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Transcript
General Disclaimer
One or more of the Following Statements may affect this Document
This document has been reproduced from the best copy furnished by the
organizational source. It is being released in the interest of making available as
much information as possible.
This document may contain data, which exceeds the sheet parameters. It was
furnished in this condition by the organizational source and is the best copy
available.
This document may contain tone-on-tone or color graphs, charts and/or pictures,
which have been reproduced in black and white.
This document is paginated as submitted by the original source.
Portions of this document are not fully legible due to the historical nature of some
of the material. However, it is the best reproduction available from the original
submission.
Produced by the NASA Center for Aerospace Information (CASI)
r°(NA , A-TM-X-71750) JET AIRCRA 'T FMISSIONSDURi4G CRUISE: PRESENT AND FUTUkF (NASA)15 p HC 43.25 CSCL 21E
N75-25945
UncIds63/07 2b6b2
JET A IRCRAFT EMI SSIONS DUR ING CRUI SE -
PRESENT AND FUTURE
by J. S. GrubmanLewis Research CenterCleveland, Ohio
',:(.CHNICAL PAPER to be presented atSeventh Aircraft Systems and Technology Meetinger isored by the American Institute of Aeronauticsand Astronautics
Los Angeles, California, August 4-7, 1975
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JET AIRCRAFT EMISSIONS DURING CRUISE - PRESENT AND FUTURE
J. S. GrobmanNASA Lewis Research Center
Cleveland, Ohio
Abstract
Forecasts of engine exhaust emissions that may for future subsonic and supersonic commercial Jetbe practicably achievable for future commercial aircraft are compared to measured or estimatedaircraft operating at high altitude cruise condi- emission levels for present-day aircraft. Thetions are compared to cruise emissions for present Department of Transportation organized the CiAPday aircraft. These results summarize Jet air- effort in order to determine the potential climaticcraft emissions studies reported in the Climatic effects of perturbations of the upper atmosphereImpact Assessment Program (CIAP) Monograph 2 causedby the propulsion effluents of a world-wide"Propulsion Effluents in the Stratosphere." The high-attitude aircraft fleet projected to the yearforecasts are based on: (1) knowledge of 1990 and beyond (ref. 1) The overall findings of
$ emission characteristics of combustors and the CIAP study reported in ref. 1 are based on the0ugmentors; (2) combustion research to emission analyses of data compiled in six monographs, which
5 reduction technology; and (3) trends in projected are to be published in mid-1975. These six mono-engine designs for advanced subsonic or supersonic graphs are entitled: (1) The Natural stratospherecommercial aircraft. Recent progress that has of 1974, (2) Propulsion Effluents In the Strato-been made in the evolution of emissions reduction sphere, (3) The Stratospphere Perturbed bytechnology will be discussed. Propulsion Effluents, (4) The Natural and Radio-
tively Perturbed Troposphere, (5) Impacts ofSummary Climatic Change on the Biosphere, and (6) Economic
and Social Measures of Biologic and Climatic Change.- Forecasts of engine exhaust emissions that may
i;n practicably achievable for future commercial Most of the cruise emission data for present-dayu.rcraft operating at high altitude cruise Jet aircraft that are presented herein were ex-conditions are compared to cruise emissions for tracted from Chapter 4 of Monograph 2 (ref. 2)present day aircraft. These results summarize Jet entitled "Emission Characteristics of Representativeaircraft emissions studies reported in the C11ma- Current Engines," The cruise emission predictionstic Impact Assessment Program (CIAP) Monograph 2 for future Jet aircraft were obtained from Chapter"Propulsion Effluents to thy. Stratosphere." The 5 of Monograph 2 (ref. 2) entitled "Forecast of Jetforecasts are based on: (1) knowledge of emission Engine Exhaust Emissions of High Altitude Commercialcharacteristics of combustors and augmentors; Aircraft Pro,,ijected to 1990" (also described in
.' (2) combustion research in emissions reduction met. 3 and 4).technology; and (3) trends in projected enginedesigns for advancer .ubsonic or supersonic Most of the discussion presented in this papercommercial aircraft, Most of the research related will pertain to the problem of reducing NOX sinceto cruise emissions is concerned with reducing the it is believed to be the most significant pollutantquantity of .nitrogen oxides emitted into the formed during high altitude cruise. The subjectstratosphere. Current subsonic and supersonic matter will be divided into the following sixcommercial aircraft produce as ouch as 20 grams of sections: (1) Cruise Emissions for Present-DayNO2 per kilogram of fuel burned during .cruise. Commercial Jet Aircraft, (2) NOx FormationExperimental combustors that have been designed to Considerations, (3) Effect of Operating Conditionsminimize emissions have achieved levels as tow as on NOX Emissions, (4) NO Reduction Research, (5)
^ommerclal6-8 gNO2/kg fuel at simulated cruise, while Design Trends for Future ;let Aircraftlaboratory burners have reached levels less than Engines, and (6) Forecasts of Future Cruise NOX1 9NO2/kg fuel. A substantial research and Emissions.development effort will be required to demonstrate
' the practicality of incorporating these low Cruise Emissions Foremissioncombustor concepts into actual engine Present-Day Conn—erc^l Jet'^Aircrafthardware and to determine the level to whichemissions may be reduced Without compromising The constituents present in the Jet engineengine performance aos 4 ,.IIity. exhaust include oxides of nitrogen (NGx), carbon
i monoxide, total hydrocarbons (unburned plus partiallyIntroduction oxidized hydrocarbons), soot (carbon), SOX (SO2
Plus S03 ), trace elements, carbon dioxide, andThis paper summarizes studies to forecast water. During cruise, the combustion efficiency of
future Jet aircraft exhaustemissions that were core engine combustors is very nearly 100 percent;performed in support of the Climatic Impact Assess- therefore carbon monoxide (CO) and total hydrocarbonment Program .(CIAP). Cruise emission predictions (THC) emissions are very small. Typical cruise
ORIGINAL' PAGE ISOF POOR QUALM
emission indices (grams of pollutant per kilogramof fuel burned) for CO and THC are 4 and 0,1,respectively. Future supersonic commercial air-craft that use augmentors during cruise would havesomewhat higher CO and THC emisslons. The CO andTHC emission indices for a future supersonic air-craft with augmentation during cruise might be ashigh as 30 and 10, respectively. Aside from thepossible need to reduce CO and THC emissions from
the cruise CO and THC emissions arered to be a significant problem with thehat techniques for reducing NOX mayncreasing carbon monoxide emissions.eoff exists between the desired emissionsNOx and CO (rof. 5). The quantity ofn) emitted during cruise is estimated to1 gC/kg fuel for current engines andgC/kg fuel for future engine designs.
Of all the constituents formed during highaltitude cruise, only NOx (NO plus NO2) and SOX(S02 plus S03) are considered to pose a seriousthreat to the global environment (ref, 1 and 6).Nitrogen oxides are of concern because of the ozonedepletion problem, and SOX is of concern because ofsulfate aerosol formation which may reduce solarradiation. The emission index for SOX , which isdirectly related to the amount of sulfur in thefuel, 1s currently about 1 9S0pplkg fuel. Lower SOXemissions may be accomplished by increasing thedegree of hydrodesulfurization at the refinery.
Cruise NOx emission indices from current sub-sonic and supersonic aircraft engines are presentedin Table 1. Available engine operating data arealso included to this Table. Most of these datawere obtained from Chapter 4, Monograph 2 (ref. 2).The data for the JT80 engine were obtained from(ref. 7), and the data for the CF6-50 engine wereobtained from (ref. 8).
In general, the NOx emission index increaseswith increasing crxnbustor inlet temperature andpressure as a result of higher compressor pressureratios and/or higher flight speeds, and approach amaximum value of about 18-20 9 NO /kg fuel for thelatest production engines for both subsonic andsupersonic aircraft. The 110, emission index isconventionally expressed as grams of NO 2 per kilo-gram of fuel burned even though most of the NO inthe exhaust is in the form of nitric oxide (N07.
NOx Formation Considerations
At full power conditions,. combustors operatewith high inlettemperatures, high inlet pressures,and high fuel-air ratios all of which contributeto the formation of NO X . The NOx formation rateincreases with increasing flame temperature, andflame temperature increases proportionately withincreases in combustor inlet temperature. Highercombustor inlet temperatures result from higherengine compressor pressure ratios or from higheraircraft flight speeds, particularly during super-sonic cruise. Nonuniform fuel distribution in theprimary zone also causes locally high flame tem-peratures.
ORIGINAL' PAGE 14
OF POOR QUALF1tYJ
The effect of flame temperature on the formationof NOx is illustrated in figure 1. These resultswere calculated for a combustor burning premixedfuel and air at an inlet temperature of about BOB K.a pressure of 6.5 aim. and a residence time of 2milliseconds. The formation of NOX increasesexponentially with increasing flame temperature.
Conventional combustors have average flametemperatures of the order of 2300 to 2500 K in theprimary zone since they are designed to operatenear an equivalence ratio of unity where nearmaximum flame temperatures occur. The equivalenceratio is the ratio of the average local fuel-airratio to the stoichiometric fuel-air ratio, whichis that required for complete combustion of thefuel. Because a conventional combustor operateswith a nonhomogeneous diffusion flame, the effectof average primary-zone flame temperature on NOxformation is not as strong as that shown in figure1. Although some reductions in NO could beachieved if flame temperature could be lowered byburning leaner fuel-air mixtures, a more effectiveapproach is to reduce locally high flame tempera-ture by improving fuel atomization and mixing. Thegreatest reductions in NOx, however, can be obtainedby using lean premixed, prevaporized fuel/airmixtures. The homogeneous fuel-air mixtures whichresult from premixing and prevaporizing arecharacterized by the strong effect of flame tempera-ture on NOX displayed in figuri 1.
