Performance assessment of a multi-fuel hybrid engine for future aircraft

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Delft University of Technology

Performance assessment of a multi-fuel hybrid engine for future aircraft

Yin, Feijia; Gangoli Rao, Arvind; Bhat, Abhishek; Chen, Min

DOI

10.1016/j.ast.2018.03.005

Publication date

2018

Document Version

Final published version

Published in

Aerospace Science and Technology

Citation (APA)

Yin, F., Gangoli Rao, A., Bhat, A., & Chen, M. (2018). Performance assessment of a multi-fuel hybrid engine

for future aircraft. Aerospace Science and Technology, 77, 217-227.

https://doi.org/10.1016/j.ast.2018.03.005

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Aerospace Science and Technology 77 (2018) 217–227

Contents lists available atScienceDirect

Aerospace

Science

and

Technology

www.elsevier.com/locate/aescte

Performance

assessment

of

a

multi-fuel

hybrid

engine

for

future

aircraft

Feijia Yin

a

,

∗

,

Arvind Gangoli Rao

a

,

Abhishek Bhat

b

,

Min Chen

c

a_Faculty_of_Aerospace_Engineering,_Delft_University_of_Technology,_Kluyverweg_1,_2629HS,_Delft,_Netherlands b_Honeywell_Technology_Solutions,_Bangalore,_India

c_School_of_Energy_and_Power_Engineering,_Beihang_University,_China

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received7September2017

Receivedinrevisedform25February2018 Accepted5March2018

Availableonline7March2018

Keywords: Hybridengine Multi-fuel Optimization Performanceassessment Lowemissions

Thispaperpresentstheperformanceassessmentofanovelturbofanengine usingtwoenergysources: LiquidNatural Gas(LNG)and kerosene, calledMulti-Fuel Hybrid Engine(MFHE).The MFHEis anew engine conceptconsistingofseveralnovel features,suchas acontra-rotating fanto sustaindistortion causedbyboundarylayeringestion, asequentialdual-combustionsystemtofacilitate“EnergyMix”in aviationandaCryogenicBleedAirCoolingSystem(CBACS)tocooltheturbinecoolingair.TheMFHEhas beenenvisagedasapropulsionsystemforalong-rangeMulti-FuelBlendedWingBody(MFBWB)aircraft. In thisresearch, westudythe uninstalled characteristicsof the MFHEcovering three aspects: 1) the effects ofCBACSonthe HighPressureTurbine (HPT)cooling air requirementand itsconsequenceon theenginecycleeﬃciency;2)thecycleoptimizationoftheMFHE;3)theperformanceoftheMFHEata missionlevel.Anintegratedmodelframeworkconsistingofanengineperformancemodel,asophisticated turbine-coolingmodel,andaCBACSmodelisused.TheparametricanalysisshowsthatusingCBACScan reducethebleedairtemperaturesigniﬁcantly(upto400K),therebydecreasingtheHPTcoolingairby morethan40%.Simultaneously,theLNGtemperatureincreasesbymorethan200 K.Thehybridengine alone reduces theCO2 emission by about27% and the energyconsumption by12% comparedtothe

currentstate-of-the-artturbofanengine.Furthermore,themissionanalysisindicatesareductioninNOx emissionby80%andCO2emissionby50%whencomparedtothebaselineaircraftB-777200ER.

1. Introduction

Aviation contributes to 5% of the total anthropogenic climate change including both the CO2 effects and the non-CO2 effects fromNOx emissions, water vapor and contrails[1]. The demand

forairtransportation is anticipatedto grow by 4.6%annuallyfor thenext20years[2],whichaggravatestheaviation’s climate im-pact. To enable the sustainable growth,the Advisory Council for AeronauticsResearchinEuropehassetambitiousobjectivesto re-duceCO2 emissionby75%andNOx emissionsby90%bytheyear

2050whencomparedtotheyear2000technology[3].

TheCO2 reduction canbe achievedin acombinationwith in-novative aircraft/engine technologies and using alternative fuels. TheGearedTurbofan[4],theIntercooledRecuperatedAero-engine [5], and the Open rotor [6] are examples of the eﬃcient engine concepts.Whereas,theNOxemissionscanbereducedbythe

inno-*

Correspondingauthor.

E-mailaddresses:F.yin@tudelft.nl(F. Yin),A.gangolirao@tudelft.nl (A. Gangoli Rao),abhishek.rkbhat@gmail.com(A. Bhat),chenmin@buaa.edu.cn (M. Chen).

vativelowNOx combustiontechniquesandbyusinghydrogen-rich

alternativefuels.

