turbojet and turbofan engine performance increases · pdf...

JOURNAL OF PROPULSION AND POWER

Vol 17 No 3 MayndashJune 2001

Turbojet and Turbofan Engine PerformanceIncreases Through Turbine Burners

F Liu and W A Sirignanodagger

University of California Irvine California 92697-3975

In a conventional turbojet and turbofan engine fuel is burned in the main combustor before the heated high-pressure gas expands through the turbine A turbine-burner concept was proposed in a previous paper in whichcombustion is continued inside the turbine to increase the ef ciency and speci c thrust of the turbojet engineThis concept is extended to include not only continuous burning in the turbine but also ldquodiscreterdquo interstageturbine burners as an intermediate option A thermodynamic cycle analysis is performed to compare the relativeperformances of the conventional engine and the turbine-burner engine with different combustion options for bothturbojet and turbofan con gurations Turbine-burner engines are shown to provide signi cantly higher speci cthrust with no or only small increases in thrust speci c fuel consumption compared to conventional enginesTurbine-burner engines also widen the operational range of ight Mach number and compressor pressure ratioThe performance gain of turbine-burner engines over conventional engines increases with compressor pressureratio fan bypass ratio and ight Mach number

I Introduction

G AS turbine engine designers are attempting to increase thrust-to-weight ratio and to widen the thrust range of engine op-

eration especially for military engines One major consequence isthat the combustor residence time can become shorter than the timerequired to complete combustion Therefore combustion would oc-cur in the turbine passages which in general has been consideredto be undesirable A thermodynamic analysis for the turbojet en-gine by the authors12 showed however that signi cant bene t canresult from augmented burning in the turbine In summary it wasshown that augmented combustion in the turbine allows for 1) areduction in afterburner length and weight 2) reduction in speci cfuel consumption compared to the use of an afterburner and 3) in-crease in speci c thrust The increase in speci c thrust implies thatlarger thrust can be achieved with the same cross section or that thesame thrust can be achieved with smaller cross section (and there-fore smaller weight) For ground-based engines it was shown thatcombustion in the turbine coupled with heat regeneration dramat-ically increases both speci c power and thermal ef ciency It wasconcluded in Refs 1 and 2 that mixing and exothermic chemicalreaction in the turbine passages offer an opportunity for a majortechnological improvement Instead of the initial view that it is aproblem combustion in the turbine should be seen as an opportu-nity to improve performance and reduce weight Motivated by thisconcept Sirignano and Kim3 studied diffusion ames in an accel-erating mixing layer by using similarity solutions This study ofdiffusion ames is also extended to nonsimilar shear layers moreappropriate for the ow conditions in a turbine passage4

Burning fuel in a turbine rotor can be regarded as too dif cultfor an initial design by many experts although thermodynamicallyit is the most desirable process because it is possible to maintainconstant temperature burning in a rotor An alternative is to con-struct burners between turbine stages which conceivably can becombined with the turbine stators In other words we can redesignthe turbine stators (nozzles) to be combustors If many such turbinestages were constructed we would then approach the ldquocontinuousrdquo

Presented as Paper 2000-0741 at the 38th AIAA Aerospace SciencesMeeting Reno NV 10ndash13 January 2000 received 15 February 2000 revi-sion received 30 August 2000 accepted for publication 13 September 2000Copyright c 2000 by F Liu and W A Sirignano Published by the AmericanInstitute of Aeronautics and Astronautics Inc with permission

Associate Professor Mechanical and Aerospace Engineering SeniorMember AIAA

daggerProfessor Mechanical and Aerospace Engineering Fellow AIAA

turbine burner concept discussed in Refs 1 and 2 There is a needto quantify the relative performance gains of such ldquodiscreterdquo tur-bine burners relative to the conventional engine and the continuousturbine-burner engines when only a small number of such inter-stage turbine burners are used In this paper we extend our stud-ies in Ref 2 on the continuous turbine-burner option for turbojetsto include discrete turbine burners for both turbojet and turbofanengines Performance comparisons are presented among the con-ventional turbojet and turbofan engines with or without afterburn-ers engines with discrete interstage burners and engines with con-tinuous turbine burners It will be shown that signi cant gains inperformance are achievable with the interstage turbine-burner op-tion for both turbojet and turbofan engines for a range of ightand design conditions The large margin in performance gains war-rants further research in this direction for future high-performanceengines

II Continuous Turbine-Burner andInter-Turbine-Burner Engines

Figure 1 shows the basic idealized con guration of a turbo-jetturbofan engine with the corresponding T -s diagrams of the coreengine and the fan bypass Air from the far upstream (statea) comesinto the inletdiffuser and is compressed to state 02 (the 0 here de-notes stagnation state) before it splits into the core engine and thefan bypass streams The stream that goes into the core engine isfurther compressed by the compressor which usually consists of alow-pressure (LP) portion and a high-pressure (HP) portion on con-centric spools to state 03 before going into the conventional mainburner where heat Qb is added to increase the ow temperature toT04 In a conventional con guration (dashed line in Fig 1) the HPhigh-temperature gas expands through the turbine which providesenough power to drive the compressor and fan and other engineauxiliaries To further increase the thrust level fuel may be injectedand burned in the optional afterburner to increase further the tem-perature of the gas before the ow expands through the nozzle toproduce the high-speed jet For a turbofan engine part of the owthat comes into the inlet is diverted to the fan bypass The pressureof the bypass ow is increased through the fan The ow state afterthe fan is marked as 03f An optional duct burner behind the fan canalso be used to increase the thrust The ow then expands throughthe bypass nozzle into the atmosphere or mixes with the ow fromthe core engine before expanding through a common nozzle