Another cause of high NOx formation levels isexcessive residence time of combustion gases inthe primary zone. The formation of NOx tends to besomewhat linear with residence time over a limitedtime span for a primary zone equivalence ratio nearunity. This effect tends to be less significant asthe equivalence ratio is reduced. The residencetime in a combustion chamber is typically of theorder of about 2 to 4 milliseconds. However, theequilibrium value of NOx is not approached untilafter several seconds. and is much higher than thequantity of NOx which is actually formed within atypical combustion choa,ber. Residence time may bereduced by either increasing velocities in theprimary zone or by providing more rapid quenchingof the combustion products,
Effect of OperatingConditions on NOx Emissions
Combustor Operating Conditions
The combustor operating variables that influencethe formation of NOX include combustor inlettemperature, combustor inlet pressure, combustorreference velocity, combustor temperature rise,inlet fuel temperature, and inlet air humidity.The variation of flame temperature with combustorinlet temperature is very close to being linear.The NOx emission index was shown to increaseexponentially with flame temperature in figure 11therefore, as expected the NOx emission index altincreases exponentially with combustor inlettemperature. Different investigators (ref. 2,Chapter 4 and ref. 9) have correlated the NOX
Thus a tralevels forsoot (carbbe about 0about 0.02
emission indices (grams of pollutant per kilogramof fuel burned) fcr CO and THC are 4 and 0.1,respectively. Future supersonic commercial air-craft that use augmentors during cruise would havesomewhat higher CO and THC emissions. The CO andTHC emission Indices for a future supersonic air-craft with augmentation during cruise might be ashigh as 30 end 1', respectively. Asiae from thepossible nerd to reduce CO and THC emissions fromaugmentors, the cruise CO and THC emissions arenot considered to be a significant problem with theexception that techniques for reducinq NO x mayresult in increasing carbon monoxide emissions.Thus a tradeoff exists between the desired emissionslevels for NO, and CO (ref. 5). The quantity ofsoot (carbon) emitted during cruise is estimated tobe about 0.1 gC/kg fuel for current engines andabout 0.02 gC/kg fuel for future engine designs.
Of all the constituents formed during highaltitude cruise, only NO (NO plus NO2) andSOx(S02 plus S03) are considered to pose a seriousthreat to the global environment (ref, 1 and 6).Nitrogen oxides are of concern because of the ozonedepletion problem, and SOx is of concern because ofsulfate aerosol formation w0 ch may reduce solarradiation. The emission index for SOx, which isdirectly related to the amount of sulfur in thefuel, is currently about 1 9S02/kg fuel. Lower 50,emissions may be accomplished by increasing thedegree of hydrodesulfurization at the refinery.
Cruise NO x emission indices from current sub-sonic and supersonic aircraft engines are presentedin Table 1 Available engine operating data arealso included in this Table. Most of these datawere obtained from Chapter 4, Monograph 2 (ref. 2).The data for the JT80 engine were obtained from(ref. 7), and the data for the CF6-50 engine wereobtained from (ref. 8).
In general, the NO x emission index increaseswith increasing crrnbustor inlet temperature andpressure as a result of higher compressor pressureratios and/or higher flight speeds, and approach amaximum value of about 18-20 g NO2/kg fuel for thelatest production engines for both subsonic andsupersonic aircraft. The NO x emission index isconventionally expressed as grams of NO2 per kilo-gram of fuel burned even though most of the NO inthe exhaust is in the form of nitric oxide (NO1.
Formation Considerations
At full power conditions, combustors operatewi s h high inlet temperatures, high inlet pressures,and high fuel-air ratios - all of which contributeto the formation of NOx. The NO x formation rateincreases with increasing flame temperature, andflame temperature increases proportionately withincreases in combustor inlet temperature. Highercombustor inlet temperatures result from higherengine compressor pressure ratios or from higheraircraft flight speeds, particularly during super-sonic cruise. Nonuniform fuel distribution in theprimary zone also causes locally high flame tem-peratures.
ORIGINAL; PAQB M
OF POOR QUAI.I Y
The effect of flame temperature on the formationof NOx is illustrated in figure 1. 'hese resultswere calculated for a combuster burring premixedfuel and air at an inlet temperature of about 800 K,a pressure of 5.5 atm. and a residence time of 2milliseconds. The formation of 140x increasesexponentially with increasing flame temperature.
Conventional combustors have average flametemperatures of the order of 2300 to 2500 K In theprimary zone since they are designed to operatenear an equivalence ratio of unity where nearmiaximum flame temperatures occur. The equivalenceratio is the ratio of the average local fuel-airratio to the stoichiometric furl-air ratio, whichis that required for complete combustion of thefuel. Because a conventional combustor operateswith a nonhomogeneous diffusion flame, the effectof average primary-zone flame temperature on NOxformation is not as strong as that shown in figure1. Although some reductions in NO ^t could beachieved if flame temperature coulA be lowered byburning leaner fuel-air mixtures, a more effectiveapproach is to reduce locally high flame tempera-ture by improving fuel atomizatiun and mixing. Thegreatest reductions in NO x , however, can be obtainedby using lean premixed, prevaporized fuel/airmixtures. The homogeneous fuel-air mixtures whichresult from premixing and prevaporizing arecharacterized by the strong eff-ct of flame tempera-ture on NO x displayed in figurr 1,
Another cause of high NO x formation levels isexcessive residence time of combustion gases inthe primary zone. The formation of NO x tends to besomewhat linear with residence time over a limitedtime span for a primary zone equivalence ratio nearunity. This effect tends to be less significant asthe equivalence ratio is reduced. The residencetime in a combustion chamber is typically of theorder of about 2 to 4 milliseconds. However, theequilibrium value of NJ, is not approached untilafter several seconds and is much higher than thequantity of NO x which is actually formed within atypical combustion 0.5,,ier. Residence time may bereduced by either increasing velocities in theprimary zone or by providing more rapid quenchingof the combustion products.
Effect of OperatingConditions on NOx Emissions
Combustor Operating Conditions
The combustor operating variables that influencethe formation of NOx include combustor inlettemperature, combustor inlet pressure, combustorreference velocity, combustor temperature rise,inlet fuel temperature, and inlet air humidity.The variation of flame temperature with combustorinlet temperature is very close to being linear.The NOx emission index was shown to increaseexponentially with flame temperature in figure 1;therefore, as expected the NOx emission index altincreases exponentially with combustor inlettemperature. Different {nvestigators (ref. 2,Chapter 4 and ref. 9) hove correlated the NOx
a
F
)I
emission index with eT3/a where T. is the combustorinlet temperature and a is an empirically determinedconstant. These different investigators have usedvalues of the constant "a" from 169 to 288 K tocorrelate their experimental data, The broad spreadin this Wiled may be attributed to differences incombustor geometry and specifically to differencesin the rrimary zone equivalence ratio and thedegree of homogenity of the fue l -air mixture in theprimary zone,
Engine/Aircraft Operating Conditions
The engine/aircraft operating conditions thataffect the NOx emission index are compressorpressure ratio, turbine inlet temperature, flightMach numbar, and cruise altitude. These variablesinfluence the NOx emission index through theireffect oncombustor operating conditions. Thecombustor inlet temperature during cruise is afunction of the combined total temperature riseacross the inlet diffuser and thn compressor. Forsupersonic cruise, the total temperature rise acrossthe diffuser due to the ram pressure rise becomesquite significant. Thus, the NOx emission indexincreases with increases in compressor pressureratio and flight Mach number. The combustor inlettotal pressure increases with increasing compressorpressure ratio, increasing Mach number, anddecreasing altitude.
NOx Reduction Research
As discussed previously, the NOx emission indexmay be reduced by reducing the flame temperatureand residence time in the primary zone. Over thelast several years, a great deal of research hasbeer, conducted by various government agencies andengine manufacturers to evolve techniques forreducing the formation of NOx in gas turbinecombustors. No attempt will be made herein tosurvey the literature on this subject. Instead,several recent fundamental and applied researchprograms will be summarized that are indicative ofthe statu q of technology for reducing NOx emissionsduring truise..
ORIGINAL PAGE IS
OF POOR QTJAL1rIYI
Fundamental Fuel-Lean Combustion Experiments
Conventional combustors are designed to burn anear stoichiometric mixture of fuel and air at fullpower operating conditions. The reduction of NOxby lowering the primary zone flame temperature maybe approached by burning leaner fuel-air mixtures.Lean burning is most effective when local hightemperature zones are eliminated by the use of ahomogeneous fuel-air mixture. This can be obtainedby premixing a prevaporized fuel upstream of thecombustor.
