling studies [ 67 ]. In contrast, a negligible dependence of detonation velocity or I sp was reported in DNS of a partially-premixed “linearised” model [ 114 ] (refer to Section 5.5 for more on these models). Despite this, it is crucial to note that Euler equation models significantly over-predicted deflagration upstream of the detonation in the premixed numerical RDE model [ 67 ], whereas the mixture upstream of the shock in the linearised model is completely unreacted [ 114 , 118 ]. This warrants further study on the differences of these modelling approaches on detonation interactions with non-premixed fuel/air injection into post-combustion gases. This is further complicated by the suggestion that the absence of viscous dissipation and diffusive mixing in the Euler equations could enhance perturbations driven by baroclinic vorticity generation which is, in turn, promoted by wrinkling in the deflagration upstream of the detonation.

Although the Euler equations cannot account for viscous effects, such as wall shear-stress and heat transfer, these have a small, but non-negligible, effect ( ∼ 7%) on predicted I sp compared to IDDES modelling including non-slip, isothermal walls in premixed RDE models [ 67 ]. The appropriate selection of wall boundary conditions will therefore likely prove to be an important factor in RDE development, with different thermal treatments significantly changing the fraction of fuel burnt upstream of the detonation wave [ 67 ]. Neglecting these physical features, results in decreased deflagration away from the detonation wave, with adiabatic walls most significantly over-predicting combustion outside of the detonation wave [ 67 ]. Despite this, detonation wave-speeds were reasonably insensitive to wall temperatures in the range of 500–800 K in the same study, and consistently over-predicting experimentally measured detonation wave-speeds [ 94 ], although temperatures significantly exceeding the autoignition temperature (up to the adiabatic wall temperatures ∼ 2000 K) were not assessed.

Incorporating viscosity and thermal wall-effects into IDDES simulations requires significant computational resources. One such study required a computational mesh of ∼ 100 million computational elements, included multiple chemical species and reactions, with numerical time-steps of 30 ns [ 94 ] and is similar to an earlier study using approximately one-third of the number of cells which required ∼ 35,000 CPU-hours to solve [ 67 ]. Several cases in an earlier study, however, required ∼ 9 million CPU-hours to produce a final solution due to the use of time-steps of 2 ns [ 67 ]. In addition to IDDES studies, viscous and diffusive effects may be accounted for in unsteady RANS modelling [ 107 ] and facilitate the inclusion of detailed chemistry (see Section 5.4) with significantly lower computational overhead than IDDES or DNS. Such RANS models cannot, however, capture the turbulent fluctuations in the instantaneous flow-field, although there is evidence that they may be able to provide sufficient accuracy for parametric studies of mixing, detonation wave structure and loss mechanisms in RDEs [ 119 , 120 ]. The interactions between detonations, deflagration and viscous and thermal wall-effects add further complexity to producing RDE models which can accurately reproduce experimentally measured engine characteristics, although the computational resources may currently prohibit broad parametric studies using high fidelity modelling approaches.

5.4 Chemical kinetics and interaction models

The majority of numerical RDEs works to date targeted H 2 /air and H 2 /O 2 systems [ 6 , 20 , 22 , 41 , 62 , 72 , 73 , 79 , 94 , 95 , 111 , 112 , 118 , 121 , 122 ], given their relatively simple chemistry in comparison with both small and large hydrocarbons. Nevertheless, limited data are also available for linearised CH 4 /air and C 2 H 4 /air systems [ 114 ].

The simplest approach to describe the chemistry is that of a one-step irreversible reaction [ 6 , 43 , 62 , 95 , 108 , 109 ]. This assumption has been widely used to numerically investigate various aspects of fully premixed canonical RDE cases and useful insights have been gained [ 6 , 32 , 95 ]. However, it is well known that such a simplification is not able to accurately quantify many detonation responses of interest (e.g. upstream deflagration phenomena [ 109 ], triple shocks structure [ 79 , 116 ]), mainly due to the sensitive Arrhenius nature of the reaction rate to temperature variations. Also, the use of ad hoc correlations of the experimental data with adjustable kinetic parameters (e.g. reaction order, activation energy) are only valid for a limited range of the system and thermodynamic parameters [ 116 ].

Simplified approaches to chemical kinetics may employ a one-step reversible reaction [ 20 , 62 ] or a two-step mechanism [ 22 , 41 ] to describe the chemistry within a system. In particular, for the one-step case, the forward reaction rate is calculated using the classical Arrhenius equation with the reaction rate constants tuned from a reference case while the backward reaction rate is calculated from the assumption of local chemical equilibrium [ 20 , 62 ]. This approach has been validated against detailed chemistry for a 1D model [ 20 ]. For canonical 2D premixed RDEs, a one-step reversible reaction is not able to accurately capture the post-detonation temperature while it is able to predict both the experimental pressure and velocity fields [ 20 ]. In addition, it was also found that this approach can be successfully implemented to describe stratification effects in three-dimensional non-premixed RDE systems [ 62 ].

For the one-step case, a number of two- and three-dimensional premixed RDE simulations employ an induction-time parameter model (IPM) to compute the chemical source terms [ 6 , 32 , 43 , 109 ]. The IPM has shown reasonable accuracy for the prediction of detonation wave propagation in premixed systems [ 108 ], as the induction time is derived from the same configuration as the CJ wave-speed [ 116 ]. In addition, it is computationally inexpensive as a global induction parameter allows for release of energy over a finite period of time. Nevertheless, the IPM lacks the flexibility to accurately describe the physics occurring in more realistic non-premixed systems [ 94 ]. The thermodynamic properties of the single product species employed in this model are dependent upon the equivalence ratio of the fuel/air mixture. Therefore, this approach cannot easily handle the spatially varying local equivalence ratio occurring in a non-premixed system [ 116 ]. This model also lacks the capability to capture the low-pressure heat release and the change in equilibrium chemistry of post-detonation products. Finally, this method requires a priori calculation of the CJ induction time, but the computed detonation velocities in detailed simulations can be significantly higher than that of CJ velocity [ 94 ]. If this approach is extended to a two-step reaction model (consisting of an induction reaction followed by an exothermic recombination reaction), two progress variables are obtained and need to be solved in lieu of individual species concentrations. This approach is termed two-parameter progress variable, and it has been successfully applied for premixed systems [ 22 , 41 ]. Nevertheless, the variation of the two source terms is extremely sensitive to the choice of the constants adopted [ 22 ]. Global chemistry has also been implemented through the well-known PDF method [ 107 ], although this approach is generally used for detailed chemistry in combustion processes [ 123 ].