Speci c thrust (ST) and thrust speci c fuel consumption rate(TSFC) are two fundamental performance measures for a jet engine

695

696 LIU AND SIRIGNANO

Fig 1 Comparison of thermodynamic cycles with and without theturbine burner

Fig 2 Desired ST and TSFC range

designed to produce thrust Speci c thrust is de ned as the thrustper unit mass ux of air

ST ma (1)

For a turbojet engine ma is the air mass- ow rate of the whole engineFor a turbofan engine the de nition of ma used in this paper isthe air mass- ow rate of the core engine The total air mass- owrate of the engine is then (1 b )ma where b is the bypass ratio ofthe fan A higher ST means a higher thrust level for the same enginecross section and thus smaller engine size and lighter weight Thrustspeci c fuel consumption rate is de ned as the fuel ow rate perunit thrust

TSFC m f (2)

where m f is the total fuel mass- ow rate for the complete engine Alower TSFC means less fuel consumption for the same thrust levelUnfortunately the two performance parameters trade off with eachother It is dif cult to design an engine that has both low TSFC andhigh ST On a TSFC-ST map as shown in Fig 2 it is desirableto move the design point of an engine toward the right-hand sideIn the discussion to follow where we study the performances ofthe various engines we will plot the TSFC-ST loci of the enginesas we vary a design parameter such as the compressor pressureratio or the ight Mach number to show that the proposed turbine-burner engines indeed move in the desired direction compared tothe conventional engines

The TSFC is determined by the thermal and propulsion ef cien-cies g t and g p respectively The thermal ef ciency is de ned as

g t(KE)gain

m f Q R

KE of exhaust gas KE of inlet air

Q R m f

(3)

where KE stands for kinetic energy and Q R is the heat content ofthe fuel The propulsion ef ciency is de ned as

g p u (KE)gain (4)

where u is the ight velocity Therefore the thermal ef ciency in-dicates how ef cient the engine is converting heat to kinetic energyas a gas generator The propulsion ef ciency tells us how ef cientthe engine is using the kinetic energy generated by the gas generatorfor propulsion purposes An overall ef ciency g 0 is then de ned as

g 0 g t g p (5)

For a given ight speed and fuel type it is clear from Eqs (2ndash4) that

TSFC 1 g 0 (1 g t )(1 g p ) (6)

Any increase in thermal ef ciency or propulsion ef ciency will bringdown the fuel consumption rate The turbofan engine signi cantlyincreases the propulsion ef ciency over its turbojet counterpart asa result of increased mass of the air ow For the same momentumgain the kinetic energy carried away by the exhaust air (including thebypass ow) is less in the turbofan engine case than in the turbojetcase In the following sections we will plot and compare the thermalpropulsion and overall ef ciencies of the different engine con g-urations Examination of these parameters helps us understand thestrengths and weaknesses of the different engine types

In the conventional turbojet con guration the basic ideal thermo-dynamic cycle is the Brayton cycle for which the thermal ef ciencyis known to be

g t 1 (1 p c )( c 1) c 1 1 [( c 1)2]M2 (7)

where p c is the compressor pressure ratio and M is the ightMach number To increase ef ciency (ie decrease TSFC) onehas to increase the pressure ratio However higher pressure ratioincreases the total temperature of the ow entering the combustorand therefore limits the amount of heat that can be added in thecombustor because of the maximum turbine inlet temperature limitAs such the conventional base turbojetturbofan engine is limitedin the maximum value of speci c thrust The afterburner increasesthe power levels of the engine but because fuel is burned at a lowerpressure compared to the main burner pressure the overall cycleef ciency is reduced Therefore the use of an afterburner increasesthe speci c thrust at the expense of the fuel consumption rate Aduct burner in the fan bypass has the same effect except that it haseven lower ef ciencies because the pressure ratio in the fan bypassis usually lower than that in the core engine

To remedy the ef ciency decrease because of the use of after-burner the turbine-burner was conceived12 in which one adds heatin the turbine where the pressure levels are higher than in the af-terburner The ideal turbine-burner option is to add heat to the owwhile it does work to the rotor at the same time so that the stagna-tion temperature in the turbine stays constant By doing this Ref 2showed that the contention between ST and TSFC can be signi -cantly relieved compared to the conventional engine It was shownthat signi cant gain in ST can be obtained for a turbojet engine with asmall increase in fuel consumption by using the turbine-burner con-cept Figure 3a shows the continuous turbine-burner cycle whichwe denote as the CTB cycle

However it is yet practically dif cult to perform combustion inthe turbine rotor at the present time An alternative for a rst designis to convert the turbine stators or nozzles into also combustorsand therefore effectively introduce inter-stage turbine burners (ITB)without increasing the engine length We may have one two ormore such ITB as shown in Figs 3bndash3d We denote such cycles asthe M-ITB cycles with M being the number of interstage burnersObviously as M goes to in nity the M-ITB cycle approaches thecontinuous turbine-burn cycle that is the CTB cycle