Experiments have been conducted at NASA LewisResearch Center to determine too minimum level towhich NOx could be reduced to do idealized fuel-leanpremixing-prevaporizing burner. Testing hasbeen performed in the laboratory flame-tubeapparatus shown schematically in figure 2. Gaseouspropane or atomized Jet-A is Injected upstream of aperforated-plate flame holder with sufficieftdistance to provide a completely prevaporizsd/premixed fuel-air mixture to the primary erne(flame zone) test section, Exhaust gas saxrples canbe extracted at varying distances downstream of theflame holder to Insure that combustion is completedat the sample measurement position. Some of theresults obtained to date are presented in figure 3where the emission index of NOx is plotted as afunction of equivalence ratio, These data whichwere obtained at an inlet temperature of 800 K, apressure of 5.5 atm., and a residence time of 2milliseconds are compared with a theoretical plotcalculated from a well-stirred reactor model.Extremely low values of NOx(<I g/kg) were obtainedat the very lean equivalence ratios (0.5). Thegood agreement with the well-stirred reactor modelpredictions indicates that good premixing wasobtained. All data shown were taken with combustionefficiencies greater than 9U; however, the lowestvalues were obtained at the edre of the combustionflammability limits and any slight perturbation inflow caused combustion blowout. Because of thisstability sensitivity, these results are consideredto be near the fundamental lower limit of NOxemissions for the type of experimental hardwareused in this investigation. It is important tonote that the operating conditions for this experi-ment were very carefully controlled and do notnecessarily duplicate conditions in an actualengine except for the levels of inlet pressure andtemperature which simul,tie a typical supersoniccruise condition.
By moving the gas sa pling probe axially (fig, 2),it was possible to detenine the effect of residencetime on NO emissions. the results plotted infigure 4 Now that at 0.` equivalence ratio, areduction in residence tine from 3 msec to 2 msecgives a 43% decrease in NOx. At 0.4 equivalenceratio, however, the same reduction in residencetime gives only a very small drop in NOx, Thus,residence time becomes less important to NOxformat n as equivalence ratio is decreased to verylean values, This is due to lower rates of NOxformation at lean conditions.
In general, most investigators have determinedthat tl,e NOx emission index varies directly withthe square root of pressure. The NOx emi_:ionindex tends to be inversely proportional to thecombustor reference Velocity since referencevelocity is inversely proportional %.a primary zoneresidence time. In conventional combustors, theNOx emission index increases directly with the
• temperature rise across the combustor (over-allequivalence ratio). 'he effect of fuel temperatureand inlet air humidity are discussed in ref. 10 and9, respectively. Reference 10 observed that theNo, emission index increases at a rate of 6 percentper 100 K increase in fuel temperature. The NOxemission index was shown to increase with decreas-ing inlet it humidity at a constant exponentialrate of e l^N (where H is humidity, 9H2O/g dry air)in (raf. 9). The humidity of the atmosphere atcruise altitudes is essentially zero.
.d
emission index with aT7/a where T 3 is the combustorinlet temperature and a is an empirically determinedconstant. These differert investigators have usedvalues of tie constant - d from 169 to 288 K tocorrelate 'heir experimental data. The broad spreadin this value may be attributed to differences incombustor geometry and specifically to differencesin the irimary zone equivalence ratio and thedegree of homogenity of the fue l -air mixture in theprimary Zone.
In general, most investigators have determinedthat t.e NO x emission index varies directly withthe square root of pressure. The NOx emi._iunindex tends to be inversely proportional to thecombustor reference velocity since referencevelocity is inversely proportional .o primary zoneresidence time. in conventional combustors, theNOx emission index increases directly with thetemperature r'se acros, the combustor (over-allequivalence ratio). 'he effect of fuel temperatureand inlet air humid':y are discussed in ref. 10 and9, rekpectively. Reference 10 observed that theNO -emission inde •, increases at a rate of 6 percentpe, 100 K increase in fuel temperature. The NOxemission index was shown to increase with decreas-ing inlet afr humidity at a constant exponentialrate of e 19N (where H is humidity, gH2O/q dry air)in (ref. 9). The humidity of the atmosphere atcruise altitudes is essentially zeru.
Engine/Aircraft Operating Conditions
The engine/aircraft operating conditions thata,fect the NO x emission index are compressorpressure ratio, turbine inlet temperature, flightMazh numb?r, and cruise altitude. These variablesinfluence the NO x emission index through theireffect on combustor operating conditions. Thecombustor inlet temperature during cruise is afunction of the combined total temperature riseacross the inlet diffuser and tho compressor. forsupersonic cruise, the total temperature rise acrossthe diffuser due to the ram pressure rise becomesquite significant. Thus, the NOx emission indexincreases with increases in compressor pressureratio and flight Mach number. The combustor inlettotal pressure increases with increasing compressorpressure ratio, increasing Mach number, anddecreasing altitude.
NOx Reduction Research
As discussed previously, the NOx emission indexmay be reduced by reducing the flame temperatureai.d residence time in the primary zone. Over thelast several years, a great deal of research hasbeer. conducted by various government agencies andengine manufacturers to evolve techniques forreducing the formation of NO x in gas turbinecombustors. No attempt wi'i be made ki_rein tosurvey the literature on this subject. Instead,several recert fv-(,,mental and applied researchprograms will be summarized that are indicative ofthe statue if technology for reducing K9 x emissionsduring ..i' P.
ORIGWAL PAGE ISOF POOR QUALITYi
fundamental Fuel-lean Combustion lx eerriments
Conventional combustors are designed to burn anear stoichtometric mixture of fuel and air at fullpower operating conditions. The reduction of hJxby lowering the primary zone flame temperature maybe approached by burning leaner fuel-air mixtures.Lean burning is most effective when local hightemperature zones are eliminated by the use of ahomogeneous fuel-air mixture. This can be obtainedby premixing a prevaporized fuel upstream of thecombustor.
Experiments have been conducted at NASA LewisResearch Center to determine the minimum level towhich NOx could be reduced in an idealized fuel-lean premixing-prevaporizing burner. Testing hasbeen performed in the laboratory flame-tubeapparatus shown schematically in figure 2. Gaseouspropane or atomized Jet -A is injected upstream of aperforated-plate flame holder with suffi,-iertdistance to provide a completely prevaporir,d/premixed fuel-air mixture to the primary aline(flame zone) test section. Exhaust qas sa,i.ples canbe extracted at varying distances downstream of theflame holder to insure that combustion is completedat the sample measurement position. Some of theresults obtained to date are presented in fiqure 3where the emission index of NOx is plotted as afunction of equivalence ratio. These data whichwere obtained at an inlet tem perature of 800 K. apressure of 5.5 atm., and a residence time of 2milliseconds are compare9 with a theoretical plotcalculated from a well-stirred reactor mudel.Extremely low values of NOx(<l g/kg) were obtainedat the very lean equivalence ratios Thegood agreement with the well-stirred reactor modelpredictions indicates that good premixing wasobtained. All data shown were taken with combustionefficiencies greater than 99 , however, the lowestvalues were obtained at the edce of the combustionflammability limits and any slight perturbation inflow caused combustion blowout. Because of thisstability sensitivity, these re:alts are consideredto be near the fundamental lower limit of NOxemissions for the type of experimental hardwareused in this investigation. It is important tonote that the operating conditions for this experi-ment were very ca refully controlled and do notnecessarily duplicate conditions in an actualengine except for the ; ,svels of inlet pressure andtemperature which simul e a tvric.al supersoniccruise condition.
By moving the gas sa oling probe axially (fig. 2),it was possible to dete nine the effect of residencetime on NO emissions. 1he results plotted infigure 4 s9ow that at 0.' equivalence ratio, areduction in residence tine from 3 msec to 2 msecgives a 431 decrease in NOx. At 0.4 equivalenceratio, however, the same reduction in residencetime gives only a very small drop in NOx. Thus,residence time becomes less important to NOxformat n as equivalence ratio is decreased to verylean values. This is due to lower rates of NOxformation at lean conditions.
In considering residence time reduction for NOxcontrol it is important to determine the minimumresidence time required fur good comoustionefficiency. Bands of constant combustion effici-ency a 3e shown in figure 4, The shaded arearepresents combustion efficiency of 99 to 99.7%.The cross-hatched portion represents combustionefficiencies of less than 99%. Data which fell inthe unshaded portion of the graph have combustionefficiency greater than 99.7%. The results ofthese experiments show that for good combustionefficiency, less NOx was produced with very lean
equivalence ratios and long residence times thanat somewhat higher equivalence ratios and shorttimes. More details of this experiment are givenin (ref. 11). A similar experiment with similarresults is being conducted under a NASA contractwich General Applied science Laboratories (ref. 12).
Another evaluation of the premix technique isbeing conducted under NASA contract to the SolarDivision of international Harvester using "quasi-combustor" type tubular test hardware, figure S.The concepts shown in figure 5 represent twodifferent approaches to acliiev p veriv lean combus-tion using premixed fuel-air. is 'Jet-InducedCirculation Combustor" concept uses ,lets of pre-mixed fuel-air to create a large recirculation ofhot gases into the flame zone which aids in main-taining combustion stability at very low equivalenceratios. The "Vortex Air Blast Combustor" conceptuses a rotati..g flow field to create a similareffect. These combustors differ from the flametube apparatus (fig. 2) in that no attempt has beenmade to completely vaporize the fu:) upstream ofthe combustor. Even though the fuel enteringthese combustors is not completely prevaporized orpremixed, preliminary results have been encouraging,and preliminary NOx emission data for the "VortexAir Blast Combustor" concept have approached thelow levels obtained in the flame-tube apparatus(fig. 2) at simulated supersonic cruise conditions.