Oneoftheother mainchallengesforfutureaviationisthe en-ergy source. Currently, aviation consumes around 1 Billionliters of Jet Fuel every day [7,8] and it is anticipated to increase with the increase inair traffic despite theimprovement inaircraft ef-ficiency. On the other hand, the oil reserves are depleting, thus creatingadiscrepancyinthesupplyanddemand,whichwilllead toasignificantincrease inthefuelcost.Thisincrease infuelcost hasalreadyincreasedthefuel shareinthetotal operatingcostof anairlinetoaround30%[9].Furtherincreaseinfuelpriceswould havenegative consequencesforairlines.Therefore,othermeansof energysourcetodrivetheaircraftengineswillhavetobetapped. Though the usage of sustainable alternative fuels in the aviation industry is not widely practiced, some commercial flights have beensuccessfullyoperatedwithbiofuels[10,11].Furthermore,the emissionsstandard setby theInternationalCivil Aviation Organi-zation for engine certification is becoming stringent. As long as theconventionalfuelisinuse,thegoalofreducing CO2 emission significantly remainsillusive; hence,alternative fuelswill playan importantrole.

https://doi.org/10.1016/j.ast.2018.03.005

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Nomenclature Abbreviations

BPR BypassRatio

CBACS CryogenicBleedAirCoolingSystem CHEX CryogenicHeatExchanger

EF EnergyFraction

EI EmissionIndex . . . g/kgkg/kg FPR FanPressureRatio

HPC HighPressureCompressor HPT HighPressureTurbine ITB Inter-stageTurbineBurner LNG LiqueﬁedNaturalGas LH2 LiqueﬁedHydrogen

LHV LowerHeatingValue . . . J/kg LPC LowPressureCompressor

LPT LowPressureTurbine

MFBWB Multi-Fuel BlendedWingBody MFHE Multi-FuelHybridEngine OPR OverallPressureRatio

SED SpeciﬁcEnergyDensity SLS SeaLevelStatic TOC TopofClimb

VED VolumetricEnergyDensity VHBR VeryHighBypassRatio

Symbols

˙

m Massﬂowrate . . . kg/s pt Stagnationpressure . . . Bar

Tt Stagnationtemperature . . . K

η

Eﬃciency

π

Pressureratio

ε

Heatexchangereffectiveness

Subscripts

3 Highpressurecompressorexit 4 Highpressureturbineinlet 46 Lowpressureturbineinlet

Fig. 1. Comparison of various energy sources for aviation [12].

2. Fuelselection

Thereareseveralcriteriainselectingafuelforaviation.Oneof themaincriteriaistheenergydensity,asreducingweightand vol-umeisofparamountimportanceforaviation.BothSpeciﬁcEnergy Density (SED, amount of energy per unit mass of the fuel) and Volumetric Energy Density (VED, the amount of energy per unit volume)are essential. InFig. 1,various energy sources regarding their SEDandVED are presented [12]. It canbe seen that Jet-A/ kerosene has good SEDandVED andtherefore suitable for avia-tion. Moreover, LiqueﬁedHydrogen (LH2)has highSED butpoor VED, implyingthat huge volume wouldbe required to carry any reasonableamountofLH2.ThismakesitchallengingtouseLH2in aviation. Additionally, using LH2 inaviation hasother challenges like safety, logistics, etc. [13]. Certainly, the advantages of using LH2 should not be neglected as the CO2 emission can be elimi-nated.Moreover,hydrogenshouldnotbeviewedasfuelbutasan energycarrier(e.g.,high-energydensebattery).From along-term perspective, LH2can be agood candidateforaviation, especially, tosatisfytheimperativerequirementforsustainability.

Furthermore,theLiqueﬁedNaturalGas(LNG),whichprimarily consistsofmethane,hasdrawnconsiderableattention.LNGis nat-uralgas that hasbeen liqueﬁed form to increase energy density andavoidpressurization.FromFig.1,itcanbe seenthatLNGlies inbetweenkeroseneandLH2,bothintermsofSEDandVED. Cur-rently,LNGis oneofthe cheapestfuels available [14].The global reservesofnaturalgasareenormous, thusimplyingthattheLNG

pricewouldbestable.Moreover, LNGisoneofthecleanestfuels, andrecentlyithasbeenshownthatLNGcanalsobegeneratedby usingrenewable energy[15,16]. TheeffectsofusingLNGforcivil aviationaresummarizedbelow.

AdvantagesofLNG:

•

Approximately 25% reduction in CO2 emission for the same energyconsumption

•

The natural gascan be mixed withair ina better way than kerosene,whichreducesNOxemission.

•

LNGisacryogenicfuelandthereforeagoodheatsink.Itcan beusedbeneﬁciallytoenhancethethermodynamiceﬃciency ofthe engine,forinstanceby intercooling,bleed cooling, air-conditioning,etc.

•

LNGischeaperthantheconventionaljetfuelintermsofMJ/$.

•

TheenergydensityofLNGishigherthankerosene DisadvantagesofLNG:

•

Unlikekerosene,LNGcannotbestoredinwings.

•

LNGhastobestoredininsulatedcylindricalorsphericaltanks, increasingtheaircraftoperatingemptyweight.

•

The volumetric energy density ofLNG is lower compared to kerosene.

•

AirportfacilitiesandlogisticsforstoringandtankingLNGare required.

•

The H2O emission (an importgreenhouse gas at higher alti-tudes and latitudes) of burning LNG is higher compared to kerosene.