In this paper we extend our cycle analysis in Ref 2 to calcu-late the preceding M-ITB cycles and also for turbofan engines We

LIU AND SIRIGNANO 697

Fig 3 TB and the M-ITB cycles

a)

b)

c)

d)

e)

f)

Fig 4 Performances of turbojet engines vs compressor pressure ratio at M1 = 2 T04 = 1500 K and T06 = 1900 K

compare engine performances at different ight Mach numbers andcompressor compression ratios for a number of engine con gura-tions including the conventional base-engine con guration with orwithout an afterburner the 1-ITB engine the 2-ITB engine andthe CTB engine con gurations Both the turbojet and the turbofanoptions are considered The basic de nitions of design and perfor-mance parameters and the method of analysis used in this paperclosely follow those in the book by Hill and Peterson5 Details ofthe computational equations and the component ef ciencies of thecore engine used in the computation are listed in Ref 2 For the caseof turbofan engines the fan adiabatic ef ciency is taken to be 088The fan nozzle ef ciency is taken to be 097

III Variation of Compressor Pressure RatiosA Supersonic Flight

Let us rst consider turbojet engines at supersonic ight whichare probably more relevant for military engines Figure 4 shows per-formance comparisons among the different con gurations for vary-ing pressure ratios at ight Mach number M 20 with maximumallowable turbine inlet temperature T04 1500 K and maximum af-terburner temperature T06 1900 K The turbine power ratios are


xed at 40 60 and 33 33 34 for the 1-ITB and 2-ITB enginesrespectively

Figures 4andash4f show the comparisons of the ST TSFC thermalef ciency g t propulsion ef ciency g p overall ef ciency g 0 andthe ST-TSFC map The speci c thrust of the base conventional en-gine drops quickly as the compression ratio increases for the reasondiscussed in the preceding section that the increased compressionraises the temperature of the gas entering the main combustor andtherefore reduces the amount of heat that can be added Althoughthe TSFC decreases initially it increases rapidly at higher pressureratios when the total amount of heat that can be added to the systemis reduced to such a low level that it is only enough to overcome thelosses as a result of nonideal component ef ciencies This is alsoseen from the thermal ef ciency plot shown in Fig 4c The thermalef ciency of the base engine rst increases as the pressure ratio in-creases but then quickly drops as the pressure ratio goes beyond 20

The propulsion ef ciency of the base engine does increase as thepressure ratio increases However the engine is really not producingmuch thrust at high compression ratios The exhaust velocity of thejet approaches the incoming ow of the engine as the pressure ratioincreases In the limit the jet velocity becomes the same as that ofthe incoming ow producing zero thrust although the propulsionef ciency becomes one The overall ef ciency of the engine stillgoes to zero because of the decreased thermal ef ciency

Adding an afterburner to the base engine dramatically increasesthe speci c thrust but it also increases the TSFC tremendously Thereason for this can be clearly seen from Figs 4c and 4d The af-terburner reduces both the thermal and the propulsion ef cienciesConsequently the base engine with the afterburner has the low-est overall ef ciency and thus the highest fuel consumption rateThe thermal ef ciency is reduced because fuel is added at a lowerpressure in the afterburner Reduction of propulsion ef ciency is aconcomitant to the increase of speci c thrust in a given engine con- guration for the increase of thrust is brought about by increasingthe jet velocity Clearly the afterburner design is really a very poorconcept to remedy the de ciency of low speci c thrust for the baseturbojet engine

As shown in Ref 2 the continuous turbine burner concept pro-vides a much more ef cient and effective way to increase the speci cthrust of a conventional engine The turbine-burner the 1-ITB andthe 2-ITB engines all provide signi cantly greater ST with mini-mum increase in TSFC as shown in Figs 4andash4f The various types ofturbine burners add heat at higher pressure ratios compared to the af-terburner Therefore they have higher thermal ef ciencies (Fig 4c)In addition the thermal ef ciency of such engines increases withpressure ratio except that of the 1-ITB engine which showed onlya slight decrease at high pressure ratios This is because unlike theconventional base engine the amount of heat added to the varioustypes of turbine-burner engines is not limited by the compressorcompression ratio because we are able to add heat in the turbinestages without going beyond the maximum turbine temperatureTherefore the speci c thrust of the turbine-burner engines remainshigh at higher pressure ratios

The CTB version apparently provides the highest speci c thrustas a result of the continuous combustion in the turbine It also hasthe highest thermal ef ciency The essential advantage of a CTBengine is that it eliminates the limit on the amount of heat that canbe added to the engine and thus the limit on the speci c thrust of theengine without sacri cing the thermal ef ciency The CTB enginewith an afterburner further increases the speci c thrust but at the costof higher speci c fuel consumption as a result of the lower thermaland propulsion ef ciencies of the added afterburner Therefore theaddition of an afterburner on top of a turbine-burner type of engineis redundant and inef cient