The minimum NO level of premixed gas phase,ombustion is limited by the lean flammabilityemit (minimum flame temperature). Even lower NOxissions might be obtained if burning with lower
f , ,me temperatures could be achieved by means ofcatalytic combustion.
1 a potential application of catalytic reactorsto ti design of a low emission aircraft gas tur-bine . m6ustor is discussed in Chapter 3, Monograph2 (ref 2). Preliminary results from the evalua-tion of catalytic reactor in a labr° s tory burner
using JP , 1 fuel (ref. 13) indicated that near 100percent ce.,ibustion efficiency could be attainedwith negligible NOx formation over a limited rangeof operating conditions. At an inlet temperatureof 650 K, pressure of 7 atm, reference velocity of13,7 m/sec, and fuel-air ratio of 0.0212 (exittemperature of about 1370 K), a NOx emission indexof less than 0.1 91102/kg was observed (less than2 ppm, which is about the level of the measurementerror of the gas analysis system). At thisoperating condition, the emission indices for COand total hydrocarbons were about 1,5 and 0.3 g/kg,
ORIGINAL PAGE ISft POOR RIiAL=
respectively. Combustion efficiency decreasedmarkedly as either fuel-air ratio was loweredbelow a value of about 0.02 or inlet temperaturewas reduced below a value of about 650 K. At aninlet temperature, above 650 K, exit temperaturesup to 1600 K were achieved with good efficiency.Temperatures above 1600 K were avoided to preventdamage to the catalytic reactor. Reactor pressuredrop was about 1% of the static inlet pressurrat 13.7 m/s reference velocity and 1600 K exittemperature,
Although these initial experimental results arequite encouraging, extensive research is requiredto establish the feasibility of developing acatalytic combustor for an aircraft gas turbineengine. Methods for obtaining complete combustionovor a wider range of operating conditions must beexplored. This might be achieved by either s-singa combination of catalyst with different operas"ngcharacteristics or by evolving a hybrid two staqecombustor consisting of a catalytic reactor anda more conventional flame stabilizer. Catalystsmust be developed that are insensitive to poisoningor deactivation in the environment of the gasturbine combustor. Substrates and methods forbonding catalyst on these substrates must bedeveloped that will insurn reliable mechanicalintegrity against thermal and vibrational stresses.Methods to prevent spelling of either catalyst orceramic substrates must be evolved to avoiddeterioration of the combustor or foreign objectdamage to the turbine. Fuel preparation (premix-ing-prevaporization) designs must be evolved thatprovide uniform fuel-air mixtures to thecatalytic reactor that avoid preignition or flamepropagation (flashback) problems. Catalyct andsubstrate materials and structures must Sedeveloped that demonstrate both good performanceand durability at higher operating temperatures.All of these areasmust be investigated ingreater detail before an honest Judgment can bemade regarding the practicality of developing acatalytic combustor for an aircraft bos turbineengine.
Applied Low Pollutant Combustor Resea rch
The fundamental law-NOx combustion conceptsdescribed in the previous section have not reachedthe state of developmant where they have beentested it properly scaled combustor hardware norhave they been evaluated for performance anddurability over the entire range of requiredoperating conditions. As a matter of fact,practical combustor designs incorporating theseconcepts will require either more than one stageof combustion or variable geometry for control ofairflow and fuel flow in order to permit satisfac-tory performance at both low and full-powerconditions. This section will briefly describetest programs being conducted in a more advancedstate of real combustor hardware.
Multizone combustors. A large part of theeffort on t e eve uat on of low pollutant emissioncombustors conducted in-house by NASA has been
P,
1
with the swirl-can-mod oar combustor shown infigure 6. Figure 6(a) is a photograph of a full-annular array of 120 swirl can modules arranged inthree radial rows. A cross-sectional view of thiscombustor is shown in figure 6(b) and the componentof the swirl can module are illustrated in figure6(c). Each module is composed of a carburetor cup,swirler, and flame stabilizer. Fuel is injectedinto the carburetorcup where it premixes with airflowing through the cup and then passes through aswirler into the wake created by the flame stabi-lizer which acts as a quasi-bluff body to the airflowing around the module. The swirling fuel-airmixture provides for a small stable flame zone inthe stabilizer wake. The combination of a smallflame zone and premixed fuel-air provides for lowresidence times and some degree of gas temperaturecontrol in the flame zone.
The quantitative NOx reductions achievable withthe swirl-can combustor are shown in figure 7. NOxemission indices for a swirl-can combustor arecompared to a more conventional single annularcombustor and a double annular combustor at varyinginlet air temperatures. These combustors weretested at 6 atm pressure and an exit temperature of1500 K. Compared to a conventional combustor, two-fold reductions in NOx are achievable with theswirl-can. The double annular combustor data alsopresented 1s from an advanced experimental designwhich contains 64 fuel nozzles arranged on twoannuli. This is slightl y greater than twice thenumber of fuel nozzles contained in a conventionallarge annular combustor. Superior fuel and airmanagement resulting from this arrangement producesdecreased levels of 110x.
Ex erlment_al clean combustor ram. The goal
of this NASA i.ew s contract program s to developand demonstrr,te technology to decrease pollutant.emissions 'ram modern aircraft turbine engines.This technology is mainly applicable to high bypassratio turbofans for advanced wide-body subsonic ,jetaircraft. .However, the combustor technologyevolved in this program is also applicable toengines for supersonic aircraft. NASA Lewis hasawarded contracts for this program to GeneralElectric and Pratt d Whitney. Each contract effortis being conducted in three separate phases. Thefirst phase involved the evaluation of variouscandidate low-pollutant combustor concepts. Thesecond phase consists of refining the more promisingconcepts evolved during the first phase, and thethird phase consists of an actual demonstration ofthe more promising low pollutant combustors in astate-of-the-art engine. The Phase I effortincluded evaluation tests at simulated supersoniccruise operating conditions.
Phase I of the Experimental Clean CombustorProgram has been completed. The combustor con-figurations tested in Phase I were mainly ,fudged bytheir idle and takeoff emissions. Several of thecombustor configurations either achieved or closelyapproached the idle emissions (CO and total hydro-carbons) goals. Significant reductions in NOx werealso achieved; however, all fell short of the NOx
ORIGINAL' PAGE ISOF POOR QUALUT
emission goal at takeoff. The more promisingcombustors achieved a NOx emission index atsimulated takeoff of about 15 9HO /kg fuelcompared to a value of about 13 g gi02/kg fuel thatis required to meet EPA emission standards. TheseEPA emission stardards which are described in (ref.14) are applicable only to low altitude flightoperations (below 91F meters). Current productionvalues for these engines during takeoff are about36 uNO21kg fuel. The best NOx emission indicesobserved during Phase I at either simulated sub-sonic or supersonic cruise were of the order of6-8 91102/kg fuel.
The primary objectives of the Phase II effortwill be to improve the overall performance anddurability of the more promising combustor con-figurations without sacrificing the improvedemission characteristics demonstrated in Phase Iand to assess engine compatibility of these combus-tors. Specific attention will be directed toimproving combustor exit temperature distribution,reducing pollutants further at ail engine operatingconditions including intermediate power settingsand improving altitude relight characteristics.Each contractor 1s currently conducting Phase IItesting and each is evaluating two combustor designs.
The two advanced technology CF6 engine combustorconfigurations being evaluated in Phase II areshown along with the standard CF6-50 combustor infigure B. Both designs utilize the concept offuel scheduling for reducing idle pollutantemissions. The pilot stages of both the radial/axial staged and the uouble annular are optimizedfor high efficiency (low CO s THC emissions) atengine idle fuel-air ratios. The main stages areoptimized for lean combustion (low NOx) at full-power fuel-air ratios. various combinations offuel scheduling can be used for off-designoperation such as approach and climb out powersettings. The radial/axial staged configurationutilities a premixed lue' sir approach in the mainstage whereas the 6 n^ ^,anular configuration usesan air-blast type n, to obtain lean combustionin the main stage.
The two advanced technology JT90 engine combustorconfigurations being evaluated in Phase 11 areshown along with the standard JT90 combustor infigure 9. As with the CF6 configurations bothdesigns use fuel scheduling as the principalapproach to controlling idle pollutant emissions.Optimization of the individual stages at idle andfull power conditions is used for overall emissioncontrol. The hybrid configuration utiliz^s aparallel (radial) fuel staging approach witn apremix technique in the pilot stage and a variationof the swirl can concept in the main stage. Thisconfiguration is an attempt to mate the lowest COS THC emission design (premix pilot stage) and thelowest NOx emission design (swirl-can-module stage)that was tested in Phase I. The vorbix configura-tion utilizes aseries-type (axial) fuel stagingapproach with standard type pressure atomizingfuel nozzles in the pilot and main stages. Themain stage has high intensity swirlers immediately
with the swirl-C•n-mod or combustor shown infigure 6. Figure 6(a) is a photograph of a full-annular array of 120 swirl can modules arranged inthree radial rows. A cross -sectional view of thiscombustor is shown in figure 6(b) and the component*of the swirl can modulo are illustrated in figure6(c). Each module is composed of a carburetor cup,twirler, and flame stabilizer. Fuel is injectedinto the carburetor cup where it premixes with airflowing through the cup end then passes through aswirler into the wake created by the flame stabi-lizer which acts as a quasi-bluff body in the airflowing around the module. The swirling fuel.-airmixture provides for a small stable flame zone inthe stabilizer wake. The combination of a smallflame zone and premixed fuel-ai r provides for lowresidence times and some degree of gas temperaturecontrol in the flame zone.