3. Themulti-fuelblendedwingbodyaircraft

Cryogenic fuels,likeLNG,needtobestoredininsulated cylin-drical or spherical tanks with the well-insulated system to pre-vent themfromleakingandboiling off.Therefore,thevolume re-quiredtocarrycryogenicfuelsincreasessigniﬁcantly,whichmakes it challenging for conventional aircraft. The Blended Wing Body (BWB) concept provides possibilities for cryogenic fuels as far as space is concerned. The BWB has been studied by many re-searchersworldwidely [17–20].The MFBWBconcept proposedin

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F. Yin et al. / Aerospace Science and Technology 77 (2018) 217–227 219

Fig. 2. Schematic of the MFBWB concept.

Fig. 3. The schematic of the hybrid engine concept using LNG & kerosene. theAHEADproject1 isone oftheseradicalconcepts.A schematic

oftheMFBWBaircraftisdepictedinFig.2.Inthisspeciﬁc conﬁg-uration,the LNGfuel tanksare placedat therear oftheaircraft. ThefeaturesoftheMFBWBarelistedasbelow[21]:

•

300passengerscapacity

•

Designrangeof14000km

•

Carryingmultiplefuels,suchasLNGandkerosene/biofuel

•

UtilizingBoundaryLayerIngestion(BLI)technology

•

LowNOx andCO2emissions

4. Themulti-fuelhybridengine

To exploit the unique opportunities provided by the MFBWB aircraft,anovelMulti-fuelHybridEngine(MFHE)concepthasbeen proposed (see Fig.3). The MFHE isconceived basedon an Inter-stageTurbineBurner (ITB) turbofanengineforthe following rea-sons. Firstly, the dual combustion chamber conﬁguration enables differentfuelstobeusedsimultaneously(LNGandkeroseneinthis paper).Thisway,thespaceusagewithin anairframecanbe opti-mizedwithrespecttoenergystoragewhilerestrictingtheincrease infuelvolumeandtheassociatedaerodynamicdrag.Secondly,the LNG isused in the ﬁrst combustor(the main burner),while the keroseneisusedintheITB.Bychangingtheenergysplitratio

be-1 _{http://www.ahead}_-euproject_.eu/_.

tween the LNG andkerosene, the reduction in engine emissions canbeoptimized.Moreover,astheVeryHighBypassRatio(VHBR) andOverallPressureRatio(OPR)becomeimperativeforaturbofan engine to improve its eﬃciency, meeting the off-design perfor-mance (e.g.the ﬂat rated temperature) is challenging. The study in [22] showsthat using an ITBin a VHBR turbofan canhelp to improvetheengineoff-designperformance.

ThedistinguishedfeaturesoftheMFHEaresummarizedas be-low:

•

Contra-Rotating Fans (CRF):TheMFBWBintendstoingest the boundarylayerflowovertheairframetoimprovethe propul-sion efficiency. To better sustain the non-uniform inlet flow causedbytheBoundaryLayerIngestion(BLI)[23],thenormal fanisreplacedbytheCRFinthehybridengine.

•

The hybrid combustion system: The reduction in NOx

emis-sion of the hybrid engine is approached by two means. The ﬁrstbeingtheadvancedlowNOx combustiontechniques

(pre-mixedcombustionintheﬁrstcombustionchamberand ﬂame-lesscombustioninITB)areused.Thesecondapproachisthat theenginewithsequentialcombustorhaslowermaximum op-eratingtemperaturecomparedtothesinglecombustorengine forthesamepoweroutput,therebyloweringtheNOxemission

further. The thorough analysis has been performed to study theemissionsofthehybridcombustionsystem,wherethe re-sults conﬁrma substantial reduction in NOx emission in the

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Fig. 4. The interactions between three sub models.

•

The Cryogenic Bleed Air Cooling System (CBACS): The Cryo-genicBleedAirCoolingSysteminthehybridengineisanother non-conventionalfeature(seetheupperpartofFig.3).Inthe modern aircraftengines,a large amountofair(about 30% of thecoreairmassflow)isbledoffthecompressortocoolthe turbineblade.Usingsuchsignificantamountofcooling penal-izes theturbine efficiencyby more than10% [25]. Moreover, theenginecoresize hasbecomemuchsmallerasthegeared turbofanengine configuration is used. The engine off-design performance, e.g., thecompressor surge margin ismore sen-sitivetothe airbleed. Therefore,the urgencyofreducingthe turbinecoolingairisstrong.The CBACSin thehybridengine isexpectedtofulfillsuchanimperative.

The coreelement ofthe CBACS is a cryogenic heatexchanger (CHEX).Intheheatexchanger,thecryogenicfuel(LNG)servesasa coolanttopre-cooltheHPTcoolingair,thereby reducingtheHPT cooling requirement. Moreover, the fuel temperatureis increased attheexitoftheheatexchanger,hencewillsavefuelconsumption. Overall,theenginecyclewouldbecomemoreeﬃcient.Thedetails aboutthehybridenginephilosophycanbefoundin[12].

5. Themulti-fuelhybridenginemodel

InordertoevaluatetheMFHEperformance,amodeling frame-work iscreated which consistsof an engine performance model, a sophisticated in-house turbine-cooling model and a cryogenic bleedaircoolingsystem(CBACS)model.Theinteractions between thesethree sub-models are described in Fig. 4.The notations in thisﬁgurecanbefoundinTable1.

