NASA Technical Reports Server (NTRS) 19960014631: High-speed engine/component performance assessment using exergy and thrust-based methods

58 Pages·2 MB·English

by  NASA Technical Reports Server (NTRS)

NASA Contractor Report 198271
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=- NASA-CR. 198271
"_ : 19960014631
High-Speed Engine/Component
Performance Assessment Using Exergy
and Thrust-Based Methods
'D. w. Riggins
University ofMissouri - Rolla
Rolla,Missouri
Grant NAGl-li89
January 1996 __-
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HIGH-SPEED ENGINE/COMPONENT PERFORMANCE ASSESSMENT
USING EXERGY AND THRUST-BASED METHODS
D. W. Riggins
University of Missouri-Rolla
Work Performed Under
NASA Grant NAG1-1189
(Hypersonic Vehicles Office)
J
-,,,_.
Nomenclature
Cv specificheat (constant pressure) (J/kgK)
3' ratio of specificheats
no 'combustion'efficiency;controlsheat release
"-_. AQ,_._ heat released into flow(J/kg)
AQcx_d totalheat energyexpendedbyvehicle (J/kg)
Lopt optimal combustor length (m)
_k rational efficiency
_.. engine-basedrational efficiency
T static temperature (K)
S entropy (J/kgK)
_/_ enginethrusteffectiveness
fia massflowrate through engine (kg/sec)
P static pressure (N/m2)
A cross-sectionalarea
u or U velocity(m/s)
irr irreversible (subscript)
incomp incomplete heat release (subscript)
ideal ideal condition(subscript)
o inlet face or flight/ambient condition (subscript)
e/E nozzle or expansionexitcondition (subscriptor identifier)
Cf skin friction coefficient
Ex exergy(availablework) (J/kg)
R gas constant (J/kgK)
v specificvolume(m3/kg)
visor viscous frictionalcomponent
tL,i_o., viscousfrictionalforce (N)
--_ F thrust (N)
FW thrust work (per kilogram)(J/kg)
-7_-
,o°
111
FWP thrust work potential
R reversible (subscript)
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iv
High-SpeedEngine/Component Performance AssessmentUsing
Exergyand Thrust-BasedMethods
_m
I. Introduction
"-_. Thesuccessfuldevelopmentofhigh-speedairbreathingenginesrequires thethorough
optimizationofthe propulsionsystemanditscomponents. Thisoptimizationprocessshould
be donewithrespect to thevehicleinwhichthe engineis embeddedjust asthe vehicleitself
should be optimized for the projected mission it is to perform. Ideally, for any speed
regime,an aerospace engine (and each individualengine component) should be designed
withinthe overallvehicledesigneffort inorder to ensure true optimization;thiswouldlead
to a specificengine for a specificvehicle. This procedure usuallyhas not been done due
to issues of increased cost and complexity. Aerospace engine selection has traditionally
been made in vehicle design efforts by examining candidate engine parameters such as
specificthrust and specificfuel consumptionand ensuringthat the proposed enginemeets
installedthrustrequirementswhileminimizingthe on-boardfuelnecessaryoverthe duration
of the mission. The designer,after carefulconsideration and analysisof candidate engine
characteristics,cangenerallyattachthe enginetothe airframewithout extensiveintegration
and still satisfactorilyachieve the mission objectivesfor lower-speed systems. For high-
speedvehicles,however,whichhaveinherentlythin performance margins,the fundamental
integration of the engine and the vehicle are of utmost importance; engine design and
engine component design should be done within the contextof the vehicle designprocess
itself. Therefore, it is not advisablein high-speedengine analysisto attempt to separate
engine (or engine component)performance assessmentfrom the vehicle.
In the context of high-speedpropulsion, if the question is asked whether engine A
or engine B is better, then the answer depends very much on the vehicle(s)(and the
mission) with which A and B are integrated with and with which they were (hopefully)
designed in conjunctionwith. In that respect, the matter of 'ranking' different engines
-_ becomes inseparable from the question of ranking different vehicles- which,in the final
analysis,will most likelybe based on technical feasibilityand overall life-cyclecost. A
-_" furtherproblem withthisparticular questionisthat high-speedengineperformance ismuch
less scalablewith engine sizethan low-speedengine performance (i.e., one might roughly
estimate that doubling the cross-sectionalarea of a turbojet woulddouble the delivered
thrust and fuel consumption; for high-speed flight,however, where scale effects can be
significant, such an approximation may be completely erroneous). More reasonable
questionsforhigh-speedpropulsionsystemanalyststo askwouldbe thefollowing: howwell _._
isa given engine (or engine component) performing, where are the performance losses
occurring,and what flowmechanisms are responsible for the lossesand to what degree?
Further, howdochanges inthe characteristicsofthe engine or an engine component affect
engine performance and how are design features of an engine component to be chosen
withinthe larger engine (or vehicle) iterative designprocedure? This investigationseeks
to shed light on two current methodswhich havebeen suggestedfor answeringthese and
related questions. These two methods are based on i) standard exergy (available work)
conceptsand ii)thrust-work-potentialconcepts. Neither method isrecent in development;
exergy has been successfullyused for many years for a wide variety of ground-based
engineeringprocesses;applicationto aerospace engineshavebeen somewhatmore limited
[1]-[7]. Thrust-potential [8]-[10](or engine thrust effectiveness) is a modification and
extension ofa much older propulsiveconcept calledthe combustor effectivenesswhichhas
been in use for at least thirtyyears. This investigationusesvery simple one-dimensional
steadyflowswithRayleighheat addition and frictioninorder toillustrate and clarifyissues