The quantitative NOx reductions achievable withthe swirl-can combustor are shown in figure 1. NOxemission indices for a swirl-can combustor arecompared to a more conventional single annularcombustor and a double annular combustor at varyinginlet air temperatures. These combustors weretested at 6 atm pressure and an exit temperature of1500 K. Compared to a conventional combustor, two-fold reductions in NO x are achievable with theswirl-can. The double annular combustor data alsopresented is from an advanced experimental designwhich contains 64 fuel nozzles arranged on twoannuli, This is slightiv greater than twice thenumber of fuel nozzles contained in a conventionallarge annular combustor. Superior fuel and airmanagement resulting from this arrangement producesdecreased levels of NOx.
Experiment±) clean combustor pr ogram, The goalof t ih's NASA ew s con—( tract program is to developand demonstr.te technology to decrease pollutantem issions 'rom modern aircraft turbine engines.This technology is mainly applicable to high bypassratio turbofans for advanced wide-body subsonic ,letai rcraft. However, the combustor technologyevolved in this program is also applicable toengines for supersonic aircraft. NA:;A Lewis hasawarded contracts for this program to GeneralElectric and Pratt 6 Whitney. Each contract effortis being conducted in three separate phases. Thefirst phase involved the evaluation of variouscandidate low-pollutant combustor concepts. Thesecond phase consists of refining the more promisingconcepts evolved during the first phase, and thethird ph ase consists of an actual demonstration ofthe more promising low pollutant combustors in astate-of-the-art engine. The Phase 1 effortincluded evaluation tests at simulated supersoniccruise operating conditions.
Phase I of the Experimental Clean CombustorProgram has been completed. The combustor con-figurations tested in Phase I were vainly fudged bytheir idle and takeoff emissions. Several of thecombustor configurations either achieved or closelyapproached the idle emissions (CO and total hydro-carbons) goals, Significant reductions in NOx werealso achieved; however, all fell short of the NO,
ORIGINAL; PAGE LSOF POOR Q UAI ny
emission goal at takeoff. The more promisingcombustors achieved a NOx emission index 1tSimulated takeoff of about 15 940 /kg fuelcompared to a value of about 13 g102/kg Nei thatis required to meet EPA emission standards. TheseEPA emission standards which are described in (ref,14) are applicable only to low altitude flightoperations (below 91 6 meters). Current productionvalues for these engines during takeoff are about36 JNO2 /k9 fuel. The best NOx emission indicesobserved during Phase 1 at either simulated sub-sonic or supersonic cruise were of the order of6-8 gNO 2 /kg fuel.
The primary objectives of the Phase 11 effortwill be to improve the overall performance anddurability of the more promising combustor con-figurations without sacrificing the improvedemission characteristics demonstrated in Phase 1and to assess engine compatibility of these combus-tors. Specific attention will be directed toimproving combustor exit temperature distribution.reducing pol l utants further at all engine operatingconditions including intermediate power settingsand improving altitude relight characteristics.Each contractor is currently conducting Phase 11testing and rach is evaluating two combustor designs.
The two advanced technology CF6 engine combustorconfigurations being evaluated in Phase 11 areshown along with the stand, rl CF6-50 combustor infigure 8. Both designs utilize the concept offuel scheduling for reducing idle pollutantemissions. The pilot ,tages of both the radial/axial staged and the uouble annular are optimizedfor high efficiency (low CO b THC emissions) atengine idle fuel-air ratios, The main stages areoptimized for lean combustion (low NO x ) at full-power fuel-air ratios. Various combinations offuel scheduling can be used for off-designoperation such as approach and climb out powersettings. The radial/axial staged configurationutili-es a premixed ,r dr approach in the mainstage whereas the r aular configuration usesan air-blast type i 3 obtain lean combustionin the main stage.
The two advanced technology JT9D engine combustorconfigurations being evaluated in Phase 11 areshown along with the standard JT90 combustor infigure 9. As with the CF6 configurations bothdesigns use fuel scheduling as the principalapproach to controlling idle pollutant emissions.Optimization of the indiviJual stages at idle andfull power conditions 's used for overall emissioncontrol. The hybrid configuration utiliz^s aparallel (radial) fuel staging approach witn apremix technique in the pilot stage and a variationof the swirl can concept in the main stage. Thisconfiguration is an attempt to mate the lowest COb THC emission design (premix pilot stage) and thelowest NO, emission design (swirl-can-module stage)that was tested in Phase 1. The vorbix con f igura-tion ut!lizes a series-type (axial) fuel stagingapproach with standard type pressure atomizingfuel nozzles in the pilot and main stages. Themain stage has high intensity swirlers immediately
z
downstream of the fuel injection point to promotevery intense, rapid mixing of the fuel and air inthe flame zone, The combination of the intensemixing and hot gases exiting from the pilot stageallow lean operation in the main stage and alsoreduce residence time due to quick quenching of thehot gases. A more detailed description of thisprogram is presented in (ref.. 15 and 16).
Design Trends for FutureCommercial Jet AircraftaftEngines_
Jet aircraft emission forecasts were based onengine design projections described in CIAPMonograph 2, Chapter 5 (ref. 2) and (ref. 4), itis predicted that future commercial subsonic jetaircraft will be equipped primarily with advancedhigh-bypass turbofan engines and that either aduct-burning turbofan or, a nonaugmented (dry)turbojet could be candidates for a future advancedsupersonic transport,
Fuels
Conventional JPor hydrocarbon type fuels willprobably be the only fuel used by commercial jetaircraft until far beyond 1990. Alternate sourcesof jet fuel such as shale oil and coal syncrudesmay become available sometime after 1985. These"synthetic" crudes generally conWn more nitrogen,oxygen, and sulfur, and less hydrogen than crude
Phowever, the undesirable compounds of
nitrogen, oxygen, andsulfur may be removed and thepercent hydrogen increased by means of varioushydrotreating and hydrocracking processes. Higherorganic nitrogen concentrations than are currentlypresent in jet fuel must be avoided because 50 to90 percent of this fuel bound nitrogen may heconverted to nitric oxide during combustion (ref.17). In princi ple, jet fuels could be producedfrom shale oil or coal syncrudes that simulate, inall important respects, those presently derivedfrom petroleum. If so, the cruise emissioncharacteristics of jet fuel derived from shale oilor coal syncrudes would not be expected to differgreatly from fuel derived from petroleum. Emissionforecasts for substitute fuels such as liquifiedhydrogen or liquified natural gas (LNG) have alsobeen included in Monograph 2, Chapter 5 (ref. 2),but it is unlikely that these fuels would be useduntil far beyond the late 1990'si therefore, thediscussion in this report is limited to the JP-fueled aircraft.
Future Subso;dc Aircraft Engines
Production or growth versions of aircraft suchas the Boeing 747, McDonnell Douglas OC-10, andLockheed 1011 will probably be in service until atleast 1990. En Ines for these aircraft (CF-6,JT9D, and R0211) manufactured after 1978 willrequire modifications in order to meet EPA emissionstandards (. ref. 14). Advanced high-bypass turbo-fan engines utilizing low emission combustortechnology could be incorporated into these aircraftbetween 1980 and 1985 if they are available. AnAdvanced Technology Transport (ATT) utilizing an
ORIGINAL' PAGE ISOI' POOR QUAI, TX
advanced high-bypass turbofan engine could beoperational in the early 1990's.
Projected values for the overall compressorpressure ratio for advanced high-bypass turbofanengines range from about 25 to 40. Compared to thelatest production engines for subsonic commercialJet aircraft (Table I), these advanced engineswould have combustor inlet temperatures andpressures, and exit temperatures ranging fromcurrent to higher values. Combustors for theseadvanced engines could be required, during cruise,to operate with inlet temperatures as high as about800K, with inlet pressures as high as about 15atm., and with exit temperatures as high as 1600 -1700 K.
Future Supersonic Alrcraft.Engines
The Concorde and Tupolev TU-144 or growthversions of these aircraft will probably continueto be in service during the 1980 to 1990 timeperiod; however, advanced supersonic transports ofgreater size A"d range would not be expected toenter service before 1990, The engine selectionfor an advanced supersonic transport will beinfluenced significantly by noise constraints.Either a duct-burning turbofan or a nonaugmented(dry) turbojet could be considered as a candidatefor this application, An .advanced supersonictransport would be expected to cruise within aMach number range of 2.2 to 2.7.
Duct-burning turbofan or dry turbojet engineswith projected overall pressure ratios (SLTO)ranging from about 10 to 25 are predicted forfuture commercial supersonic aircraft. Compared tothe operating conditions for the Concorde's Olympus593 engines (Table 1), these advanced engineswould have combustor Inlet temperatures andpressures, and exit temperatures ranging fromcurrent to higher values. Combustors for theseadvanced engines could be required, during cruise,to operate with inlet temperatures as high as900 - 1000 K, with inlet pressures as high as10 - 15 atm., and with exit temperatures as highas 1600 - 1800 K.