The CTB engine has both the highest thermal ef ciency and thehighest speci c thrust of all nonafterburner engines at all pressurelevels Unfortunately the higher speci c thrust level of the CTBengine inevitably incurs a lower propulsion ef ciency (Fig 4d) be-cause of the high-exhaust jet velocity needed for the higher speci cthrust as mentioned in the preceding paragraph Therefore the over-all ef ciency and the speci c fuel consumption is relatively higher

than the base engine However the speci c fuel consumption rate ofthe CTB engine is signi cantly lower than the base engine with anafterburner whereas the CTB engine provides comparable or evenhigher speci c thrust than the base engine with the afterburner forthe speci ed maximum afterburner temperature T06 1900 K Formilitary engines that may allow a higher T06 the base engine withan afterburner may have higher thrust but then again would havemuch lower fuel ef ciency because the extra heat would be added atthe lowest pressure This shows that the CTB engine is a far betterchoice than the afterburner engine for missions that require veryhigh speci c thrust

For missions of median speci c thrust levels the 1-ITB and 2-ITBengines provide a means of controlling the total amount of heataddition and thus the speci c thrust because combustion is limitedto only part of the turbine stages The thermal ef ciencies of the1-ITB and 2-ITB engines are at the same level of the CTB enginealthough slightly lower at HP ratios (Fig 4c) The controlled thrustlevels however offer us the bene t of higher propulsion ef cienciescompared to the CTB and CTB with afterburner versions as can beseen in Fig 4d Consequently the 1-ITB and 2-ITB engines providebetter overall ef ciency and thus better fuel economy as shown inFigs 4e and 4b Comparing Fig 4f with Fig 2 we see that all ofthese turbine-burner and inter-turbine-burner engines are moving inthe desirable direction on the TSFC-ST map

As the number of ITBs increase the engine performance ap-proaches that of the CTB engine With the CTB and ITB enginesthe ST is almost independent of compression ratio At high com-pression ratios the 2-ITB and the CTB engines provide the sameST levels of an afterburner type of engine but with much less fuelconsumption rate The 1-ITB 2-ITB and potentially the M -ITBengines t nicely in between the base engine and the CTB en-gine designs because they maintain the high thermal ef ciencyof the CTB engine while keeping a good balance of the speci cthrust and the propulsion ef ciency It is to our advantage to con-trol the number of ITBs or the amount of fuel added in the TBto reach the best balance of TSFC and ST for a given missionrequirement

B Subsonic Flight

Next the same types of engines are examined for subsonic ightFigures 5andash5f show the performance parameters at M 087Clearly the relative standing of the various engine types remainthe same as in the supersonic case Therefore the same discussionsin the preceding paragraph apply However we do notice that atlower ight Mach numbers the base engine is capable of operatingat higher compressor pressure ratios than in the supersonic ightcase because of the lower ram pressure rise as a result of ightspeed In the M 2 case the base engine ceases to produce anythrust at compressor pressure ratios beyond 55 because at that timethe overall compression of the incoming ow brings the main com-bustion inlet temperature to a high level that no heat can be addedto the engine without exceeding the 1500 K maximum turbine inlettemperature limit The turbine-burner type of engines however canstill operate at this or even higher compression ratios because heatcan still be added in the turbine burner or inter-state turbine burners

The most signi cant difference between the subsonic and super-sonic cases is in the propulsion ef ciencies It is well known thatturbojets lose propulsion ef ciency at low ight Mach numbersFigure 5d shows that the propulsion ef ciencies of all of the en-gines become smaller at M 087 than at M 2 This is under-standable because we know that the turbine-burner engines producemuch higher thrusts than the base engine but to produce the higherthrust level at the low ight M penalizes these engines becausepure turbojets have low propulsion ef ciency at low ight Machnumbers

C Turbofan Con guration

The turbofan con guration offers a resolution for the low propul-sion ef ciency of the turbojets by imparting momentum to a fanbypass ow In this way the average exhaust velocity of the coreengine and the fan becomes low for the same thrust because of the


a)

b)

c)

d)

e)

f)

Fig 5 Performances of turbojet engines vs compressor pressure ratio at M 1 = 087 T04 = 1500 K and T06 = 1900 K

increased total amount of propulsive mass of air The added fanbypass that increases the total air ow however does not signi -cantly increase the engine weight and size (except for the addedfan duct) because the core engine remains almost the same Theturbine-burner engine type as a gas generator is capable of produc-ing much greater power than the base engine type because of itshigher thermal ef ciency and capacity of greater heat addition Itis best for the turbine-burner type of engines to be used in a con- guration in which this high power can be utilized most ef cientlyto produce thrust It is expected that the turbine burners combinedwith a high-bypass turbofan engine will give the best combinationof both high thermal and propulsion ef ciencies while at the sametime high speci c thrust

Figures 6 and 7 show performance comparisons for turbofan en-gines with a bypass ratio of 5 and 8 respectively at ight Machnumber M 087 The fan pressure ratio is 165 for both casesThe relative standings of the various engines are the same as beforeHowever we notice that the propulsion ef ciencies of all of the en-gines increase compared to the turbojet counterparts The turbine-burner types of engines bene t more from the fan bypass ow thanthe base engine type This can be seen by comparing Fig 6f toFig 5f Compared to the turbojet engine case shown by Fig 5f theturbine-burner type of engines show a more desirable trend in the

TSFC-ST map because they move more in the high ST directionwith less increase in TSFC This trend is further enhanced when weincrease the bypass ratio from 5 to 8 as shown in Fig 7f At a pres-sure ratio of 70 or higher the 1-ITB engine is able to produce around30 more speci c thrust with almost the same fuel consumptionrate of the base engine at its optimum pressure ratio in the rangeof 30 to 40 as can be seen in Figs 7a and 7b At pressure ratiosbeyond 40 the base engine starts to suffer from large decreases ofspeci c thrust At pressure ratios over 70 the core engine of thebase engines with or without the afterburner starts to yield negativethrust because it cannot produce enough power to drive the fan Forthis reason no points are plotted for the base engines for compressorpressure ratios over 70 in Fig 7

