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Conceptual Development of Quiet Turbofan Engines for Supersonic Aircraft

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Conceptual Development of Quiet Turbofan Engines for Supersonic Aircraft JOURNAL OF PROPULSION AND POWER Vol.19,No.2, March– April 2003 Conceptual Development of Quiet Turbofan Engines for Supersonic Aircraft DimitriPapamoschou ¤ Universityof California,Irvine, Irvine, California 92697-3975 and MarcoDebiasi † TheOhio State University, Columbus, Ohio 43235 Thisis ajointthermodynamic and acoustic study of engines for next-generation supersonic aircraft. It explores xed-cycleconcepts withpotential for quiet takeoff and ef cient cruise. The owpathis simple,without mechanical suppressors.The strategyis totake a representativestate-of-the-artmilitary turbofan engine and increase its bypass ratioto a moderatevalue. The enginecore staysthe same.Three exhaustcon gurations are considered:mixed ow,coaxial separate ow,and eccentric separate ow.Engine-cycle analysis predicts thermodynamicperformance andnozzle exhaustconditions at takeoff and Mach 1.6 cruise. Subscaleexperiments duplicate the staticexhaust conditionsat takeoffpower andmeasure the far-eld sound. Flyover perceived noiselevels are estimatedfor a twin- engineaircraft in the 120,000-lbclass. In terms ofeffective perceived noiselevel (EPNL),the eccentric arrangement is6.5dB quieter thanthe mixed-ow arrangementand 5 dBquieter thanthe coaxialcon guration. Spectral- and time-domainanalyses indicate that the eccentric exhaustis free ofstrongMach wave radiation. The acousticbene t ofthe eccentric arrangement,combined with faster climbafforded by the modied engine,leads to areductionof 14dBin EPNL. Compared to the baselineengine, the specic fuelconsumption of the modied engineis about 13%less atsubsonicclimb and 3% less atsupersoniccruise. Nomenclature tot = total =ightconditions D = diameter 1 f = frequency M =Machnumber Introduction m =mass owrate P OMMUNITY noisefrom aircraft has profound environmental r =distancefrom jet exit andeconomicconsequences. First-generation subsonic jetlin- = thrust C T erswere very noisy because of the high exhaust velocity of their t =timefrom liftoff engines.Efforts to suppressnoise using mixing enhancement had U = velocity onlymoderate impact. 1 Itwasnot until the introduction of thehigh- x =horizontaldistance from brake release bypass-ratioturbofan that noise was reduced remarkably, by 20– y = altitude 30dB.This was simply achieved by thesame thrust being produced ® =geometricangle of attack witha largermass owrate, hence lower exhaust speed. The associ- ° =climbangle atedgains in propulsiveef ciency led to much lower fuel consump- µ =polarangle relative to jetcenterline tion,making the high-bypass turbofan the only choice for commer- Á =azimuthangle relative to vertical plane cialaircraft developed in the 1980s and beyond. The increase in à =polarobservation angle of airplane bypassratio was enabled by development of high-temperature ma- terialsfor the turbine blades. For given size of thegas generator, the Subscripts powerthat can be delivered to the bypass stream is directlyrelated com= compressor totheturbine inlet temperature (TIT). eng= full-scaleengine Developmentof economically viable supersonic transports exp= subscaleexperiment hingeson solvingthe problem of communitynoise without penaliz- fan = fan tip ingaircraft performance. The same issue affects to someextent mil- LO = liftoff itaryhigh-performance aircraft because communities surrounding p =primary(core) exhaust militarybases are becoming increasingly sensitive to noise. So far, thebulk of thesupersonic noise suppression effort has encompassed s =secondary(bypass) exhaust 2;3 TOM =takeoffmonitor mixingenhancement and ejector approaches, whichtypically lead tolargeand heavy powerplants. 4 Onemay wonder if supersonic en- gineswill follow the same evolution as subsonicengines, leading Presentedas Paper2002-0368 at theAIAA 40thAerospace Sciences tosupersonichigh-bypass turbofans. However, the issue is notas Meetingand Exhibit, Reno, NV ,14–17 January2002; received 6May2002; simple.High-bypass ratio generally causes worse, not better, ef - revisionreceived 21November2002; accepted forpublication 4 December ciencyat supersonicspeeds. Figure 1 showscalculations of thrust 2002.Copyright c 2003by DimitriPapamoschou and Marco Debiasi. Pub- ° specic fuelconsumption (TSFC) andfan diameter vs bypassra- lishedby theAmerican Instituteof Aeronautics and Astronautics, Inc., with tio(BPR) andfan pressure ratio (FPR) atcruiseMach number of permission.Copies of thispaper may bemade forpersonal or internaluse, 1.6.The calculation, based on anengine-cycle analysis mentioned onconditionthat the copier pay the $10.00 per-copy fee tothe Copyright laterin the paper, assumes TIT 1600 K (2400±F),a valueclose to Clearance Center,Inc., 222 Rosewood Drive, Danvers, MA 