Forecasts of Future Cruise NO, Emissions
Recommendations for future cruise NOx reductionsare presented in references 1. and 6 based on theresults of the various CIAP studies. Reductionsin the cruise IIOx emission index for currentengines of anywhere from six to sixty-fold arerecommended. The actual reductions required would
dependent on the future size of the aircraftfleet, cruise altitudes and the amount of cruisetime in the stratosphere. Supersonic cruise wouldhe entirely within the stratosphere; however, sub-sonic cruise in the stratosphere would occur onlyfor a portion of the flight envalupe. Therefore,NOx reductions necessary for futu re subsonic air-craft are very dependent on the percent of theirflight envelope that occurs within the stratosphere.
1
In
downstream of the fuel injection point to promotevery intense, rapid mixing of the fuel and air inthe flame zone. The combination of the intensemixing and hot gases exitiny from the pilot stageallow lean operation in the main stage and alsoreduce residence time due to quick quenching of thehot gases. A more detailed description of thisprogram is presented in (ref. 15 and 16).
Desi gn Trends for Futurecomment a et rcra t homes
Jet aircraft emission forecasts were based onengine design projections described in CLAPMonograph 2, Chapter 5 (ref. 2) and (ref. 4). Itis predicted that future commercial subsonic jetaircraft will be equipped primarily with advancedhigh-bypass turbo fan engines and that either adcct-burning turbofan or a nonaugmented (dry)turbojet could be candidates for a future advancedsupersonic transport.
Fuels
Conventional JP or hydrocarbon type fuels willprobably be the only fuel used by commercial jetaircraft until far beyond 1990. Alternate sourcesof jet fuel such as shale oil and coal syncrudesmay become available sometime after 1985. These.'synthetic" crudes generally con W n more nitrogen,oxygen, and sulfur, and less hydrogen than crudepetroleum. However, the undesirable compounds ofnitrogen, oxygen, and sulfur may be removed and thepercent hydrogen increased by means of varioushydrotreating and hydrocracking processes. Higherorganic nitrogen concentrations than are currentlypresent in jet fuel must be avoided because 50 to90 percent of thi. fuel bound nitrogen may beconverted to nitric oxide during combustion (ref.17). In princiole, jet fuels could be producedfrom shale oil or coal syncrudes that simulate, inall important respects, those presently derivedfrom petro i o:um. 11 so, the cruise emissioncharacteristics of jet fuel derived from shale oilor coal syncrudes would not be expected to differgreatly from fuel derived from petroleum. Emissionforecasts for substitute fuels such as liquifiedhydrogen or liquified natural gas (LNG) have alsobeen included in Monograph 2, Chapter 5 (ref. 2),but it is unlikely that these fuels would be useduntil far beyond the late 1990's; therefore, thediscussion in this report is limited to the JP-fueled aircraft.
Future Subso.'.c Aircraft Engines,
Production or growth versions of aircraft suchas the Boeing 747, McDonnell Douglas DC-10, dndLockheed 1011 will probably be in service until atleast 1990. Engines for these aircraft (CF-6.JT9D, and R8211) manufactured after 1978 willrequire modifications in order to meet EPA emissionstandards (ref. 14). Advanced high-bypass turbo-fan engines utilizing low emission combustortechnology could be incorporated into these aircraftbetween 1980 and 1985 if they are available. AnAdvanced Technology Transport (VT) utilizing an
ORIGINAL; PAGE ISOF PooR QUALav
advanced high-bypass turbofan engine could beoperational in the early 1990'%.
Projected values for the overall compressorpressure ratio for advanced high-bypass turbofanengines range from about 25 to 40. CxWared to thelatest production engines for subsonic commercialjet aircraft (Table 1), these advanced engineswould have combustor inlet temperatures andpressures. and exit temperatures ranging fromcurrent to higher values. Combustors for theseadvanced engines could be required, during cruise.to operate with inlet temperatures as high as about800 K. with inlet pressures at high as about 15atm., and with exit temperatures as high as 1600 -1700 K.
Future Supersonic Aircraft Engines
The Concorde and Tupolev TU-144 or growthversions of these aircraft will probably continueto be in service during the 1980 to 1990 timeperiod; however, advanced supersonic transports ofgreater size o-A range would not be expected toenter service uefore 1990. The engine selectionfor an advanced supersonic transport will beinfluenced significantly by noise constraints.Either a duct-burning turbofan or a nonaugmented(dry) turbojet could be considered as d candidatefor this application. An advanced supersonictransport would be expected to cruise within aMach number range of 2.2 to 2.7.
Duct-burning turbofan or dry turbojet engineswith projected overall pressure ratios (SLTO)ranging from about 10 to 25 are predicted forfuture commercial supersonic aircraft. Compared tothe operating conditions for the Concorde's Olympus593 engines (Table 1), these advanced engineswould have combustor inlet temperatures andpressures. and exit temperatures ranging fromcurrent to higher values. Combustors for theseadvanced engines could be required. during cruise,to operate with inlet temperatures as high as900 - 1000 K, with inlet pressures as high as10 - 15 atm., and with exit temperatures as highas 1600 - 1800 K.
Forecasts of Future Cruise NO, Emissions
Recommendations for future cruise NOx reductionsare presented in references 1 and 6 based on theresults of the various C1AP studies. Reductionsin the cruise NO, emission index for currentengines of anywhere from six to sixty-fold arerecommended. The actual reductions required would
dependent on the future size of the aircraftfleet, cruise altitudes and the amount of cruisetime in the stratosphere. Supersonic cruise wouldbe entirely within the stratosphere; however. sub-sonic cruise in the stratosphere would occur onlyfor a portion of the flight envelope. Therefore,NOx reductions necessary for Tutu-e subsonic air-craft are very dependent on the percent of theirflight envelope that occurs within the stratosphere.
Protected Low-Emission Combustor Technolo
The projected combustor technology that might beutilized in the advanced propulsion systems de-scribed in the previous section are based on aprojection of the emission reduction technologydiscussedin reference 2 (Chapter 5). The evolutiorof low emission combustor technology available bythe 1980 to 1985 time period will be motivated bythe need to meet the 1979 EPA emission standards(ref. '4), These emission standards presentlypertai„ only to subsonic aircraft; however,additional standards for supersonic aircraft hakealso been proposed. These standards are currentlyonly applicable to aircraft opperations below 915meters (landing-takeoff cyclo). Many of theconeep`,s being investigated to reduce NOx for theproposed
ctiveE in reducingtNOxoduringlcruise. also to
Research programs such as the NASA "ExperimentalClean Combustor Program" described previously areapplying low emission technology to combustorredesign. The representative engine manufacturersare also engaged in independent research effortsaimed at the development of low emission combustorsthat would comply with the proposed EPA standards.The type of low-pollutant combustor hardware beingevolved and evaluated in research efforts such asthe NASA "Experimental Clean Combustor Program"are probably representative of the level oftechnology that could be available within the nextdecade. More optimistic predictions of the levelof low-pollutant combustor technology that mightbe available 'n the future are predicated on theconversion of fundamental concepts such as fuel-leanpremixing-prevaporizing burners into practicalcombustor designs.
Cruise NO, Emission Index Forecast
The NOx emission indices listed in Table I forcurrent commercial jet aircraft would be character-istic of existing production engines manufacturedprior to 1979 that would continue to be in serviceduring the 1980 to 1985 time period. These engineswould not be required to meet the proposed 1979 EPAemission limits since the regulation would onlyapply to engines either manufactured or certifiedafter 1978. The range of cruise NOx emission indexvalues of 10-20 gNO2/kg fuel listed in Table I forthe latest subsonic and supersonic commercialaircraft represents a pessimistic projection forfuture engines. The pessimistic forecasts mightresult if water injection were to he used partiallyor totally for the reduction of NOx during takeoff,in order to meet EPA low altitude emission standards,which would not result in a comparable reduction inNOx during cruise,
The more probable range of values for the cruiseNOx emission index that might be achievable inadvanced engines within the next decade were basedon the low-pollutant combustor technology .beingevolved in efforts such as the NASA "ExperimentalClean Combustor Program." Values for the NOemission index of 3-8 9NO2/kg fuel were predicted
for future subsonic aircraft using advanced high-bypass turbofan engines, and values of 3-14 91NOa4 gfuel were predicted for future supersonic aircrafttusing advanced duct-burning turbofan or dry turbo-jet engines. The spread in these predictions isdue to the range of possible combustor operatingconditions for advanced engines, and the uncertaintyas to the degree to which these combustors designswill be able to incorporate fuel-lean premixing-provaporizing burner concepts.
Optimistic predictions based on the eventualdevelopment of a combustor burninv premixed-prevaporized fuel near the lean flammability limitindicate that cruise NO
' emission indices of one or
less may be attainable In engines for futuresubsonic and supersonic commercial jet aircraft.hese predicted values represent a goal to be
approached in practical combustor design. AlthoughNO emission indices of one or less have beenaciteved to laboratory-type burners, a considerableeffort will be required to convert this fundamentalresearch into practical combustor technology,
Concluding Remarks
It is premature to arrive at a judgment as towhether or not future commercial aircraft engineswill be able to attain cruise NOx emission indicesthat are as much as 60 times lower than values forcurrent aircraft engines, Chemical kineticscalculations and fundamental laboratory blrrnertests conclude that these lower NOx emissionindices are theoretically possible. However, agreat deal of ingenuity on the part of combustordesign engineers will be required to convertfundamental concepts such as the premixed-prevapor-ized fuel-lean burner into practical combustorhardware, Research and development programs suchas the "NASA Experimental Clean Combustor Program"are attempting to establish a technology base forthe design of low emission combustors. Asubstantial amount of development time and testingwill be required to translate experimentaltechnology into production technology that fulfillsthe safety, reliability, and economic requirementsof a commercial aircraft.