relating to the thrust-potential and exergymethods (whenapplied to high-speedaerospace
engines).
In order to establish the performance base-line for an aerospace propulsion system,
consideranengine (suchas ascramjet)operatingat somegiveninflowconditionswithsome
fLxedamount of heat 'spent, in the engine (corresponding to fuel used in a real engine).
The engine has somereal flowlosses(i.e.,lossesintotal pressure or, equivalently,entropy
increases due to irreversible mechanisms), incomplete combustion (less than 100%
combustion efficiency),and somespecificfinitenozzle exit area. An engineer tasked with
improvingthe performance of this engine(at these conditions)has (possibly)three waysto ..
perform this task;i) decrease the irreversibilitieswithinthe engine, ii)increase heat release
(increase the combustion efficiency),and iii) increase the nozzle exit area. These three _
. 2
routes are coupled -for instance,increasingheat release bymodifyingthe combustor may
resultin greatertotalpressurelosses(more irreversibilities)orincreasingnozzleexitarea
mayincreasethe irreversibilitiesthroughgreater friction,etc. Natureprovidestheoretical
_a
limitsforthefirsttwoof thesemethods:1)theflowcannotbemorereversiblethanthatof
_. the completelyreversibleengine,i.e., the enginewhichhas no total pressurelosses or,
equivalently,no irreversibleentropyincreases,and2) the maximumpossiblereleasedheat
into theflowisequaltotheexternallyprovidedheatinput(completecombustion).Further,
the nozzle exitdegree-of-expansionis limited(constrained)by the externalaerodynamic
drag. Althoughthelatterlimitissomewhatdifferentin characterthanthe firsttwolimits,
it will be seen to be an importantconstraintwhen assessing engine or component
performanceandcomponentdesigncharacteristics.It canbe arguedthat,in the absence
of external informationor weight issues, the nozzle degree-of-expansionis unlimited
(theoretically)by naturesuchthataninfiniteexpansionof thenozzle is thenaturallimit,
at leastfrom the engine-alonestandpoint. In anyevent, these three criteria(degreeof
irreversibility,degree of incompletenessof heat-release, and degree of expansion)are
criticalinassessingtheactualperformanceof theenginefor somegiveninflowconditions
andheat input.
Bothexergyandthrust-work-potentiaalre basedonwork availabilityconcepts,i.e.,
both describe systemworkwhichis potentiallyavailableas measured from someset of
referenceconditions.Parametersbasedoneitherof themethodscanbe showntodecrease
inaflowdueto irreversibilitiesandto increasewithheat(energy)addition.Thisbehavior
is necessaryif a performanceparameteris to be usedfor meaningfuland comprehensive
engine(or enginecomponent)design. This canbe illustratedby consideringtwosimple
scramjet combustorsboth with total pressure losses and scheduled heat releases; one
combustorwithgreater totalpressurelossesmayhaveassociatedgreaterheatrelease such
that itisa better 'performer' than theother combustorwhichhaslesstotal pressure lossbut
less heat release. Obviously,a comprehensive performance parameter must be able to
... distinguishsuch a trade-off; both exergy and thrust-work-potentialhave this ability. In
contrast, combustion efficiencyand the total pressure ratio are performance parameters
"" which,while useful and informative,are not comprehensivein nature.
3
Exergyat an enginestationisusuallydefined asthe maximumreversibleworkwhich
canbe obtained fromthe flowasmeasuredfromthe reference (usuallyambient) conditions.
It is aclosed-cyclequantitywhichaccountsforthe effectoflossesandheat releaseupstream
of the station of interest at which it is calculated. Losses in exergyare, by definition,
directlyproportional to irreversible entropy gains;exergylossesin individual components -
due to specificirreversible mechanisms(aswellas Carnot losses)can be readilyassessed.
In addition, exergylossdue to incompleteheat addition canbe easilycomputed, at least for
the flow-fieldsexamined here whichhave simpleRayleighheat addition.
The rational efficiencyof an engine componentis defined as the ratio of the exergy
exitingthe component to the total exergyentering the component [4],[5]. This implies,for
bothphysicaland mathematicalconsistency,that the rational efficiencyofthe overallengine
is the ratio of exergyexitingthe engineto the exergyentering the engine (through both air
and fuel). However,sincethe thrust workisthe trulyusefulworkofthe engine (rather than
the exergy),references [3]-[5]define a 'true rational efficiency'of the overall engine. This
is described as the ratio of the engine thrust work to the exergy entering the engine.
Nevertheless,componentperformance and losseswithinthe engineare computedusingthe
original exergy-basedrational efficiency (exergy out over exergyin). This inconsistency
between how performance and lossesare measured for the overall engine (in terms of
•thrust) and howperformance and lossesare measured for an individualcomponent within
the engine (in terms of exergy) violates the fundamental principle that a useful and
comprehensive performance parameter mustbe consistentinformwhether applied overan
engine component or over the entire engine. Such consistencyis necessary because the
segmentationofa high-speedengineintocomponentsisan arbitraryprocessfromthe stand-
point of performance assessment. For example,the beginningof the nozzlecan be viewed
equallyas a downstream extensionof the combustor;there is no fluid-dynamicdistinction
between the two components. In fact, the entire engine can and should be viewed as a
singleentityfor performance assessment;sucha perspectivewillalwaysresult in a superior
overall engine design. In this sense, each component in the engine, however identified, .-
should ultimatelybe assessedinterms of howwellit contributes to the achievement of the
overallpurpose of the engine, This mandates a synergisticcomponent design process. -_