Finite-rate kinetics and the associated kinetic mechanisms are needed to capture complex phenomena such as near-limit propagation leading to quenching of the detonation wave [ 116 ]. This is mainly because the use of a one-step reaction precludes the influence of chain-branching-termination mechanisms that are invariably multi-step in nature. In this regard, an advanced approach is the induction-length model, which concerns determining the induction length for adiabatic propagation and using it to estimate global detonation parameters such as the cell size of steady propagation and the wave curvature at quenching [ 116 ]. This study showed that at least a four-step mechanism is required to achieve acceptable predictions in CJ detonation.

Models of RDEs using H 2 /air, H 2 /O 2 , CH 4 /air and C 2 H 4 /air mixtures have employed detailed chemistry and simplified configurations [ 68 , 72 , 73 , 79 , 111 , 112 , 114 , 118 , 122 ], although only limited studies are available in comparison with simplified (one- or two-step) chemistry, given the relatively large computational expense required and the current computational resources. A set of 8–9 chemical species and 18–21 elementary reactions are generally employed for H 2 systems [ 72 , 112 ], while 21–22 species and 34–38 reactions are used for simple hydrocarbons systems [ 114 ]. These studies highlighted that the use of detailed chemistry is needed to accurately predict the energy-release pattern in RDEs and complex characteristics, including re-ignition, number of triple points and transverse waves [ 68 ].

5.5 Linearised model detonation engines

The chamber is pre-filled with premixed fuel/oxidiser, and then the detonation is initiated.

The chamber is pre-filled with an inert gas, then premixed fuel/oxidiser is injected and the detonation is initiated simultaneously.

The chamber is pre-filled with oxidiser, then fuel is injected and the detonation is initiated simultaneously.

literature review of jet engine

An example linearised model detonation engine [ 79 ].

LMDEs have been used to characterise the detonation process, by allowing both sides of the chamber to be imaged through quartz walls, or the density field imaged through the use of the Schlieren technique [ 79 , 126 ]. It has been found that the critical fill height of an LMDE is about 10 λ , which is consistent with Eq. (4) for RDEs [ 27 , 126 ]. It has been found that the presence of background gases, such as the inert gas used to pre-fill the chamber, strongly affected the detonation process, causing the reaction zone to slightly trail the detonation wave [ 125 ]. This produced fluctuations in the wave velocity, adversely affecting the detonation propagation [ 125 ]. This would seem to be consistent with mixing of detonated and undetonated reactants producing Kelvin-Helmholtz instabilities in an RDE, as noted in Section 3.1 [ 3 , 22 , 72 , 73 ]. It was also found that low pressure zones in an LMDE attenuate reflected shocks [ 124 ]. This suggests that, should a shock wave be reflected off an irregular feature in an RDE’s annulus, then the shock would not serve as a significant source of thermodynamic loss [ 124 ].

Computer modelling of an LMDE indicated that the propagation of a detonation wave was not affected by the turbulence caused by in-chamber mixing of fuel and oxidiser [ 118 ]. However, the presence of this turbulence did cause the reaction zone to trail the detonation wave [ 118 ]. A model of an LMDE was also used to test the result of applying different back pressures, such as might occur if a nozzle or a turbine was attached to an RDE [ 114 ]. This indicated that increased back pressure also increased the detonability of the fuel mixture, but also restricted the acceleration of the products, which, in some cases, led to the production of tertiary shock waves to sufficiently compress the flow to match the exit plane conditions [ 114 ]. However, as noted previously in Section 2, nozzles have very limited benefit [ 53 ], and, as noted in Section 4 the effect of secondary and tertiary shocks on a turbine may be problem.

6. Future outlook

Nozzles have been shown to have limited benefit to the thrust generated by RDEs. However, varying the angles of the walls of an RDE, either independently or together, may simulate the effect of a nozzle to provide a slight benefit to performance. It remains unknown what effect such modifications to the conventional cylinder might have.

Comparisons of thrust to weight ratios between experimental RDEs and conventional rocket engines show similar values, indicating that an RDE could represent a method of propulsion in space. This has not been widely explored as an option, and would benefit from experimental work in vacuum conditions or microgravity conditions.

It has been suggested that there may be a maximum equivalence ratio at which an RDE will operate, but further investigation is required to determine if this is a universal limit, and identify ways to lower the limit.

Triple points appear to have significant effect on the propagation of the detonation wave but little work has been done on determining the constraints, besides chemical composition, on the formation of stable and consistent triple points as well as the effect of those parameters on other characteristics of the triple points such as peak pressure and propagation direction. Findings would be beneficial in terms of properly defining the parameters that affect λ as well.

Very few studies have provided a mathematical relationship between the detonation cell width and the geometry requirements of the chamber. More supporting work to help refine and verify or dispute the relationships that have been established needs to be done, so that in the future, specialised design needs can be catered for through knowing the geometry and cell width of fuel types.

Varying the channel width has been noted to affect the stability of the detonation wave in an RDE. As such, this is likely to affect the performance of such devices. Further research is required to determine what the optimal width would be for different design requirements.

It is established that RDE chambers need to be at least twice as long as the fuel fill height, and increasing the length four to six times the fill height improves the efficiency. However, depending on the ratio of inlet pressure to nozzle pressure, such a length increase may also result in reduced I sp . Further research is required to determine an appropriate balance of these effects, and the effect chamber length has on other design parameters.

So-called “centrebodiless” designs have been explored, and proposed for use in afterburners. However, they have not been modelled or tested with heated high velocity air, as would be typically found at the outlet of a conventional jet engine, so their potential performance remains unknown.