Refere:,ces
1. Grobecker, A.J., Coroniti, S.C. and Cannon, R.H.Jr.: Report of Findings, the Effects ofStratosphere Pollution by Aircraft. DOT-TST-75-50, Dec. 1974.
2. English, J. Morley (General Chairman), CIAPMonograph 2, Propulsion Effluents to theStratosphere. Proposed 00T Report.
3. Grobman. Jack and Ingebo, Robert D.: JetEngine Exhaust Emissions of High-AltitudeCommercial Aircraft Projected to 1990. NASATMX-3007, March 1974.
4. Grobman, Jack and Ingebo, Robert 0.: Forecastof Jet Engine Exhaust Emissions for FutureHigh Altitude Commercial Aircraft. NASA TMX-71509, Paper Presented at DOT Third Conferenceon CIAP, Cambridge, Mass. Feb. 26 - Mar. 1, 1974..
h
1
_.r
Pro ected Lqw-Emission Combustor Technol2W
The projected combustor technology that might beutilized in the advanced propulsion systems de-scribed in the previous section are based on aprojection of the emission reduction technologydiscussed in reference 7 (Chapter 5). The evolutionof low emission combustor technology available byChia 1900 to 1985 time period will be motivated bythe r4, 4 to meet the 1979 EPA emission sta..dards(ref. 4). These emission standards presentlypertai.. only to subsonic aircraft; however,additional standards for supersonic aircraft he ealso been proposed. These standards are current'yonly applicable to aircraft oerations below 915meters (finding-taktofl c - 1. Many of theconcep.s being investigated to reduce NO x fur theproposed EPA landing-takeoff cycle would also beeffective in reducing NOx during cruise.
Research programs such as the NASA "ExperimentalClean Combustor Program" descrihed previously are
applying low emission technology to combustorredesign. The representative engine manufacturersare also engaged in independent research effortsaimed at the development of low emission combustorsthat would comply w:ih the proposed EPA standards.The type of low-pollutant combustor hardware beingevolved and evaluated in research efforts such asthe NASA "Experimental Clean Combustor Program"are probably representative of the level oftechnology that could be available within the nextdecade. More optimistic predictions of the levelof low-pollutant combustor technology that mightbe available 'n the future are predicated on theconversion ur fundamental concepts such as fuel-leanpremixing-prevaporizing burners into practicalcombustor designs.
Cruise NO„ Emission Index Forecast
The NO x emission indices listed in Table I forcurrent commercial jet aircraft would be character-istic of existing production engines manufacturedprior to 1979 that would continue to be in serviceduring the 1980 to 1985 time period. These engineswould not be required to meet the proposed 1979 EPAemission limits since the regulation would onlyapply to engi-es either manufactured or certifiedafter 1978. The range of cruise NOx emission indexvalues of 18-20 91402/kg fuel listed in Table I forthe latest subsonic and supersonic commercialaircraft represent , a pessimistic projection forfuture engines. The pessimistic forecasts mightresult if water injection were to to used partiallyor totally for the reduction of NO x during takeoff,in order to meet EPA low altitude emission standards,which would not result in a comparable reduction inNOx during cruise.
The more probable range of values for the cruiseNOx emission index that might be achievable inadvanced engines within the next decade were basedon the low-pollutant combustor technology beingevolved in efforts such as the NASA "ExperimentalClean Combustor Program." Values for the NOisemission index of 3-8 gNO2/kg fuel were predicted
for future subsonic aircraft using advanced high -bypass turbofan engines. and values of 3-14 g/060e gfuel wort predicted for future supersonic aircrafftus i ng advanced duct-burning turbofan or dry turbo-jet engines. The spread in these predictions Isdue to the range of possible combustor operatingconJitions for advanced engines. and the uncertaintyas to the degree to which these combustors designswill be able to incorporate fuel-lean premixing-preva i,vrizing burner concepts.
Optimistic predictions based on the eventualdevelopment of a combustor burn inn pr*wiied-prevaporized fuel near the lean flammability limiti ndicate that cruise Noss emission indices of one orless may be attainable to engines for futurevAbsonic and supersonic commercial jet aircraft.'hose predicted values represent a goal to beapproached in practical combustor design. AlthoughNO' ission indices of one or less have beenecAieved in laboratory-type burners, a considerableeffort will be required to convert this fundamentalresearch into practical combustor technology.
Loncluding Remarks
It is premature to arrive at a judgment as towhether or not future comme rcial aircraft engineswill be able to attain cruise NOx emission indicesthat are as much as 60 times lower than values forcurrent aircraft engines. Chemical kineticscalculations and fundamental labo ratory b•irnertests conclude that these lower NOx emissionindices are theoretically possible. However, dgreat deal of ingenuity on the part of combustordesign en7ineers will be required to convertfundamental concepts such as the p remixed-prevapor-ized fuel-lean burner into practical combustorhardware. Research and developmeit programs suchas the "NASA Experimental Clean Combustor Program"are attempting to establish a technology base forthe design of low emission combustors. Asubstantial amount of development time and testingwIll be required to translate experimentaltechnology into production technology that fulfillsthe safety. reliability, and economic requirementsof a commercial aircraft
Refere,ices
1. Grobecker, A.J., Coroniti, S.C. and Cannon, R.H.Jr.: Report of Findings, the Effects ofStratosphere Pollution by Aircraft. DOT-TST-75-50, Dec. 1974.
2. English, J. Morley (General Chairman). CLAPMonograph 2. Propulsion Effluents in theStratosphere. Proposed DOT Report.
3. Grobman. Jack and Ingebo, Robert D.: JetEngine Exhaust Emissions of High-AltitudeCommercial Aircraft Projected to 1990. NASATMX-3007, March 1974.
4. Grobman. Jack and Ingebo, Robert D.: Forecastof Jet Engine Exhaust Emissions for FutureHigh Altitude Commercial Aircraft. NASA TMX-71509. Paper Presented at DOT Third Conferenceon CLAP. Cambridge. Mass, Feb. 26 - Mar. 1. 1974.
i
S. Verkamp, F.J., Verdouw, A.J„ and Tomlinson. 17. Hazard, H.R.: Conversion of Fuel Nitrogen toJ.G.: impact of Emission Regulations on Future NOx in a Compact Combustor, ASME Paper No.Gas Turbine Engine Combustors. AIAA Paper No, 73-WA/G7-2, Technical Paper presented at ASME73-1277, Paper presented at the AIAA/SAE 9th Winter Annual Meeting, Detroit, Michigan,Propulsion Conference, Las Vegas, ilevada. November 11-15, 1073.Nov 1973,
6. Booker, Nonry G., Chairman, Climatic impactCommittee: Environmental Impact of StratosphericFlight, National Academy of Sciences, 1975.
7. Diehl, Larry A., and James D. Haldeman:Gaseous Emissions from a JT8D-109 TurbofanEngine at simulated Cruise Flight Conditions,Proposed NASA TMX,
B. Bahr, D.W.: Attainment of Ultra-Low NOxEmissions Levels in Aircraft Turbine Engines.Paper presented at Fourth Conference on theClimatic !'pact Assessment Program, DOTTransportation Systems Center, Feb, 4-7, 1975.
9. Marchionna, Nicholas R., Diehl, Larry A. andTrout, Arthur M,: Effect of Inlet-AirHumidity, Temperature, Pressure, andReference Mach Number on the Formation ofOxides of Nitrogen in a Gas Turbine Combo^tor.NASA TNO-7396 October 1973,
10, Marchionna, Nicholas R.: Effect of IncreasedFuel Temperature ow Emissions of Oxides ofNitrogen from a Gas Turbine Combustor BurningASTM Jet-A Fuel, NASA TMX-2931. January 1974. -
11, Anderson, David N,: Effects of EquivalenceRatio and Dwell Time on Emissions from anExperimental Premixing-Prevapori zing Burner.NASA TMX71592, Paper presented at 20th AnnualInternational Gas Turbine Conference, Houston,Texas, March 1975.
12. Roffe, G. and Ferri, A.: Prevaporization andPremixing to Obtain Low Oxides of Nitrogen inGas Turbine Combustors. NASA CR-2495, 1975,
13. Blazowski, William S., and Bresowar, Gerald E,:Preliminary Study of the Catalytic CombustorConcept as Applied to Aircraft Gas Turbines,Technical Report AFAPL-TR-74-32, May 1974.
14, Anon,: EPA Aircraft Emission Standards forControl of Air Pollution, Federal Register,Vol. 38, No. 136, Part 2, July 17, 1973,pp. 19087-19103.
15. Niedzwiecki, R.W. and Jones, R.E.: TheExperimental Clean Combustor Program -Description and Status. NASA TMX-71547. Paper
• presented at the SAE Air Transport Meeting,Dallas, Texas, April 30 - May 2, 1974.