5.1. Theengineperformancemodel

The conﬁguration of the hybrid engine is unconventional; therefore,aﬂexible modelingenvironmentalisrequiredtomodel such an engine concept. In the currentanalysis, the Gas turbine SimulationProgram(GSP)isusedtomodelthehybridengine[26].

Table 1

ThedescriptionsofthevariablesinFig.4.

Notations Descriptions Units

Tc Initial cooling air temperature K Tc,new Reduced cooling air temperature via CBACS K

Pc Cooling air pressure Pa

mc Cooling air mass ﬂow kg

Tf Fuel temperature (LNG) K

Tf,new Increased fuel temperature via CBACS K

mf Fuel mass ﬂow kg

Th Hot gas temperature K

Ph Hot gas pressure Pa

ThemodellayoutisshowninFig.5.Themaingaspathconsistsof inlet,fan,LowPressureCompressor(LPC),HighPressure Compres-sor (HPC), main combustionchamber (burning naturalgas),High Pressure Turbine (HPT), Inter-stage Turbine Burner (ITB, burning kerosene/biofuels), Low Pressure Turbine (LPT), core nozzle and bypassnozzle.Anadiabaticduct(componentnumber23inFig.5) isappliedtoconsiderthebypasspressureloss.Thehybridengine usesaContra-rotatingFans(CRF),whichisdrivenbyageared sys-tem. Thelossescausedbysuchagearedsystemareconsideredas a fractionofthe totallosses throughthe LPT, representedby the LPT mechanicalefficiency.Furthermore,inthebleedcontrol com-ponents(numbered as2–6and9),theturbinecoolingisspecified asafractionofthecoreairmassflowrate,whichisextractedfrom specificHPCstagesdependingonthepressureofthehotgas.The componentnumbered8isagenericschedulertosetuptheengine thrustrequirementatgivenoperatingconditions[27].

Thefundamentalequations,whichGSPfollowstocalculatethe componentcharacteristicsandtheoverallengineperformance,can be found from standardtextbooks on gasturbine theory [28,29]. Themodelingprocedurefollowstheconventionofstartingwitha reference point (also named asthe engine design condition) fol-lowedbytheoff-designperformance calculations.Thecomponent eﬃcienciesandthepressurelossesatthedesigncondition(cruise) ofthehybridenginearegiveninTable2.Fortheoff-design perfor-mance calculation,the generic compressor and turbinemaps are scaled accordingto the design condition. For the contra-rotating fans, adifferent map was created usingthe data takenfrom the available public literature [30],and thenscaled basedon the de-signperformanceoftheCRFinthecurrentstudy.Themapforthe CRFisshowninFig.6.

The performance requirements ofthe hybrid engine at differ-entoperatingpointsaregiveninTable3.Thesevaluesarederived fromthe designofMFBWBaircraftover differentﬂightsegments intheaircraftmissionanalysis.

5.2. Theturbinecoolingpredictionmodel

ThetechnologicaldevelopmenttrendisdrivinguptheOPRand TIT of the modern gas turbines. Consequently, the effect of the bleed aircoolingontheengineperformance isbecoming

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Table 2

Baselinecomponentperformanceparameters.

Component Performance parameter Notation Datum value Unit Fan polytropic eﬃciency ηf an 93 %

LPC polytropic eﬃciency ηL P C 93 %

HPC polytropic eﬃciency ηH P C 91 %

Main combustor combustion eﬃciency ηC C 99.9 %

pressure ratio πC C 0.95 [–]

HPT (uncooled) polytropic eﬃciency ηH P T 93 %

ITB combustion eﬃciency ηI T B 99.7 %

pressure ratio πI T B 0.97 [–]

LPT (uncooled) polytropic eﬃciency ηL P T 92.5 %

HP shaft mechanic eﬃciency ηmH P 99.5 %

LP shaft mechanic eﬃciency ηmL P 99.3 %

Bypass duct pressure loss pt/ptin 2 %

Fig. 6. Scaled contra-rotating fan map.

Table 3

Theperformancerequirementsfromhybridengine.

Operating points Ambient condition Mach number Thrust [kN] Max static Sea Level Static (SLS) ISA 0 280 Hot day takeoff SLS, ISA+15 K 0 280

Take-off SLS ISA 0.2 250

Top of climb (TOC) 12 km, ISA 0.8 56

Cruise 12 km, ISA 0.8 50

Ground idle SLS ISA 0 20

inglyimportant.Duringthecycleanalysisphase,thecoolingbleed air isoften modeled by simpliﬁed correlations, which canfail to capturethenonlinearfeatureshencehavingariskof overestimat-ingtheengineperformance.

Inthecurrentanalysis,theeffectsofthebleed coolingsystem playsigniﬁcantrolestoassesstheengineperformance.Therefore,a physics-basedin-houseturbinecoolingpredictiontoolusing semi-empirical correlations for heat transfer and pressure losses has beenused.Detailsofthedevelopedmodelshavebeenelaborated in[31,32].