It has been demonstrated that the thrust produced by RDEs scales non-linearly with engine size, but they are not yet approaching the size required to replace most existing gas turbines. It remains unknown if an RDE can be scaled up sufficiently to provide the thrust levels offered by contemporary gas turbine engines.

It has been suggested that a turbine could be attached to an RDE. However, the effects of the various shocks on a turbine have not been explored. In particular, the oblique shock (Feature B in Figure 5a ) has been shown to propagate out of the chamber, and is likely to have significant effect on the viability of using a turbine.

The invsicid Euler equations have been demonstrated to over-predict deflagration in three-dimensional computational models of premixed RDEs, even with the use of detailed chemistry. Their validity in non-premixed RDE configurations, with deflagration upstream of the detonation and the potential to produce lifted detonation waves, still requires rigorous assessment.

Viscous and thermal wall-effects in RDEs have significant effect on RDE performance characteristics, and may be essential in accurately reproducing experimentally measured values. Understanding of the appropriate numerical modelling approaches of these effects, however, is still immature, owing to the computational resources required for sufficiently fine resolution of near-wall grids.

The computationally predicted wave-speeds and plenum pressures in RDEs are significantly different to those measured experimentally. It has been proposed that this could be partially due to baroclinic vorticity, resulting from interactions between detonation waves, fresh reactants, deflagration reaction-zones and post-combustion products, although this is yet to be analysed in detail in either full RDEs or linearised models.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Journal of Aeronautics & Aerospace Engineering Open Access

ISSN: 2168-9792

Research - (2020)Volume 9, Issue 1

Technologies for Aircraft Noise Reduction: A Review

Author info »

This article presents a review of current aircraft noises and technologies to reduce noise as well as an estimate of the technology’s readiness level. Aircraft noise remains the key inhibitor of the growth of air transportation and remains an acute environmental problem that requires advanced solutions. To deal with this problem, aircraft manufacturers and public establishments are engaged in research on technical and theoretical approaches for noise reduction concepts that should be applied to new aircraft. This review paper discusses a selection of enabling technologies and their implications on acoustics and noise and gives a perspective on future trends and new directions in aeroacoustics required to address the challenges.

Noise reduction; Aircraft noise; Technology trends; Aeroacoustics; Sound propagation

Abbreviations

ANC: Active Noise Control; DLR: Deutsches Luft- Raumfahrt; AWB: Aeroacoustics Wind Tunnel Brauschweig; HLD: High Lift Device; RAIN: Airframe and Installation Noise; HBR: High Bypass Ratio; UHBR: Ultra High Bypass Ratio; DDTF: Direct Drive TurboFan; CRTF: Counter-Rotating Turbo Fan; GTF: Geared TurboFan; RESOUND: Reduction of Engine Source Noise through Understanding and Novel Design; OR: Open Rotor

Introduction

Jet engine noise suppression has become one of the most important fields of research due to airport regulations and aircraft noise certification requirements. Further reductions in aircraft noise will be harder to achieve, and the problem becomes more difficult with anticipated increases in noise due to increased aircraft operations. It has been the implementation of innovative technology solutions related to engine and shape design that have resulted in the noise reductions. When an aircraft gets to fly it produces friction and turbulence that causes sound waves. In general, as the faster flight of the aircraft becomes more turbulent and friction will occurs. As long as the flaps and landing gear of aircraft are used, more noise is created because more drag is being generated. The quantity of noise which is generated can be different according to the way the plane is flying.

The elimination of aircraft noises is the long-term goals of the industry, universities and government agencies. The noise generated by the airframe is a factor of several parameters affecting the noise level of aircraft; the main source of noise is in the engine. In general way noise reduction techniques can be arranged into passive and active methods. Passive control involves reducing the radiated noise by energy absorption, while the active method involves reducing the source strength or manipulating the acoustic field in the duct to get noise reduction.

There are significant sources of noise in the fan or compressor, turbine and jet or exhaust jets. The noise generation of these components increases with the relative velocity of the airflow. The exhaust jet noise has the significant part of the noise in comparison with compressor or turbine, so reducing it has a profound effect than a similar reduction in above mentioned. Jet exhaust noise is generated when a mixture of produced gases with a turbulent cases are being released that also being affected by the shearing action due to the relative velocity between the exhaust jet and the atmosphere.

The turbulence which is generated near the exhaust exit is the reason of high frequency noise (small eddies) and more at the lower exhaust, turbulence makes low frequency noise (large eddies) also, a shock wave is created as the exhaust velocity exceed the velocity of sound. Reducing noise could be achieved when the rate of mixing getting faster or the relative velocity exhaust to the atmosphere decreases. The noise of the compressor and the turbine is due to the interaction of pressure and turbulence fields for rotary blades and fixed vanes. In the jet engine, the exhaust jet noise is of a high level that the turbine and compressor noise is negligible in most operating conditions. However, low landing gears reduce exhaust jet noise and low pressure compressor and turbine noise will be increased for the cause of internal power.

Another source of noise is the combustion chamber which is located inside the engine. However, due to being buried in the engine core, it does not have dominated influence. Progress in noise reduction technology such as smooth acoustically inlet and chevrons has made these improved engines available on existing aircraft, and at the same time meeting challenging the requirements for noise. Looking for the future, it is unclear whether the process of increasing productivity will generally continue with decreasing fuel consumption and reducing community noise.

Literature Review

The primary aim of the present paper is to provide a review on the main noises of aircrafts and theologies to reducing them. Emphasis is placed on evolution of these technologies that are widely used by the major aerospace research establishments.

Types of noises

Airframe noise: In the early 1970s Research into airframe noise reduction and prediction was started. This investigation was done by Crighton [ 1 ]. Crighton defined airframe noise as the nonpropulsive noise of an aircraft in flight and includes the noise of a glider. The empirical data recorded on aircraft noise assisted in the formulation of an experimental airframe noise prediction method published by Fink [ 2 ]. Airframe noise is defined as the noise generated as a result of the airframe moving through the air. The main components of airframe that lead to airframe noise generation are high lift devices and landing gears. In 1970s some initial airframe noise studies were carried out, and reference provides a good summary of this airframe noise research work [ 3 , 4 ]. During the 198Os, a lack of funding for research on airframe noise caused the technology to remain at the 1970s level. In the 199Os, research about an airframe noise has been kept on again in the USA where analytical and experimental works were conducted effort between NASA and the aircraft industry. Noise testing was performed on scaled aircraft models and adequate noise localization techniques were developed [ 5 , 6 ].