01923;include D thecode 0748-4658/ 03$10.00in correspondencewith the CCC. today’s limitsof turbinematerials. It isseenthat the TSFC slightly ¤ Professor,Department ofMechanical andAerospace Engineering. dipsand then rises with increasing bypass ratio. The fan diameter Associate FellowAIAA. increasesroughly with p.1 BPR/ meaningincreased drag and †PostdoctoralResearcher, Mechanical Engineering.Member AIAA. weightof the vehicle. The quantitative C information shown in Fig. 1 161 162 PAPAMOSCHOU AND DEBIASI thusmaking the technique very effective at suppressingMach wave emissiontoward the ground. More generally, the MWE resultsillus- tratethe potential for noise reduction by shapingthe mean owof theprimary and secondary streams. Our study represents the initial stepsof abroadereffort to reduce supersonic and subsonicjet noise bymeanpro le shaping of realistic engine ows. Thispaper examines, at a fundamentallevel, the thermodynamic andacoustic performance of a xed-cycle,moderate-bypass super- sonicengine. It will be shownthat signi cant noise reduction relative totoday’s militaryturbofan engines is achievable with an eccentric separate-ow exhaust that reduces the convective Mach number of thecore stream. EngineCon gurations Weconsidera supersonictwin-engine aircraft with maximum takeoffweight of about540 kN (120,000 lb) The assumed lift-to- dragratio is 5 attakeoff and 10 at supersoniccruise, values roughly 20%better than those of the Aerospatiale Concorde. 16 The study startswith a representativemilitary turbofan engine for this kind ofairplane, increases its bypass ratio to moderate value, and as- sessesnoise and performance of three con gurations: the mixed- owturbofan, the separate- ow turbofan with coaxial exhaust, and theseparate- ow turbofan with eccentric exhaust. The comparison basisis thefollowing. 1)All engines have the same supersonic cruise thrust. 2)All engines have the same core characteristics (mass owrate towithin10%, overall pressure ratio, and turbine inlet temperature). Fig.1 TSFCand fan diameter ,normalizedby turbojet value, vs BPR Thebaseline engine is a militaryturbofan with BPR 0:3, andFPR for Mach 1.6 cruise: ,baselineengine and , modi ed D engines. FPR 5:0,static thrust of 126 kN (28,000 lb) and cruise thrust of30DkN(6700 lb) at Mach 1.6 and altitude of 16,000m. The static thrustis dictatedby thefederal requirement for the aircraft to climb willchange somewhat with the assumptions of thecycle analysis, atanangleof 1.4 deg with one engine inopertaive. 17 The modi ed forexample, component ef ciencies, but the qualitative trends will enginesare increasedmass owrate derivatives of thebaseline en- not.Figures 1 indicatethat, at today’s limitsof TIT, bypassratios gine,with bypass ratio 1.6. The size, speci c fuelconsumption, and beyond3.0 would lead to poor performance at supersoniccruise. exhaustconditions of the engines are derived from thermodynamic Designersof supersonic engines, therefore, face two con icting analysisof aBraytoncycle with component ef ciencies and spe- requirements:high-bypass ratio on takeoff/ landingfor reduced noise cic heatratios listed in T able1. (SeeRef. 18 for more information andlow- or zero-bypass ratio for ef cient supersonic cruise. One onthecycle analysis.) For all engines, 25– 30% of thecompressor possibilityis thevariable-cycle turbofan engine, but it entails com- airis used for turbine cooling, 1% of thecompressor air is bledto plexityfar greater than that of today’s engines.Another approach is systemsoutside the engine, and 1.5% of the turbine work drives toseekan intermediate bypass ratio that satis es both requirements auxiliarysystems. These guresare representative of theoperating inaxedcycle. Because the bypass ratio would be moderate, it be- conditionsof modern engines. T otalpressure loss due to turbine comescrucial how one uses the bypass stream to reduce noise. One coolingis estimated at 7%timesthe mass fraction of coolingair. 19 conguration is themixed- ow turbofan, currently used on all mili- Forthe mixed- ow design, the core and fan streams mix at con- taryengines, in which the bypass and corestreams mix before exit- stantpressure, constant total enthalpy, and Mach number 0.4 before inga commonnozzle. The other option is theseparate- (unmixed-) expandingto ambient pressure. The overall pressure ratio (OPR) owturbofan, which is very common on subsonictransports. The andTIT formaximum static thrust were selected at 30 and1800K, unmixeddesign allows shaping of the bypass exhaust so thatthe by- respectively.The requirement that all engines have the same super- passstream substantially reduces Mach wave emission from the core soniccruise thrust sets the size of eachengine. Speci cally, it makes stream.Previous work on theMach wave elimination (MWE) tech- thefan diameter of eachengine dependent on the OPR andTIT cho- niqueshowed signi cant gains in
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