Theturbinecooling modelreadsinthe temperatureand pres-sureofthecoolant,thetemperatureandpressureofthehotgas,as well asthe maximum allowablemetal temperature. By following theprocedureinFig.7,theamountofcoolingairrequiredandthe resultingpressurelossescanbe calculated.As onecannotice,the turbine-coolingmodelrequires informationon thegeometryofa turbinevaneorblade.Inthecurrentmodel,atypicalturbineblade conﬁguration used inthe modern aero engines, asdemonstrated inFig.8,isapplied.Themaximumallowablemetaltemperatureis 1450K.

5.3. TheCryogenicBleedAirCoolingSystem(CBACS)model

Theintegration oftheCBACSandtheengine maingaspath is demonstrated in Fig.9, where, the HPT cooling bleed air is

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Fig. 8. Schematic of an advanced aero engine cooled high-pressure turbine blade. cooledbyLNGvia acryogenicheatexchanger(CHEX).Oneofthe noticeablephenomenaobserved intheCHEXisthephasechange of LNG. When the temperature of LNG reaches its boiling point (120Kat1Baratmosphericpressure),LNGstartstovaporize.This phase change is beneﬁcial in increasing the heat transfer

coeﬃ-Table 4

Speciﬁcationsofheatexchangerdesign. Design variables Values Inlet bleed air temperature, K Tt3

Exit bleed air temperature, K 600 Inlet fuel temperature, K 120 Inlet bleed air pressure, Bar pt3

Air/fuel mass ﬂow rate, kg/s Determinedby cyclecalculation Bleed air pressure loss (p/pt3), % ≤3

Table 5

Heatexchangergeometricalspeciﬁcations.

Description Parameter

Length (straight pipe) [m] 2.46 Outer shell diameter [m] 0.26 Inner tube diameter [m] 0.026 Wall thickness tube [m] 0.001 Number of units [–] 3

cient. However, it hasanegative effectonthe pressuredropdue totheaccelerationoftheﬂow.

Theair–LNGCHEXfortheMFHEhasbeencarefullydesignedby Fohmann[33].Thedesignconditionisatcruise.Ashell-tube con-figuration withfins in a counter-flow arrangement is considered following a typical two-phase flow heat exchanging mechanism. The heat exchanger design parameters are specified in Table 4. Theyarepressure,temperature, andmassflow rateofbothfluids at the inlet. The temperature of the bleed airat the exitis an-otherdesignvariabletodeterminethetotalheattobetransferred. Theseparametershavebeentakenbasedontheparametric analy-sisconductedabove.Thecycleperformancecalculationdetermines themassflowrateofLNGandcoolingair.Itshouldbenotedthat the pressure drop of the bleed air should be significantly lower than thecombustor pressuredropto enable successfulmixingof thefilmcoolingairwiththemainstreamflowthroughtheturbine. InFig.10,thelayoutoftheheatexchangerdesign(Fig.10(B))and thecross-section(Fig.10(A))aredepicted,withthemain geomet-ricalspecificationsgiveninTable5.

After the dimension of the heat exchanger is calculated, the performance ofthe heat exchanger atother operating conditions isdetermined.Thecharacteristicsoftheheatexchangerare repre-sented by its effectiveness. The deﬁnition of the heat exchanger effectiveness follows the

ε

-NTU method presented by Shah and

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Fig. 10. Schematic of the heat exchanger: (A) Cross section of the heat exchanger, (B) the complete layout [33].

Sekulic in [34]. The maximum temperature difference together withthemassflowrateandtheheatcapacityoffluidsdetermine themaximumheatflux,representedbyEqn.(1),

qmax

=

min

˙

mair

·

h

(

Tt3

)

−

h

(

Ti_LNG

)

,

m

˙

LNG

·

h

(

Tt3

)

−

h

(

Ti_LNG

)

(1)

whereminindicatestheminimumheatfluxbetweentwofluids;h representsthespecificenthalpyofgivensubstance;m is

˙

themass flowrateofgivenfluid;the subscript3 indicatesthe inlet condi-tionoftheair,andi_LNG indicatestheinletconditionofLNG.After definingthemaximumheatflux,theeffectivenessoftheCHEXcan becalculatedbyEqn.(2),

ε

=

q qmax

=

˙

mLNG

· (

h

(

Te_LNG

)

−

h

(

Ti_LNG

))

qmax

=

m

˙

air

· (

h

(

Tt3

)

−

h

(

Tt32

))

qmax (2)

wherethe subscripts3 and 32 indicatethe inlet andexit ofthe bleed airsection (in line withFig. 3); e_LNG andi_LNG indicate theLNGatexitandinletrespectively.

In order to incorporate the CHEX performance characteristics intotheengineperformancemodel,aCHEXperformancemapwas generated as shown in Fig. 11. The map describes the variation in heat exchanger effectiveness as a function of mass ﬂow rates inairandLNG.The designeffectiveness oftheheat exchangeris 50%.This performance map isimplemented inthe hybridengine performancemodel.Itcanbeseenthattheeffectivenessdecreases foroff-designconditions.

6. Engineperformanceevaluation

Usingthemodelsdescribedintheprevioussection,the perfor-manceofthehybridengineisinvestigated.

Fig. 11. Thevariationoftheeffectivenessoftheheat exchangerversusthe ﬂuid massﬂowrate.