An airframe noise occurs when air passes over the plane’s body and it wings. This cusses friction and turbulence, and make noise. Even gliders make a noise when in flight and they have no engines at all. Planes land with their flaps down and their landing gear deployed. This creates more friction, and produces more noise, than when the flaps are up and the landing gear is stowed.

The aerodynamic noise which is created by all the non-propulsive components of an aircraft is classified as airframe noise. For advanced high-bypass engine powered commercial aircraft, the airframe noise has the major role in the overall amount of flight noise levels during landing approach stages, when the highlift devices and the landing-gear are ready to be used. Five main mechanisms are known to significantly contribute airframe noise: (i) the landing-gear multi-scale vortex dynamics and the consequent multi-frequency unsteady force applied to the gear components, (ii) the flow unsteadiness in the recirculation bubble behind the slat leading-edge, (iii) the vortex shedding from slat/main-body trailingedges and the possible gap tone excitation through nonlinear coupling in the slat/flap coves, (iv) the roll-up vortex at the flap side edge, (v) the wing trailing-edge scattering of boundary-layer turbulent kinetic energy into acoustic energy. Since the Seventies most of these mechanisms have been addressed both empirically and theoretically.

Aerodynamic noise which is created from airframe components identified as a most important contributor to commercial aircraft noise emissions. The intense regulatory context governing civil aviation has caused of research in optimize of noise generated by airframe and other aircraft components in large amount. Adaptive techniques and Flow control are two possible solutions for noise reduction, when other methods are not effective. Such unconventional techniques include boundary layer excitation, exploitation of cavity resonance effects and flow distortion in airframe components.

It was triggered by this US initiative and the intended extension of a very large aircraft; in 1995 Airbus industry was volunteer for sponsoring two airframe noise related research projects. These include the full scale noise testing of the landing gears and scaled aircraft model high lift devices in wind tunnel [ 7 , 8 ].

Dobrzynski W, et al. In 1998 worked in results of initial and basic experiments conducted on a model scale high-lift wing-section in DLR’s AWB provided detailed information on source noise characteristics, led to a better understanding of the dominating mechanisms on slats or flaps and revealed perspectives for noise reduction [ 9 ].

Werner Dobrzynski, et al. in 2001, Since airframe noise has become a significant contributor to the overall radiated noise from commercial aircraft during landing approach, a research project was initiated to investigate the noise of wing HLD, known to represent one major source of airframe noise. Noise source studies where performed on both a 1/7.5 scaled complete model and an A320 full scale wing section, employing far field microphones and source localization techniques, to quantify airframe noise levels and identify the major aero acoustic sources. Potential source areas were instrumented with unsteady pressure sensors to study local source characteristics in detail [ 10 ].

In 1999 Leung Choi Cho and Pierre Lempereurn announced a brief description of research project which took them for three years, ‘reduction of airframe and installation noise (RAIN) [ 11 ] (Figure 1) .

aeronautics-aerospace-engineering-wing

Figure 1: Sources of noise on a typical wing.

Fan noise: Reduction of fan noise radiation to the far field can be followed by five general concepts: (i) reducing the interaction mechanisms between an optimal design of the rotor blades and the stator vanes, or to reduces the velocity deficit in the rotor wakes with the flow control techniques, (ii) reduce the aerodynamic response to an impinging gust by tuning of the stator cascade parameters in order to, (iii) to drive only few propagating (cut-on) duct modes by tuning of the rotor blades and stator vanes numbers, (iv) use of passive/active duct wall treatments in order to reduce noise during transmitting from the duct, (v) manipulation of sound diffraction mechanism in exhaust nozzle and at the inlet lip through advanced nacelle devices. Since the first two noise mitigation concepts requires analytical models that highlight the mutual influence of all the design parameters.

In turbofan aero-engines, noise is created by the interaction between flow non-uniformities and stator vanes and rotating bladed. In modern high-bypass-ratio turbofans, the noise generated by the fan system has the main role than the one generated by the turbine stages and the compressor. Since there is connection between the duct acoustic modes and aero acoustic excitation mechanisms.

Through the duct under the condition, at supersonic blade tip, the rotor-locked shock wave system makes propagative several pure tones at rotational shaft harmonics frequency, the so called “buzzsaw” noise.

Fan noise is a powerful performance of the fan pressure ratio and rotational tip speed. The reliable approach to reduce fan noise is to mitigate the pressure ratio and tip speed, but this will increase the engine diameter to recover thrust. Optimization examinations demonstrate that the best fan speed for takeoff is where the rotational tip speed is just below Mach=1 to eradicate shock induced noise. After achieving this engine design; the fan pressure ratio becomes the controlling factor for broadband noise. Reducing pressure ratio and fan tip speed, reduce the number of noise sources, which makes noise reduction design features more effective [ 12 ]. European Brite-Euram project called RESOUND (Reduction of Engine Source Noise through Understanding and Novel Design) was launched in 1998. A task of this project was dedicated to laboratory experiments relative to passive/active design [ 13 ].

Currently active noise control approach (ANC) that has been studied by many authors [ 14 - 20 ]. The use of the well-known concept of noise reduction in fan noise involves of attempting to cancel the interaction modes by generating the identical out-ofphase spinning modes. Typical ANC studies are generally based on two possibilities: (1) as active sources use of flush-mounted loudspeakers; (2) the active source is an airfoil equipped with actuators (active airfoil). Using a sophisticated experimental setup shows the capability of these ANC techniques to the noise reduction. Unfortunately, because of weight, applications to turbofans are not straightforward, complexity of such devices and aerodynamic penalties (Figure 2) .

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Figure 2: Turbofan engine.