IV Variation of Fan Bypass RatiosWe argued in the preceding section that it is best to maximize the

propulsion ef ciency in order to make use of the high energy gasproduced at high thermal ef ciency by the core gas generator It isthen useful to see the effect of bypass ratio of a turbofan engine asa design parameter in more detail Figure 8 shows the performanceparameters vs the bypass ratios at a compression ratio of 40 Al-though these curves now take different shapes from curves in the


a)

b)

c)

d)

e)

f)

Fig 6 Performances of turbofan engines vs compressor pressure ratio at M1 = 087 T04 = 1500 K T06 = 1900 K macr = 5 and frac14f =165

preceding section on the variation of pressure ratios they still showthe same relative standing of the various engines To be noticedhowever is that the speci c thrust gain by the turbine-burner en-gines over the base engine widens signi cantly as the bypass ratioincreases (Fig 8a) while the speci c fuel consumption rate de-creases to approach the level of the base engine (Fig 8b) This is aclear indication that the turbine-burner engines bene t more fromthe increased bypass ratio than the base engine con rming our pre-ceding discussions In addition we notice that the base engines stopproducing positive thrust for bypass ratios over 10 The large-bypass ow drains all of the power from the core engine in such situationsThe turbine-burner type of engines is capable of operating with amuch larger-bypass ratio with decreasing fuel consumption rate andno sign of decreased speci c thrust In fact the 1-ITB engine appearsto operate optimally with a bypass ratio of 13 With that bypass ratiothe 1-ITB engine produces more than 50 thrust with no more than10 increase in fuel consumption rate than the base engine with itsoptimal bypass ratio around eight

V Variation of Fan Pressure RatiosIt is clear from the preceding discussions that the fan bypass

ow improves the ef ciencies of the turbine-burner engines morethan those for the base engines Another way to increase the en-

ergy supply to the bypass ow is to increase the fan pressure ratioFigure 9 shows the performance comparisons when the fan pressureratio is varied from 11 to 19 for a bypass ratio of 8 Other param-eters are the same as before The relative standing of the enginesfor all of the performance parameters remain the same as beforeFigures 9a and 9b show that the turbine-burner engines gain morefrom increased fan pressure ratio than the base engines The speci cthrusts of the turbine-burner engines increase with fan pressure ra-tio at faster rates than those for the base engines while the speci cfuel consumption rates continue to decrease without leveling off asquickly as those for the base engines

VI Variation of Flight Mach NumberFigure 10 shows the performance comparisons for the various

turbojet engines with ight Mach number in the range of 0 to 25One can see that all of the engine types exhibit decrease in speci cthrust and increase in speci c fuel consumption rates as the ightMach number increases However the turbine-burner engines showa slower decrease in speci c thrust than the base engines Further-more the turbine-burner engines are capable of operating at higher ight Mach numbers than the base engines extending the operationenvelop


a)

b)

c)

d)

e)

f)

Fig 7 Performances of turbofan engines vs compressor pressure ratio at M1 = 087 T04 = 1500 K T06 = 1900 K macr = 8 and frac14f = 165

Figure 11 shows the performance comparisons for the various tur-bofan engines of bypass ratio of 8 with ight Mach number in therange of 0 to 25 The behavior of the turbofan engines at different ight Mach numbers are qualitatively similar to those of the turbo-jets However the conventional base engine does not operate well atall for supersonic ight whereas the turbine-burner engine contin-ues to operate well in supersonic ight range However we note thatthis is on the assumption that the aerodynamic performance of thefan does not deteriorate at supersonic speed This may be dif cultwith current fan technology for high bypass ratios

VII Variation of Turbine Inlet TemperatureOne can argue that the advantages of the turbine burner CTB

or ITB may be eroded when the turbine inlet temperature canbe raised by improved cooling andor high-temperature materialsFigures 12 and 13 show the performance of the turbojet engines ying at M 2 and the turbofan engines ying at M 087respectively for different turbine inlet temperatures from 1400 to1800 K Except for the fact that the base engines do not performwell at low turbine inlet temperatures the curves for the differentengines are almost parallel with exactly the same relative standingsthroughout the inlet temperature range This clearly indicates that

the turbine-burner engines bene t equally from higher turbine inlettemperatures as the base engines do The general trends are thatraising the turbine inlet temperature increases the speci c thrust ofthe engines with a small increase in fuel consumption rate There-fore we should not view the development of the turbine-burnerengine technology being inconsistent with the push to increase theturbine inlet temperature An increase in turbine inlet temperaturewill equally improve the performances of both the base engine typeand the turbine-burner engine type

VIII Variation of Turbine Power RatiosFor the 1-ITB and the 2-ITB engines an important design con-

sideration is the distribution of turbine power among the segmentsof turbines separated by the interturbine burners For instance the1-ITB design separates the complete turbine into two segments withthe ITB in the middle A natural location of the ITB would be be-tween the conventional HP turbine and the LP turbine although itdoes not have to be In addition to considerations governed by otherdesign constraints the choice of the power distribution of the HPand LP turbines becomes an important cycle parameter in the caseof the 1-ITB engine because heat is now added in between the twoturbines which can signi cantly affect the thermal ef ciency of the


a)

b)