6.1. Theengineperformanceoptimizationatcruise

The cycle optimization is performed to minimize the specific fuel consumption atcruise. The designspace andconstraintsare definedinTable6.Theoptimizationhasbeenconductedwith re-specttotheITBenergyfraction(definedbyEqn.(3), where“ker” denotes kerosene)from0to0.3. Theinlet massflowrateiskept constant in order to compare engines with the same diameter (andthussimilarinstallationpenalties).Theoptimizationresultsof

Table 6

Deﬁnitionofenginedesignspaceandconstraints.

Design space formed by design parameters Optimization constraints

Fan Pressure Ratio (FPR) [1.2, 1.5] Overall Pressure Ratio (OPR) ≤70 LPC pressure ratio [1.4, 5.0] FN [kN] =50 HPC pressure ratio [8, 20] Inlet mass ﬂow rate [kg/s] constant HPT inlet temperature (Tt4) [K] [1400, 1900] ITB energy fraction 0, 0.1, 0.2, 0.3

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Table 7

TheoptimizedhybridenginecycleincludingCBACSatcruisecondition. ITB energy fraction

0 0.1 0.2 0.3 Design parameters BPR 15 15 15 15 FPR 1.48 1.48 1.48 1.48 LPC pressure ratio 5 5 5 5 HPC pressure ratio 9.48 9.48 9.48 9.48 HPT inlet temperature [K] 1593 1521 1451 1387 LPT inlet temperature [K] 1177 1187 1200 1215 Engine performance Thermal efficiency [%] 52.5 51.7 50.7 49.7 Propulsive efficiency [%] 82.6 82.6 82.6 82.6 LNG mass flow rate [kg/s] 0.54 0.50 0.45 0.40 Kerosene mass flow rate [kg/s] 0 0.06 0.13 0.20 Bleed air temperature [K] 602 603 604 604 LNG temperature [K] 528 478 424 366

Fig. 12. Variationinfueltemperature(LNG)attheexitoftheexchangerversusITB energyfractionwithrespecttothenon-CBACSITBturbofanengineatthesameITB energyfraction.

themulti-fuelhybridengine arepresentedin thefollowing para-graphs.

ITB EF

=

m

˙

bio

×

L H Vker

˙

mLNG

×

LHVLNG

+ ˙

mker

×

LHVker

(3)

Theoptimalperformance ofthehybridengine atdifferentITB energy fractionsis presented in Table 7. As the ITBenergy frac-tionincreases, theHPTinlettemperaturereducesfrom1593Kto 1387 K. The LPT inlet temperatureof each engine cycle remains nearly constant at around 1200 K. The bleed air temperature is about600 K andis nearly constant regardlessof the ITB energy fraction(asthisisthedesignrequirementfortheheatexchanger). TheenginethermalefficiencydecreasesastheITBenergyfraction increases, as more heat is added at lower pressure in the cycle. Themassflowrateofkerosene increaseswithincreaseintheITB energyfractionwhereasthemassflowrateofLNGdecreases.

In orderto assess the impact ofCBACS onthe design perfor-mance of the hybrid engine, the engine performance in Table 7 iscomparedtotheengineswithmulti-fuelconﬁgurationsbut ex-cludingthe CBACS system. The variationin fueltemperature and the resultingturbine cooling requirementis presented in Fig. 12 andFig.13respectively.ComparedtotheoriginalLNGtemperature (120 K),thefueltemperatureattheexitoftheheatexchanger in-creasesby200 KattheITBenergyfractionof0.3andupto400 K attheITBenergyfractionof0(Fig.12). Also,theturbinecooling airrequirementcanbereducedbyaround45%,ascanbeobserved inFig.13becauseofthelowerbleedairtemperatureviaCBACS.

Fig. 13. VariationinturbinecoolingmassﬂowrateversusITBenergyfraction.The negativesignindicatesthereductionofcoolingmassﬂowratewithrespecttothe non-CBACSITBturbofanengineatthesameITBenergyfraction.

Table 8

Theoperatinglimitsofthehybridengine.

Variables Notation Value Description High pressure spool speed [%] N2 100 Max Low pressure spool speed [%] N1 100 Max Fan surge margin [%] SM_fan 10 Min LPC surge margin [%] SM_LPC 20 Min HPC surge margin [%] SM_HPC 25 Min HPC exit temperature [K] Tt3 1000 Max

HPT inlet temperature [K] Tt4 1800 Max

LPT inlet temperature [K] Tt46 1450 Max

Table 9

Enginecharacteristicsatsealevelstaticconditions. ITB energy fraction

0 0.1 0.2 0.3

FN [kN] 280 280 280 280

Flat rated temperature ISA-30 K ISA-3 K ISA+13 K ISA+18 k 6.2. Theveriﬁcationoftheoptimizedenginecycle

This section is focused on the veriﬁcation of the optimized engine cycle concerning the ITB energy fractions from 0 to 0.3. The analysisisperformedtoexamineifeachoftheenginecycles wouldmeettheperformancerequirementsatoff-designconditions elaborated inTable 3.The operatinglimitsinTable8 are consid-ered. The LPT inlet temperature (Tt46) is limited below 1450 K

suchthatnoLPTcoolingwouldberequired.