Flap noise: It’s too long that flap side edge flows have been recognized as important factor in airframe noise. Vertical flow around the side edge of a deployed flap is one of the most effective sources of airframe noise at landing and takeoff conditions. Additionally, vortex breakdown at high flap angles is observed as an additional noise source mechanism.

The noise source mechanisms are the cause of the vortex structure of the cross flows in the flap side edge region [ 21 - 25 ]. This concept has caused the concepts for noise mitigation like flap side edge fences, seeking to reform the properties of the vortex structure in a desirable approach to reduce the noise from these currents. While there are difficulties in the use of this concept in real aircraft, such as the cost and added weight, its effectiveness in reducing noise –associated with flap has been shown to be very clear [ 26 - 28 ]. These successful demonstrations include both simplified flaps and realistic aircraft configurations. Typically, side edge fences can reduce noise by up to 4 dB in the middle to high frequency domain in which flaps are known to be major noise sources.

It has been proved in wind tunnel experiments that the fences only alter the local flows in that the overall lift characteristics of the flaps and the high lift systems is not influenced by the fences in any significant way [ 27 ]. The vortex structure in the cross flow will be appeared in the surface pressures in the form of distinct spectral humps [ 29 ]. In 2003 Yueping Guo shows that reduction of flap-related noise by shifting the source spectra downward in frequency can be achieved. Analytical prediction for the frequency change has been given and has been shown to agree with data quite satisfactorily. It should be noted that with the weakness of the source current, fences might also reduce noise [ 30 ] (Figure 3) .

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Figure 3: Flap of the wing.

Jet noise: Mixing of the high-velocity exhaust stream with the still air causes Jet noise, which causes friction. When these two Streams at different velocities are mixed, significant amount of turbulence is created, with the intensity of the turbulence, and hence the noise increases as eighth power of the velocity difference [ 31 - 33 ]. Modern bypass engines, which introduce a layer of moderately fastmoving cold air between the hot exhaust and the ambient air, are quieter than early jet engines, which didn’t use this technology.

Engine noise is created by the sound from the moving parts of the engine and by the air coming out of the engine at high speed and interacting with still air, creating friction. Most of the engine noise comes from the exhaust or jet behind the engine as it mixes with the air around it. Modern bypass engines introduce a layer of moderately fast-moving cold air between the hot exhaust and the still air. This makes them quitter than the engines on earlier jets, which didn’t use the bypass technology.

The degree to which people experience aircraft noise on the ground has a lot of do with atmospheric condition. Temperature wind speed and direction, humidity, rain, cloud cover all have a part to play. The reverberation of sound waves caused by the weather can make noises seem louder. Sometime the aircraft flying at the altitudes that would not normally produce noise may be heard in certain atmospheric condition. The noise that coming from airplane is caused by two things: from air going over its body (or ‘airframe’) and from its engines.

Over the years there was considerable decrease in Jet noise, mainly because of an increase in (BPR) in turbofan engines, which reduces the velocity gradient therefore, the shear stresses within the shear layer of exhausted jets. In modern high-BPR engines, an increase in the nacelle diameter has caused the aircraft to operate by reducing exhaust flow velocities without affecting the thrust. The engine exhaust velocity has to decrease in order to reduce the engine noise during takeoff. The exhaust nozzle is designed to have variable area in order to ensure fan operability at the low power, with cruise bypass ratio of 12 and take-off bypass ratio of 18. The low engine rotational speed during approach enabled by the variable nozzle mitigates the rearward fan noise and the airframe drag requirements. The fan design, however, must now accommodate the wide range of flows related to the performance of low pressure ratio fans at different flight conditions. A change in fan design methodology was required to enable the fan to cope with the various conditions imposed by the variable area nozzle (Figure 4) .

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Figure 4: Nozzle Jet flow.

Technologies for noise reduction

Active noise control: Active noise control, also known as active noise cancellation is the reduction of sound wave by adding reverse sound wave. A noise cancelation speaker send out sound with amplitude as same as the noise sources but with inverted phase . waves combine to constitute new wave and effectively cancel each other out. ANC has become more and more popular in recent years. At 1991 J. C. Stevens and K. K. Ahujat in Georgia Institute of Technology, Atlanta, Georgia worked in active noise control. This popularity is due, in part, to the advancement of electronics and signal-processing techniques which take advantage of increased computer power. In particular, adaptive filtering method has natural applications in active noise control [ 34 ] (Figure 5) .

aeronautics-aerospace-engineering-cancellation

Figure 5: Active noise cancellation.

Acoustic boundary control: An acoustic boundary control method has been developed by Hirsch and Sun that proposes to implement a distributed array of acoustic sources at the structuralacoustic interface in conjunction with a sensor array [ 35 ]. Hirsch, Jayachandran and Sun proposed the acoustic boundary control approach for preventing internal sound areas which mixes the lowered power requirements of ANC. A mathematical model of curved composite trim panels has been mentioned in the article [ 36 ].

Shape optimization: It has been shown that shape optimization tools can be used effectively to design the inlet duct to reduce the radiated sound in distant area. The main idea of the shape optimization is to minimize the far field acoustic radiation by controlling the geometry of an engine duct.

Novel acoustic treatments and shape design of turbofan engine ducts to attenuate such noise are vital for the noise reduction of modern aircraft engines. These designs usually depend on extensive empirical tests, which are very expensive and time consuming.

In the past, research activities in the field of noise optimization systems have been carried out. It has been shown that in the case of noise reduction of radiated sound in the far field, these shape optimization tools can be effectively used. The controlling the geometry of an engine duct, could be main idea of the shape optimization in order to to minimize the far field acoustic radiation.

Chenais had examined the mathematical aspect of the problem [ 37 ]. For the existence of an optimal shape for systems, He mentioned the conditions necessary by coercive elliptic partial differential equations. More recently, there had been research in minimization of viscous through shape modifications in [ 38 ]. Extensive research work had been done on shape identification for acoustic scattering problems [ 39 ].

Yanzhao Cao in 2002 showed the results that it may present one viable alternative for far field noise reduction. The extension of this work to the case with mean flow represents a natural extension of these results which we hope to address in the near future [ 40 ].