Fig 8 Performances of turbofan engines vs fan bypass ratio at M 1 =087 T04 = 1500 K T06 = 1900 K frac14c = 40 and frac14f = 165

a)

b)

Fig 9 Performances of turbofan engines vs fan pressure ratio at M1 =087 T04 = 1500 K T06 = 1900 K frac14c = 40 and macr = 8

a)

b)

Fig 10 Performances of turbojet engines vs ight Mach numberT04 = 1500 K T06 = 1900 K and frac14c = 40

a)

b)

Fig 11 Performances of turbofan engines vs ight Mach numberT04 = 1500 K T06 = 1900 K frac14c = 40 macr = 8 and frac14f = 165


a)

b)

Fig 12 Performances of turbojet engines vs turbine inlet temperatureat M 1 = 2 T06 = 1900 K and frac14c = 40

engine In a particular design of a multiple ITB engine the powerdistribution among the segments of turbines that sandwich the ITBsshould be optimized for a given mission In this section we consideronly the 1-ITB engine because it has only one parameter to vary andyet it shows the essence of the physical signi cance of the powerdistribution

Consider the 1-ITB turbojet con guration at a ight Mach num-ber 2 with a compressor compression ratio of 30 and a maximumturbine inlet temperature T04 1500 K For convenience we willcall the turbine section before the ITB to be the HP turbine andthe section behind the ITB the LP turbine although they may notnecessarily correspond to the conventional de nition of the HP andLP turbines as just mentioned We de ne a power ratio of the HPturbine n as the ratio of the power of the HP turbine to the com-bined total power of the HP and LP turbines Figure 14 shows theperformance parameters as we vary n from 0 to 1 as compared tothe conventional engine with and without an afterburner the 2-ITBengine with a xed 33 33 34 power ratio for its three segmentsof turbines and the CTB engine The performances of all of theseengines except the 1-ITB engine are shown as straight lines in the gure because they are independent of n For the 1-ITB enginehowever the performance parameters vary with n

When n is small it means that we have a low-power HP (theITB is very close to the front of the turbine) The temperature dropafter the HP is small and thus the amount of heat we can add inthe ITB is limited for a given maximum inlet temperature of the LPturbine We therefore expect the performance of the 1-ITB engine toapproach that of the conventional base engine when n is small Inthe limit when n 0 the 1-ITB engine becomes the conventionalbase engine because the amount of heat added to the ITB is zeroFigure 14 shows that the performance of the 1-ITB engine falls righton top of the conventional base engine for n 0 As n increasesboth the ST and the TSFC increases However the ST rises muchfaster than the TSFC as shown in Figs 14a and 14b In fact the STincreases signi cantly with little increase in TSFC for small n Onthe TSFC-ST map this is shown by almost a straight line movingto the right with only a small upward motion This very desirable

a)

b)

Fig 13 Performances of turbofan engines vs turbine inlet temperatureat M 1 = 087 T06 = 1900 K frac14c = 40 macr = 8 and frac14f = 165

change is a result of the signi cant increase of the thermal ef ciencyof the 1-ITB engine as n increases from 0 shown in Fig 14c Themaximum thermal ef ciency is reached at about n 040 Beyondthat g t begins to decrease This is because at higher n the pressuredrop across the HP turbine is high The ITB will then operate atlower pressure reducing the thermal ef ciency of the cycle In factwhen n goes to 1 the 1-ITB engine becomes a conventional turbojetwith an afterburner of temperature T06 T04 This is clearly shownin Fig 14 where the performance of the conventional turbojet withthe afterburner is calculated with a T06 T04 1500 K

The preceding discussions show the physical signi cance of thepower distribution of the turbine segments in a multiple ITB designIt should be optimized for each particular mission and choice ofcon guration In the rest of the discussions however we will restrictto a n 04 for the 1-ITB engine which incidentally is close to theoptimum for the preceding turbojet example For the 2-ITB enginewe will still use the 33 33 34 distribution

IX Optimal Design Comparisons of the Base Enginesand Turbine-Burner Engine

We have compared performances of the various engine con gu-rations with identical design parameters and ight conditions Thestudies indicate that the turbine-burner engine offers advantagesover their base engine counterparts for all of the design parame-ters just considered It is clear however from the preceding studiesthat the base engines and the turbine-burner engines do not operateoptimally for the same set of design parameters and ight condi-tions The turbine-burner engines obviously favor higher compres-sor pressure ratios higher bypass ow rate and higher fan pressureratios Therefore it is appropriate that we design our turbine-burnerengines to operate at their favorable conditions and compare theirperformances to those of the base engines also operating with theiroptimal design parameter ranges Short of doing any systematic op-timization we compare the two kinds of engines operating at theirown favorable conditions deduced from the single parameter studiesobtained from preceding sections


a)

b)

c)

d)

Fig 14 Variation of turbine power ratio for the 1-ITB turbojet engine at M 1 = 20 T06 = T04 = 1500 K frac14c = 30 the 2-ITB engine is with xed33 33 34 power ratio