Theenginecharacteristicsatthesealevelstaticconditionscan be found in Table 9. We can notice that even though the de-signperformance of thesingle combustor engineis thebest, the enginefailstodeliverthethrustrequiredatsealevelstatic condi-tions.AstheenergyadditionintheITBincreases, theenginecore becomes morepowerful. Theoptimizedenginecanmeetthe con-straintswhiledeliveringthethrust.Thesameprocedurehasbeen performedtoexamineifeachenginecanmeettherequirementin Table3.Eventually,theenginedesignedwiththeITBenergy frac-tionof0.3wasselectedandwillbe usedforthemissionanalysis inthenextsection.

6.3. Evaluationofthehybridengineperformanceatcruiselevel Toevaluatetheperformanceoftheoptimizedhybridengine cy-cle,threebaselineengineslistedbelowareused.Theperformance oftheseengines atcruise condition isdisplayed inTable 10. The

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Table 10

Thebaselineengineperformanceatcruisecondition.

GE90-94Ba _GEnx-1B64 _GTF-2035 Design parameters BPR 8.1 9.1 15 FPR 1.65 1.65 1.44 OPR 40 41 70 Tt4 [K] 1380 1438 1900 Engine performance Thrust [kN] 69 54 50 TSFC [g/kN/s] 15.6 14.1 13.2 Kerosene [kg/s] 1.08 0.76 0.66

a _The_GE90-94B_engine_data_source_is:_https://web_.stanford_.edu_/~cantwell_/AA283_{_}

Course_Material/GE90_Engine_Data.pdf.

performanceoftheGTF-2035enginehasbeenoptimizedatcruise conditionforthesamethrustrequirementasthehybridengine.

•

GE90-94B,representinganenginefromtheyear2000

•

GEnx-1B64,representingthecurrentstateofart(SOA)engine technology

•

GTF-2035,animaginaryVeryHighBypassRatio(VHBR) turbo-fanenginefortheyear2035

Acomparisonhasbeenmadebetweenthehybridenginewith the ITBenergy fraction of 0.3 in Table 7 andthe three baseline engines presented in Table 10. A direct TSFC comparison is not possibleduetothedifferentenergysourcesusedintheMFHE. In-stead,theenginesarecomparedbyenergyconsumptionasdeﬁned inEqn.(4),

Energy consumption

= ˙

mfuel

·

LHVfuel (4)

where LHV is the Lower Heating Value of the given fuel type (42.8 MJ/kgforkerosene and50MJ/kgforLNG);m

˙

fuel isthefuel

mass ﬂow rate. The energy consumption of the hybrid engine wouldbetheenergysummationofkeroseneandLNG.

Thecomparisonsaremadefortheenergyconsumption,theCO2 emission,andtheH2Oemission(Fig.14).AscomparedtotheGE90 engine,thehybridenginescoreshigherinallthreeaspects.When comparedtotheGEnxengine,thehybridengineisabout12% en-ergy eﬃcient and is able to reduce the CO2 emission by about 27% due to LNG. The H2O emission is increased by about 20%. Whencompared to thefutureengine technology(GTF-2035), the hybridenginecanreducetheCO2 emissionsby17%atthecostof 1%higherenergyconsumption.TheH2Oemission,inthiscase,is about40%higher.

6.4.Performanceatmissionlevel

Thissection focuseson the performance of thehybrid engine withMFBWBaircraftforagivenmission.Eventhoughtheengine cycle optimization was conducted for different ITB energy frac-tions, afteriteration withthe MFBWB design process, thehybrid enginewithan ITBenergyfractionof0.3wasselected.Therefore, themissionanalysishasbeenmainlyconductedforanITBenergy fractionof0.3.Asimplifiedmissionprofile,consistingofflight con-ditionsattake-off,topofclimbandcruise,isused.Threecitypairs,

Fig. 14. ThevariationintheenergyconsumptionofthehybridLNG–kerosenehybrid enginetothreebaselineengines:GE90-94B,GEnx-1B64,andartiﬁcialGTF-2035.

representingdifferentﬂight distances,are presented.For compar-ison,long-rangeBoeing787-8andBoeing777-200ERareselected asbaseline aircraft,representingtheyear2000and2015 technol-ogyrespectively.

Tocalculateemissionsofthehybridengine,theemissionindex presented in Table 11 are used. The emission index of CO2 and H2O are calculated assuming a complete combustion process for LNG and kerosene. The NOx emission index has been calculated

usingadetailedchemicalreactionnetworkmodel[24],whichwas developed in the AHEAD project for emission predictions of the hybridengine.

ThePianoX[35],anaircraftperformanceanalysistool,isused togeneratemissionprofilesforB787andB777.Withthis calcula-tiontool,emissionsofanexistingaircraftforagivenflightmission can be predicted with fairly good accuracy. Assuming the same amount of payload and the same flight distance as used in Pi-ano XforBoeing777-200ERandBoeing787-8,theemissionsand energyconsumption of thehybrid engine are comparedwith re-spect to per unit payload andkilometer distance. Theresults for various aircraftare presentedin Tables 12–14. Inaddition to the hybridengine,thesuperioraerodynamiccharacteristicsofthe MF-BWBaircraftreducethethrustrequirementandthereforethefuel consumptionevenfurther.