Ultra High Bypass Ratio (UHBR)

In 1970 Boeing 747-100, the HBR turbofan engine was entered commercial service; soon, it was followed by McDonnell-Douglas, Lockheed who was other wide-body aircraft, and the newly formed Airbus consortium. A major advance in environmental protection was achieved, because these engines, which were produced by Pratt & Whitney, General Electric, and Rolls-Royce, they consume significantly less fuel. At the front of a turbofan engine there is a massive fan which creates the lion’s share of thrust (up to 80 percent on an ordinary commercial jetliner) and accounts for two airflows: the main flow, that passes through the engine core and be involved in combustion, and the latter flow, which drives the engine core through the nozzle. By increasing this secondary flow, Increase of the BPR (the ratio of the cold airflow to the hot airflow) is achieved. At a given level of thrust, increasing the BPR decreases the exhaust gas speed and therefore the noise it generates. By increasing the BPR to reduce noise, the optimum fan pressure ratio reduces and the specific thrust drops, as a result the optimum fan tip speed also is reduced. A decrease in tip speed with the increased fan diameter together, causes to a drop in shaft speed and an increase in shaft torque.

Noise reduction research has been summarized with participation of DLR which aims to reduce aircraft engine noise at the source, because this is the most effective and economical way to reduce noise. The main part of noise reduction potential can be seen in the Ultra- High Bypass Ratio (UHBR) engine concept where can be observed that the average flow Mach number at the blade tip is reduced. This large reduction in fan rotation speed requires that the fan to be driven by the turbine through a gear box to maintain the high turbine speeds necessary for proper aerodynamic efficiency. It is necessary to large bypass ratios together with much reduced jet velocities for reduction of jet noise down to an acceptable level. With the reduction in jet noise other noise sources become dominant which also need to be reduced to achieve the 10 dB reduction target. Methods of Active noise control have been studied in order to reduce the total fan noise of aircraft engines. There is accomplishment in radiated sound power up to 34 dB with loudspeakers mounted flush with the duct wall impressive tone level. Active stators do not reduce the space available in the nacelle for passive liners; therefore they are a promising concept. Recent laboratory experiment have shown that current flow induced sound sources generated by flow disturbances at the blade tips are another means to avoid the weight and space penalties associated with the conventional loudspeakers. In UHBR engines, the low pressure turbine has the main role of overall noise. In order to fully assess the effectiveness of noise reduction studies involving blade and vane design advanced sound measurement and modal analysis techniques are required [ 41 ]. Pascovici in 2008 suggested a model for coupling engine and aircraft performance with noise algorithms of three ultrahigh bypass engines. Various parameters have been examined also a comparison with the baseline engines Trent 772 and CFM56-7b has been done too. The purpose of these analysis, comparison, and calculation, was to determine the viable improvements calculated from a change in cycle just as problems that are related with these new concepts (DDTF, CRTF, and GTF) [ 42 ] (Figure 6) .

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Figure 6: Ultra high bypass ratio.

The fan noise can be reduced effectively by the use of the equipment of an optimally designed acoustic liner in the engine nozzle. To this end, some design challenges must be addressed, including the choice of acoustic liner material and layer structure.

To reduce noise within the turbofan bypass duct, the use of acoustic liners is already common, and it is usual practice to consider the effect of liner configuration as a noise reduction measure. The basic idea of the shape optimization is to minimize the far-field acoustic radiation by controlling the geometry of an engine duct. The embedded propulsion system allows smaller engine diameter and thus increased non-dimensional (length/diameter) duct length. The longer inlet and exit ducts causes engine noise reduction by allowing additional acoustic liners, compared to ordinary nacelles, to absorb the engine noise. Another promising technique for fan noise reduction is to increase the acoustic treatment area on the tip of the rotor. Existing engines only use acoustic liners in fan ducts and the inlet, and sometimes in the inter-stage region. To provide maximum insertion losses around a desired target frequency, they usually use honeycomb materials with porous or felt metal face sheets. NASA has explored that metal foams can be used to provide optimum bulk liner properties which also provide engine requirements over a range of temperatures for either the fan ducts or the core [ 43 ] (Figure 7) . The concept of active absorption was first put forward by Olson and May who mentioned an electronic sound absorber providing pressure release on the back face of a resistive sheet. In the 1980s, Guicking and Lorenz [ 44 , 45 ] confirmed this concept by experimental. Several researches have sought to implement hybrid absorption technology, leading to patent applications [ 46 ]. Thenail and Furstoss [ 47 , 48 ] developed an active treatment consisting of a layer of glass wool layer backed by an air cavity closed through an active surface. Beyene and Burdisso obtained active boundary conditions by using impedance adaptation in a porous rear face layer [ 49 ]. More recently; Cobo et al. [ 50 ] illustrated the feasibility of designing thinner hybrid passive/active absorbers using micro perforated panels instead of the conventional porous materials.

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Figure 7: Duct flow in Turbofan engine.

Chevron nozzles have drawn a lot of attention recently as they are currently one of the most popular passive jet noise reduction devices. Investigations reveals that, by adding chevrons to the nozzle significant amount of noise reduction will occur. In medium and high bypass turbofan engines, chevron nozzles represent the current state in jet noise reduction technology. These nozzles possess triangular serrations along the trailing edge, which induce stream wise vortices into the shear layer.

Serrating trailing edge geometry, chevron nozzles are the cause of enhancement in mixing between adjacent streams, reducing the velocity gradient across the jet plume. The penetration rate in the individual chevron lobes is lower than that for the tabbed nozzles, and so resulting vortices are weaker.