First consider the turbojet engines For the conventional engineswe choose the following design parameters compressor pressureratio p c 30 turbine inlet temperature T04 1500 K and after-burner temperature T06 1900 K For the turbine-burner engines wechoose p c 60 Other parameters are the same The performancesof these engines are then plotted in Fig 15 These gures show thatthe turbine-burner engines are clearly superior to their base enginecounterparts The percentage gain in speci c thrust of the 1-ITBengine over the base engine increases from about 10 to about 30as the ight Mach number increases from 0 to 2 The speci c fuelconsumption rate is almost identical for subsonic ight and onlybegins to increase slightly over that for the base engine when the ight Mach number exceeds 1 At Mach number 2 the 1-ITB engineis capable of producing 30 more thrust while incurring less than5 increase in speci c fuel consumption rate the CTB engine iscapable of producing 166 more thrust with only 18 increase inspeci c fuel consumption rate compared to the base engine The2-ITB engine falls in between the 1-ITB and the CTB engines inperformance

Next consider the turbofan engines We choose p c 30 p f

165 and b 8 for the conventional engines and p c 60 p f

175 and b 12 for the turbine-burner engines Figure 16 show theperformance of these engines vs ight Mach number These guresagain show that the turbine-burner engines are far superior to theirbase engine counterparts Now the 1-ITB engine provides about50 increase in speci c thrust than the base engine even morethan the thrust of the base engine with the afterburner whereas itsspeci c fuel consumption rate is lower than or equal to that of thebase engine without the afterburner for the entire subsonic ightrange At Mach number 1 the 2-ITB engine produces 80 morespeci c thrust while incurring only about 10 increase in speci cfuel consumption rate the CTB engine is capableof producing120more thrust with about 15 increase in speci c fuel consumptionrate compared to the base engine More signi cantly the 2-ITB andthe CTB engines are capable of operating over the entire 0 to 2 ight

a)

b)

Fig 15 Performance comparisons for turbojet engines vs ight Machnumber T04 = 1500 K and T06 = 1900 K frac14c = 30 for the conventionalengines frac14c = 60 for the turbine-burner engines


a)

b)

Fig 16 Performance comparisons for turbofan engines vs ight Machnumber T04 = 1500 K and T06 = 1900 K frac14c = 30 frac14f = 165 and macr =8 for the conventional engines frac14c = 60 frac14f = 175 and macr = 12 for theturbine-burner engines

Mach range The conventional base engines cannot operate beyondMach 125

With this type of improvement in performance it is certainlyworthwhile to face the added challenge of designing a CTB or ITBwith ef cient high-bypass and high-speed fans and high-pressurecompressors to complement the CTB and ITB designs

X ConclusionsA thermal analysis has shown the advantages of CTB and dis-

crete ITB engines for both the turbojet and turbofan con gurationsBurning in the turbine passages reduces the tradeoff between STand TSFC It allows signi cant increases in ST with only smallincreases in TSFC The fundamental bene t of a CTB or ITB en-gine is to be able to produce gases of high kinetic energy at highthermal ef ciency providing a very desirable gas generator as thebasis of high-performance engines applicable to both military andcommercial applications

The parametric studies in this paper have shown the following1) The turbine-burner engines are capable of and favor operations

at high compressor pressure ratios Although conventional enginesmay have an optimal compressor pressure ratio between 30ndash40 forsupersonic ight beyond which they have diminished thrust andvery high TSFC the turbine-burner engines are capable of oper-ating at pressure ratios higher then 60 Increased compressor ra-tios generally increase ST and decrease TSFC of the turbine-burnerengines

2) Because of the extended compressor pressure ratio range andalso of the fact that the propulsion ef ciency improves at highspeed for jet propulsion the performances of the turbine-burner en-gines are signi cantly superior at high ight speed to conventionalengines

3) The turbine-burner engines bene t more from ef cient large-bypass fans than the conventional engines The bypass pressure ratiocan be optimized for a given mission To combine the bene t of boththe high-speed ight and high bypass ef cient high-speed fans mustbe designed

4) The turbine-burner engines bene t equally well asconventionalengines do from high-turbine inlet temperatures that may result fromdevelopment of new materials andor turbine cooling technologies

5) The power distribution among the segments of an ITB engineshould be optimized for a given mission and a given con gurationThere exists an optimal distribution for the best thermal ef ciency

When a turbine-burner engine is designed by taking advantageof the preceding features potential performance increases are ex-tremely high compared to the conventional engines For the turbofanexample studied in Sec IX of this paper the 1-ITB engine providesmore than 50 increase in ST with equal or lower TSFC over theconventional base turbofan engine

AcknowledgmentsThe authors thank the National Science Foundation for its support

under Grant number CTS-9714930 The Grant manager is F FisherDiscussions with Mel Roquemore and his colleagues at the Air ForceAeronautical Laboratories have been very helpful

References1Sirignano W A Delplanque J P and Liu F ldquoSelected Challenges in

Jet and Rocket Engine Combustion Researchrdquo AIAA Paper-97-2701 July1997

2Sirignano W A and Liu F ldquoPerformance Increases for Gas TurbineEngines Through Combustion Inside the Turbinerdquo Journal of Propulsionand Power Vol 15 No 1 1999 pp 111ndash118

3Sirignano W A and Kim I ldquoDiffusion Flame in a Two-Dimensional Accelerating Mixing Layerrdquo Physics of Fluids Vol 9 No 9 1997 pp2617ndash2630