ThecomparisonispresentedinFig.15andFig.16.InFig.15,it canbeobservedthatcomparedtoB777-200ER,theproposedLNG– keroseneMF-BWBaircraftcanreducetheNOx emissionsbymore

than80%,CO2by50%,andH2Oby20%.Thetotalenergy consump-tion islower by 40%forthe MFBWBaircraft.When compared to theB787-8,asimilartrendcanbeobservedasdepictedinFig.15. AnexceptionistheH2Oemission.Asthemissionrangedecreases, theLNG–keroseneBWBmightemitmoreH2OthanB787-8.

The reduction inCO2 emission is beneﬁcial formitigatingthe climateimpact,whereas,theeffectsoftheincreasedH2Oemission on the climate is more complicated. On the one hand, the wa-tervaporitselfisagreenhousegas;ontheotherhand,moreH2O mightincreasesthepossibilityofcontrailsformation,whichisan importelementintheclimateimpactofaviation.Thorough analy-sishasbeenconductedbytheAHEADteamtoevaluatetheoverall

Table 11

Emissionindexusedforthehybridengine.

Take-off TOC Cruise

1st combustor (LNG) ITB (kerosene) 1st combustor (LNG) ITB (kerosene) 1st combustor (LNG) ITB (kerosene)

EI NOx[g/kg] 8.25 14.52 3.5 8.8 2.1 7.4

EI CO2[kg/kg] 2.75 3.16 2.75 3.16 2.75 3.16

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Table 12

Theemissions andenergyconsumptionperpayload perunitdistance (SYD-DXB 12000km).

B777-200ER B787-8 LNG–kerosene BWB Energy consumption, kJ/payload/km 8.3 7.1 4.95 NOxemission, mg/payload/km 3.12 1.93 0.4

CO2emission, mg/payload/km 610.65 522.78 297.45

H2O emission, mg/payload/km 242.32 207.45 201.58

Table 13

Theemissions andenergyconsumptionperpayloadperunitdistance (AMS-EZE 11000 km).

H2O emission, mg/payload/km 222.8 191 194.3

Table 14

Theemissionsandenergyconsumptionperpayload perunitdistance (MAD-PVG 10000 km).

H2O emission, mg/payload/km 193.4 168.7 190.2

Fig. 15. Comparison of the MFBWB to B777-200ER.

climateimpactofthehybrid engine.Theresultsconﬁrm thatthe multi-fuel hybrid engine together with the MFBWB reduces the climateimpactby morethan20% comparedtothe contemporary aircrafttechnologylevel[36].

7. Conclusionsanddiscussions 7.1. Conclusions

Thispaperpresentstheperformance analysisofanovel multi-fuelhybridengine.Both on-designandoff-designperformance of thisnovelengineconcepthasbeeninvestigated.Following conclu-sionscanbedrawnfromtheresearchcarriedout:

•

The proposed engine architecture with inter-turbine burner allowstheusage oftwoenergysources (LNG andBiofuel) si-multaneously.

•

Byintroducingacryogenicbleedaircoolingsystem,thebleed airtemperaturecanbe reducedby 400 K,andtheLNG

tem-Fig. 16. Comparison of the MFBWB to B787-8.

peraturecanbeincreasedbymorethan200K.Asaresult,the turbinecoolingairmassﬂowratecanbedecreasedby45%.

•

Comparedtothecurrentstateoftheartturbofanengine,the hybridenginereducesenergyconsumptionbyaround12%and theCO2 emissionby27%.

•

ComparedtoB777-200ER,theMFBWBreducestheNOx emis-sionsbymorethan80%,CO2emissionby50%.

•

Compared to B787-8, the maximum reductions in the NOx emissions, CO2 emission, and the energy consumption are about 80%, 40%, and 30% respectively. However, there is a slightincreaseintheH2Oemission.

•

The hybrid engine along withMF-BWB aircraft paves a new approachofmakingaviationmoresustainable.

7.2. Discussions

Inthispaper,we haveanalyzedthedesignandoff-design per-formanceofthemulti-fuelhybridengineintheuninstalled condi-tion.The resultsprove thepotentialofMFHEconceptintermsof the emissionsreduction.The installation effectsofthe MFHE en-ginewithBLIsystemshould be investigatedfurtherasit hasthe potentialtoincreasethesystempropulsiveefficiency.However,the effectofBLIon theintakepressureratio,flow distortion,andfan efficiency should be taken into account while evaluating the in-stalledengineperformancewithBLI.

Conﬂictofintereststatement

Thereisnoconﬂictofinterest.

Acknowledgements

Theauthorswouldliketoacknowledgethesupportofall con-sortiummembersoftheAHEADproject.Furthermore,theauthors alsowouldliketothankK.G.Fohmannforhiscontributionsonthe heatexchangermodeling.

Funding

Researchleadingtotheseresultshasreceivedfundingfromthe European Union Seventh FrameworkProgramme (FP7/2007-2013) undergrantagreementNo.284636.

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