Compared with other noise mitigation technologies, chevron nozzles are the most effective reducing engine exhaust noise tools, with minimal penalty on engine performance. However chevron nozzle seems to be an interesting solution to the jet noise problem, There is not much empirical work has been done with chevron nozzles. Researches seem to be in computational aero acoustics, because of vastly improved numerical methods based on of very powerful computers. Chevron nozzles are known to be excellent attenuators of jet noise. Conventional chevron nozzles use triangular serrations at the trailing edge of the nozzle. According to Bridges and Brown [ 51 ], the chevron count controls the azimuthal spacing between the axial vortices, whereas chevron penetration controls the strength of the axial vortices and chevron length controls the distribution of vortices within axial vortices Bridges and Brown [ 51 ] studied the influence of geometrical chevron parameters on flow and noise characteristics and far field on a parametric family of chevron nozzles. A high chevron count resulted in good lowfrequency reductions without considerable high-frequency penalty; Callender et al. [ 52 ] empirically examined single and dual flows for baseline inner nozzle and three chevron nozzles over an extended range of operating conditions. Chevrons with different numbers of lobes and levels of penetration were performed to find out the effect of these geometrical parameters on far-field acoustics. Chevron nozzles are the most effective at lower frequencies and at aft directivity angles based on Spectral and directivity results from heated coaxial.

Rask et al. [ 53 ] conducted experiments to determine the acoustic emissions from chevron nozzles operating at under-expanded conditions. It has been shown that the chevron nozzle was result in lower shock noise levels by 2.1 dB for the Mach 0.85 condition. It was also found that the chevron nozzle reduced the shock cell spacing, resulting in a higher frequency shock noise.

Callender et al. [ 54 ] conducted empirical researches about the effect of chevron nozzles on the near-field acoustics were for a separate flow exhaust system. Chevron count and levels of penetration were different to provide insight into the influences of these parameters on the acoustic near-field.

It was understood that chevrons are effective at low frequencies where the peak noise region was mitigated by 5-7 db. The nozzle penetration was more important than the number of chevron lobes for noise mitigation in the near-field. Khritov et al. [ 55 ] presented computational and experimental results containing turbulence and jet noise for baseline nozzles, chevron nozzles, and coaxial nozzles with chevrons. Experiments also showed a weak effect of external flow on the noise level in a coaxial nozzle jet.

Numerical predictions of single-stream chevron nozzle flow and far-field acoustics presented by Engblom, et al. [ 56 ]. Birch et al. [ 57 ] employed RANS-based jet noise prediction model to a series of chevron nozzle flows and the predictions were compared with experimental data. Chevrons have been shown to affect the flow in two important ways. Massey et al. [ 58 ] presented a computational flow field and predicted jet noise source analysis for asymmetrical fan chevrons on a separate flow nozzle at take-off conditions.

Uzun and Hussaini [ 59 ] presented the simulation of the near-nozzle region of a moderate Reynolds number cold jet flow exhausting from a chevron nozzle. Simulation of flow through symmetric chevrons with a 5° penetration angle was done, by them. The chevron nozzle flow and the free jet flow outside were simultaneously calculated by a high-order accurate, multi-block, large eddy simulation code with ∼100 million grid points. The enhanced shear layer mixing were captured by the simulation due to the chevrons and the resulting noise generation that happens in the mixing layers of the jet within the first few diameters downstream of the nozzle exit.

Shur et al. [ 60 ] reported noise mitigation concepts such as beveled nozzles chevron nozzles, and dual nozzles with enhancement in numerical system to represent complex nozzle flows more faithfully. The simulations were performed on PC clusters on a grid size of 2–4 million nodes with a goal accuracy of 2–3 dB for both directivity and spectrum. However, the limitation in frequency prediction are significant for chevron nozzles, they showed that exiting computational aero acoustic models are capable of predicting the noise of complex jets with affordable computational resources (Figure 8) .

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Figure 8: Boeing 777 chevron nozzle.

Micro-tab device

In 2010, Brian CK et al. conducted an investigation about a twodimensional numerical study, in the case of acoustic influence of micro-tab device on airframe noise mitigation. While the noise generated by leading-edge slat and trailing-edge flap rise as long as deflection angles are increasing, it is possible to reduce such high-lift noise by using reduced settings without sacrificing the aerodynamic performance during procedure, micro-tab device connected to the pressure side of the flap surface is intended as a means to this end. The resolution of the computation was selected so that the details of flow were captured in the critical noise generation area [ 61 ].

Flow-induced unsteady blade forces

Mathias Steger et al. found that additional sound field is the causes of the interaction between the rotor blades and these jets. The number of nozzles is as the same as the number of vanes in the stator due to create the same azimuthal modes as the stator. A slight decrease in overall sound power was made in a first optimization attempt, by shifting the azimuthal jet location relative to the stator vane. Most likely an optimization with respect to the axial position, nozzle diameter, and mass-flow rate of the jet will bring a significant reduction in the initial noise field from the rotorstator interaction [ 62 ].

Under certain conditions, this secondary sound field may offset the main sound field as was shown empirically for a low-speed fan by Schulz et al. [ 63 ] and numerically by Ashcroft and Schulz [ 64 ]. This method is now applied numerically to the fan of an aero engine with the objective to show that ANC is possible and to find the optimum position for the required flow rate and nozzles.

Acoustic liners

Novel acoustic treatments and design of turbofan engine shape ducts to attenuate such noise are important for the noise reduction of modern aircraft engines. These designs usually rely on extensive experimental tests, which are very time consuming and expensive. Acoustic liners are common to reduce noise within the turbofan bypass duct, and it is common practice to consider the effect of liner configuration as a noise reduction measure.

One effective way of reducing aero-engine noise is to use acoustic liners [ 65 – 69 ]. Due to weight restrictions, Noise reduction by acoustic liners has become difficult to achieve. Optimization the shape of turbofan duct is an alternative technology which is being considered in recent years. A noise optimization and prediction system for turbofan inlet duct designs, is developed by Zheng et al. [ 70 ] With the integration of an in-house software suite of computational fluid dynamics (CFD) codes, and a commercial software suite of codes, into an in-house optimizer, Soft liner and geometry optimizations of an axi-symmetric intake are performed by Pan et al. [ 71 - 74 ]. McAleer et al. [ 75 ] investigated the influence of duct geometry on noise propagation, however, the bypass design stage receives less attention during the bypass design stage, due to the sophisticated models needed for this require larger computational times, making analyses on duct geometry less possible within an industrial timescale. Among different optimization techniques which are mentioned in the literature, recently the adjoint method has become one of the widely used techniques for solving a variety of steady and unsteady optimization problems. Rumpfkeil combined a discrete-adjoint Newton–Krylov algorithm with a hybrid Navier–Stokes (NS)/Fowcs Williams and Hawkings farfield noise for shape optimization of a trailing edge flow to control aerodynamic noise [ 76 - 78 ]. Cao et al. [ 79 ] investigated the acoustic shape optimization of a fan inlet in the frequency domain by using a discrete adjoint method with the wave equation governed by a simple Helmholtz equation without considering the influence of a mean flow.