4Fang X Liu F and Sirignano W A ldquoIgnition and Flame Studiesfor an Aceelerating Transonic Mixing Layerrdquo AIAA Paper 2000ndash0437 Jan2000

5Hill P G and Peterson C R Mechanics and Thermodynamics ofPropulsion 2nd ed Addison Wesley Longman Reading MA 1992 pp141ndash216


Fig 1 Comparison of thermodynamic cycles with and without theturbine burner

Fig 2 Desired ST and TSFC range

designed to produce thrust Speci c thrust is de ned as the thrustper unit mass ux of air

ST ma (1)

For a turbojet engine ma is the air mass- ow rate of the whole engineFor a turbofan engine the de nition of ma used in this paper isthe air mass- ow rate of the core engine The total air mass- owrate of the engine is then (1 b )ma where b is the bypass ratio ofthe fan A higher ST means a higher thrust level for the same enginecross section and thus smaller engine size and lighter weight Thrustspeci c fuel consumption rate is de ned as the fuel ow rate perunit thrust

TSFC m f (2)

where m f is the total fuel mass- ow rate for the complete engine Alower TSFC means less fuel consumption for the same thrust levelUnfortunately the two performance parameters trade off with eachother It is dif cult to design an engine that has both low TSFC andhigh ST On a TSFC-ST map as shown in Fig 2 it is desirableto move the design point of an engine toward the right-hand sideIn the discussion to follow where we study the performances ofthe various engines we will plot the TSFC-ST loci of the enginesas we vary a design parameter such as the compressor pressureratio or the ight Mach number to show that the proposed turbine-burner engines indeed move in the desired direction compared tothe conventional engines

The TSFC is determined by the thermal and propulsion ef cien-cies g t and g p respectively The thermal ef ciency is de ned as

g t(KE)gain

m f Q R

KE of exhaust gas KE of inlet air

Q R m f

(3)

where KE stands for kinetic energy and Q R is the heat content ofthe fuel The propulsion ef ciency is de ned as

g p u (KE)gain (4)

where u is the ight velocity Therefore the thermal ef ciency in-dicates how ef cient the engine is converting heat to kinetic energyas a gas generator The propulsion ef ciency tells us how ef cientthe engine is using the kinetic energy generated by the gas generatorfor propulsion purposes An overall ef ciency g 0 is then de ned as

g 0 g t g p (5)

For a given ight speed and fuel type it is clear from Eqs (2ndash4) that

TSFC 1 g 0 (1 g t )(1 g p ) (6)

Any increase in thermal ef ciency or propulsion ef ciency will bringdown the fuel consumption rate The turbofan engine signi cantlyincreases the propulsion ef ciency over its turbojet counterpart asa result of increased mass of the air ow For the same momentumgain the kinetic energy carried away by the exhaust air (including thebypass ow) is less in the turbofan engine case than in the turbojetcase In the following sections we will plot and compare the thermalpropulsion and overall ef ciencies of the different engine con g-urations Examination of these parameters helps us understand thestrengths and weaknesses of the different engine types

In the conventional turbojet con guration the basic ideal thermo-dynamic cycle is the Brayton cycle for which the thermal ef ciencyis known to be

g t 1 (1 p c )( c 1) c 1 1 [( c 1)2]M2 (7)

where p c is the compressor pressure ratio and M is the ightMach number To increase ef ciency (ie decrease TSFC) onehas to increase the pressure ratio However higher pressure ratioincreases the total temperature of the ow entering the combustorand therefore limits the amount of heat that can be added in thecombustor because of the maximum turbine inlet temperature limitAs such the conventional base turbojetturbofan engine is limitedin the maximum value of speci c thrust The afterburner increasesthe power levels of the engine but because fuel is burned at a lowerpressure compared to the main burner pressure the overall cycleef ciency is reduced Therefore the use of an afterburner increasesthe speci c thrust at the expense of the fuel consumption rate Aduct burner in the fan bypass has the same effect except that it haseven lower ef ciencies because the pressure ratio in the fan bypassis usually lower than that in the core engine

To remedy the ef ciency decrease because of the use of after-burner the turbine-burner was conceived12 in which one adds heatin the turbine where the pressure levels are higher than in the af-terburner The ideal turbine-burner option is to add heat to the owwhile it does work to the rotor at the same time so that the stagna-tion temperature in the turbine stays constant By doing this Ref 2showed that the contention between ST and TSFC can be signi -cantly relieved compared to the conventional engine It was shownthat signi cant gain in ST can be obtained for a turbojet engine with asmall increase in fuel consumption by using the turbine-burner con-cept Figure 3a shows the continuous turbine-burner cycle whichwe denote as the CTB cycle

However it is yet practically dif cult to perform combustion inthe turbine rotor at the present time An alternative for a rst designis to convert the turbine stators or nozzles into also combustorsand therefore effectively introduce inter-stage turbine burners (ITB)without increasing the engine length We may have one two ormore such ITB as shown in Figs 3bndash3d We denote such cycles asthe M-ITB cycles with M being the number of interstage burnersObviously as M goes to in nity the M-ITB cycle approaches thecontinuous turbine-burn cycle that is the CTB cycle

In this paper we extend our cycle analysis in Ref 2 to calcu-late the preceding M-ITB cycles and also for turbofan engines We



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