16, Grobman, Jack, Anderson, David N., Diehl,Larry A., and Niedzwiecki,. Richard W.:Combustion and Emission Technology (Chapter IV);Presented at Aeronautical Propulsion Conferenceat NASA Lewis Research Center, May 13-14, 1975;Proposed NASA SP. l
8
S. Verkamp, F.J., Verdouw, A.J., and Tomlinson, 17 hazard, N.A.: Conversion of fuel Nitrogen toJ.G.: Impact of Emission Regulations on Future NOx in a Compact Combustor, ASME Paper No.Gat Turbine Engine Combustors. AIM Paper No. 73-WA/G7-2, Technical Paper present#1 at ASK73-1277, Paper presented at the AIM/SAE 9th Winter Annual Meeting, Detroit, Michigan,Propulsion Conference, Las Vegas, Nevada, November 11-15, 1073.No% 1973.
6. Booker, Henry G.. Chairman, Climatic impactCommittee: Environmental Impact of StratosphericFlight. National Acadriy of Sciences, 1975.
7. Diehl, tarry A., and James D. HoldemanGaseous Emistions from a JT80-109 TurbofanEngine at Simul p ted Cruise flight Zonditlons.Proposed NASA TMX.
R. Bahr, D.W.: Attainment of Ultra-Low NOxEmissions Levels in Aircraft Turbine Engines.Paper pre:ented at Fourth Conference on theClimatic :^pact Assi-ssment Program, DOTTransportation Systems Center, Feb. 4-1, 1975.
9. Marchionna, Nip holes R., Diehl, Larry A. andTrout. Arthur M.: Effect of Inlet-Air:tumidity, Temoerature, Pressure, andReference k:a,;h Number on the Formation ofOxides of i trogen in a Gas Turbine Comb—tor.NASA TNO-7396 October 1973.
10. Marchionna, Nicholas R.: Effect of IncreasedFuel Temperature on Emissions of Oxides ofNitrogen from a Gas Turbine Combustor BurningASTM Jet-A Fuel. NASA TMX-2931. January 1974.
11. Anderson, David N.: Effects of EquivalenceRatio and Dwell Time on Emissions from anExperimental Premixing-Prevaporizing Burner.NASA TMX11592. Paper presented at 10th AnnualInternational Gas Turbine Conference, Houston,Texas, March 1975.
12. Roffe, G. and Ferri. A.: Prevaporization andPremixing to Obtain Low Oxides of Nitrogen inGas Turbine Combustors. NASA CR-2495. 1975.
13. Blazowski, William S., and Bresowar, Gerald E.:Preliminary Study of the Catalytic CombustorConcept as Applied to Aircraft Gas Turbines,Torhnical Report AFAPL-TR-74-32. May 1974.
14. Anon.: EPA Aircraft Emission Standards forControl of Air Pollution. Federal Register,Vol. 38, No. 136, Part 2. July 11, 1973,pp. 19087-19103.
15. Niedzwiecki, R.M. and Jones, R.E.: TheExperimental Clean Combustor Program -Description and Status. NASA TMX-71547. Paperpresented at the SAE Air Transpor! Meeting.Dallas, Texas, April 30 - May 2, 1974.
16. Grobman, Jack, Anderson, David N., Diehl.Larry A., and Niedzwiecki, Richard W.:Combustion and Emission Technology (Chapter IV);Presented at Aeronautical Propulsion Conferenceat NASA Lewis Research Center, May 13-14, 1975;Proposed NhSA SP.
TABLE I. -CRUISE NOx EMISSIONS FROM CURRENT COMMERCIAL JET AIRCRAFT
TABLE I. - CRUISE NO x EMISSIONS FROM CURRENT COMMERCIAL JET AIRCRAFT
AIRCRAFT ENGINE CRUISEALTITUDE,
KM
CRUISEMACH
NUMBER
COmdUSTORINUIT
TEMPERATURE,
COMBUSTORINLET
PRESSURE,ATM
NOEMMI?ION
INDEX,q NOjkg
SUBSONIC
BOEING 707 P&N JT3D 10.7 0.85 --570 --4 6
BOEING 777 P&N JT80 10.7 0.80 610 5.7 8
BOEINu 747 P&N JT90-70 10.7 0.84 710 10 19
McDONNELL G. E. CF-6-50 10.7 0.85 730 11.4 16,5DOUGLASDC-10
LOCKHEED 1011 ROL LSROYCE 10.7 I 0.85 720 10.4 ?0 jRB 211
SUPERSONIC
OLYMPUS 17.7 2.0 820 6.5 18CONCORDE593
TUPOLEV 144 KUZNETOV ---- --- ---- 18NK-144
0a^MW
1
W
##F"1111 01NG PAGE BLANK NOT
9
1
JW
W
OZm
DZ
OC% ZO
mCO LA
I
LA
WW
XOZ
CA
1600 1800 2000 2200 2400 2600FLAME TEMPERATURE, K
Figure 1. - Effect of flame temperature on NOx emissionIndex for an Ideal premixing-prevaporizing combustor;combustor Inlet temperature, 800 K; pressure,5.5 atm.; and residence time, 2 milliseconds.
80
70
60
JW
T 5o0zv^
XW0
n40
z0
M _00 V)
1 NW
WxOz
20
10
0 1 1
1600 1800 2000 2200 2400 2600
FLAME TEMPERATURE. K
Figure 1. - Effect of flame temperature on NO x emissionindex for an ideal premixing-prevaporizing combustor;combustor inlet temperature, 800 K; pressure,5.5 atm.: and residence time, 2 milliseconds.
o ^ ^
i k
FUEL
10 CM
TTGAS
LSAMPLE
PREHEATED 210 CM^^ -I
AIRFLOW PERFORATED
FLAMEHOLDER
Figure 2. - Premixed primary zone lest section.
100
THEORETICAL
50•
• e•
NOx
•
EMISSION INDEX, 10
g NO2GASEOUS-FUEL DATA
kg FUEL 5
e LIQUID FUEL DATA
Cr
1
, 5 t I I I.4 .5 .6 .7 .8 .9 1.0
EQUIVALENCE RATIO CS-73168
Figure 3. -Variation of NOx emission Index with equivalence ratio Ina premixing-prevaporizing burner; Inlet temperature, BOOK; Ares-sure, 5,5 atm.; and residence time, 2 milliseconds,
i
FUEL
10 Chi 11 F
F GAS
J"PREHLATED 210 CM--*--^
SAh1PEE
AIRFLOW lPERFORATE DFLA MEHOLDER
Figure 2. - Premixed primary zone test section.
100
THEORETICAL50
•
NOx
EMISSION INDEX, 10GASEOUS-FUEL DATAg NO2
kg FUEL 5
• LIQUID-FUEL DATA
. 1
.5.4 .5 .6 .7 .8 .9 1.0
EQUIVALENCE RATIO CS-7315e
Figure 3. - Variation of NO x emission index with equivalence ratio ina premixing-prevaporizing burner; Inlet temperature, 800 K; pres-sure, 5. 5 atm. ; and residence time, 2 milliseconds.
^ {
CD
coOD
k
. ^
^.
k ® ^ ^ p
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) -- \ )\:
^^@ b ) & ƒ j ƒ..
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)/NO )-,@^^ @@ tk^ !^k x(
'ilk^^ k/7 ^§ =a
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t1 ^ V CV^
^•
^
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N
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^
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omc
ce
W ^^ 3 } o,
C
V 'rriC p^
x6/ N
c no o .u
O
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y^ Nj E
►r Gi
O ^ h A
n c
Wcc
N6! dr
C 7:
r ^ p
W
VoK
CU
K^ O
Nr
L/1 V
\W
OQ LA-
r-
+ce r ^ ,c ac r"K zoQ lJ ^J a LA.
U
° zWuZD
C3
^ Z
W N
Z
N ti O O
O
U®Z
Q3OW 1 I
I
Ln
S6
I ^ cr4 Z3
AP
W X
z I
rW
1
0z N JO Z W
N O jir
W
^i
Z ^_
r
l ^
CD v+
^I
I
•
(ai PHOTO Of FULL ANNULAR COMBUSTOR.
FUEL
PREMIX•, -^.-^.
120 FUEL INLETS",...,.r3' LOCALIZED y
AIRFLOW--+ RECIRCULATIONZONES
(b) CROSS-SECTIONAL VIEW OF FULL ANNULAR COMBUSTOR.
FUEL
1
AIRFLOW---►^ i - FLAME^+► STABILI7FR,,1
r-rSWIRLIERCARBURETOR
Itl MODULE COMPONENTS.
Figure 6. - NASA Exp,rimental Swir!°Can-.Modular Combustor.
500 600 700 800 900
INLET-AIR TEMPERATURE, K
Figure 7. - Variation of NOx emission index withcombustor inlet temperature for single andmulti-zone combustors. Combustor Inletpressure, 6 atm; exit temperature, 1500 K.
i
24
a 20EYo^ZE 16o,
0zz 12ONN$_
S
o" 8z
GO,M00
W
COMBUSTOR
SINGLE ANNULAR
BLE-ANNULAR
SWIRL-CAN
COMBUSTOR
e SINGLE ANNULAR
/ DOUBLE-ANNULAR
L-CAN
I A
^I
W
GO^Muc
EVkj
v+
9E 1
0z_
z 1O_N
x0z
500 6G2 700 800 900
INLET-AIR TEMPERATURE, K
Figure 7. - Variation of NO x emission index withcombustor inlet temperature for single andmulti-zone combu,tors. Combustor inletpressure, 6 atm .. exit temperature, 1500 K.