Stanescu et al. [ 80 ] developed the work of the optimal designs for the shape of fan inlet within the framework of linearized full potential equation and its discrete adjoint formulation [ 81 ]. However, their work just involves one design variable and their acoustic models are simple (Figure 9) .

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Figure 9: Acoustic liners.

Swept and leaned

Rotor-stator interaction is one of the mechanisms in noise generation in an aero engine; this includes periodic impingement of the rotor wake on the stator. As future designs are heading towards higher bypass ratio the interaction process is also expected to become more significant. Swept stators reduce fan noise by increasing the phase changes from hub-to-tip of the unsteady aerodynamics producing the sound and by increasing the effective distance from the fan to the stator vanes [ 82 ]. In general, the modern aircraft engines are designed using combination of the structural noise reduction technologies and passive methods which are assumed to install and absorbed the noise treatment in engine ducts [ 83 , 84 ]. Among the first group of noise reduction approaches in complying of the cutoff condition, choosing the optimal axial clearance between rotor and stator as on the one hand the increasing of axial clearance leads to noise mitigation and on the other hand to negative increasing of the engine weight. Recently scientific papers have been reported in which the configuration of fan design with the swept and leaned stator vanes were considered in terms of noise mitigation as compared to the conventional radial vanes [ 85 ]. Fan stator leaned and swept vanes are provided in order to weaken the mechanism of interaction between the stator vanes and the rotor wake.

One of the first published articles related to this subject shows that the stator vane angle equal to 45.2in the rotation reduces noise by 9 db [ 86 ]. Envia [ 87 ] describes general physical phenomena of noise mitigation in fans with swept-and-leaned stator vanes. Compared to radial stator vanes, the swept stator vanes provide an increased axial gap at the tip that is useful for noise mitigation. Additionally, the vane leaning leads to a great number of rotors wake-stator vanes span wise intersections. As a result, there is an additional decrease in the amplitude of sound wave.

Noise reduction technologies for future

What will be the challenges beyond 2020? In the last sections, various technologies presented, or to be applied, to conventional engine architectures, i.e., so-called “tube and wings” equipped with turbofans. However, the challenge to reduce fuel consumption is so great that new architectures are required. As mentioned before, Ultra High Bypass Ratio engines (UHBR) are being studied, but with difficult integration issues, because the fan diameter is even greater than it is currently used. With this option, noise reductions essentially require pushing for the same technologies more than the above technologies. In this case, the main machine noise, such as turbine noise, combustion noise or even compressor noise would need to be considered.

In addition to UHBR, another strategy could also be to keep on increasing BPR using the Open Rotor architecture (OR). The most critical issue is Noise, along with safety: while mostly tonal noise in the propeller plane radiated by single propellers. Actually, the radiated frequencies combine all of the possible linear combinations between the two blade passing frequencies and this spectrum is propagated in all directions. Currently there is ongoing research about facing this drawback and in order to lower this excessive noise several tricks are being investigated. From a programmatic perspective, Clean Sky research program is the main framework for such integrated research, by the end of the decade which will allow the engine manufacturer Snecma to produce a demonstrator.

This article has reviewed the current state of noises which are produced in aircrafts, and main mechanisms involved in aerodynamic noise reduction. This review paper has focused on various methods to reduce aircraft noise. Examples of these technologies have been presented, such as Active noise control and to calculate optimized shape body of duct or wings, Acoustic boundary control can reduce noises of engine and also to consider Landing gear noise can be efficient, the installation of chevrons mixer on exhaust nozzles, effects of higher by pass ratio, and Microtab device also were investigated. This is especially valuable, for instance, to evaluate the effect of a noise reduction device on the aircraft operating cost. A review of the main role technologies for airframe-, jet and fan-noise reduction and those currently under evaluation is also reported. While many scientific and technological elements have not been addressed, we believe that this work may be useful for a quick access to information in the field of aircraft noise reduction.

Author Info

Citation: Sadeghian M, Bandpy MG (2020) Technologies for Aircraft Noise Reduction: Review Paper. J Aeronaut Aerospace Eng. 9:218. doi: 10.35248/2168-9792.20.9.219

Received: 06-Jan-2020 Accepted: 17-Jan-2020 Published: 24-Jan-2020 , DOI: 10.35248/2168- 9792.20.9.219

Copyright: © 2020 Sadeghian M, et al. This is an open access article distributed under the term of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Chinese Journal of Aeronautics

Review review of hybrid electric powered aircraft, its conceptual design and energy management methodologies.

The paper overviews the state-of-art of aircraft powered by hybrid electric propulsion systems. The research status of the design and energy management of hybrid aircraft and hybrid propulsion systems are further reviewed. The first contribution of the review is to demonstrate that, in the context of relatively underdeveloped electrical storage technologies, the study of mid-scale hybrid aircraft can contribute the most to both theoretical and practical knowledge. Meanwhile, the profits and potential drawbacks of applying hybrid propulsion to mid-scale hybrid airplanes have not been thoroughly illustrated. Secondly, as summed in the overview of design methodologies, the multi-objective optimization transcends the single-objective one. The potential of the hybrid propulsion system can be thoroughly evaluated in only one optimization run, if several objectives optimized simultaneously. Yet there are few researches covering the conceptual design of hybrid aircraft using multi-objective optimization. The review of the most popular energy management strategies discloses the third research gap—current methodologies favoured in hybrid ground vehicles do not consider the aircraft safety. Additionally, both non-causal and causal energy management are needed for performing a complicated flight mission with several sub-tasks.

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Peer review under responsibility of Editorial Committee of CJA.

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