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Intensive Course on

Aero Gas Turbine Design(Components & Sub Systems)

2010 11

Shri V.SundararajanEx-Director, GTRE

ENGINE DIVISION, BANGALORE COMPLEX,

HINDUSTAN AERONAUTICS LIMITED

BANGALORE 560 093

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INDEX

Sl.No. Particulars PageNo.

1 GasTurbineEngineAnOverview 3

2 Compressor 28

3 TurboProp&TurboShaftEngines 41

4 CombustionChambers 46

5 Turbines 56

6 ExhaustSystem 64

7 Afterburning 70

8 FuelSystem 78

9 FullAuthorityDigitalEngineControlSystem 86

10 StartingAndIgnitionSystem 107

11 PerformanceDeduction&Prediction 113

12 AltitudeTestingVisVisFlyingTestBedForGasTurbineEngineDevelopment 143

13 Airframe EngineIntegration 149

14

GasTurbine

Engine

Manufacturing

Techniques

157

15 PerformanceTestingAndAnalysis 174

16 ClassificationofCriticalityofAeroEngineComponents 177

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GAS TURBINE ENGINE AN OVERVIEW

Contents

PrincipleofJetPropulsion

ClassificationofGasTurbineEngines

GasTurbineEnginesPrincipleofOperation

GasTurbineEnginesComponentsandSubSystems

EngineTesting

Gasturbine

materials

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HISTORYOFGASTURBINES

Principleof

Jet

propulsion

JetPropulsionisapracticalapplicationofNewtonsIIILawofmotion

Foreveryforceactingonabodythereisanequalandoppositereaction

Inthecaseofaircraftpropulsionthebodyisatmosphericairthatiscausedtoaccelerateasitpassesthroughtheengine

APropulsionsystemisamachinethatproducesthrustorpowertopushanobjectforward

TheGasortheworkingfluid isacceleratedbytheengineandreactiontothisaccelerationproducesa forceontheengine

The

Propulsion

System

Propulsion=pro+pellere

pro:beforeorforwards

pellere:meaningtodrive.

Propulsionmeanstopushforwardordriveanobjectforward.

Apropulsionsystemisamachinethatproducesthrusttopushanobjectforward.Agas,orworkingfluid,isacceleratedbytheengine,andthereactiontothisaccelerationproducesaforceontheengine.

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PurposeofThePropulsionSystem

TheAirplanepropulsionsystemmustservetwopurposes.

The thrust from the propulsion systemmust balance the drag of the airplanewhen theairplaneiscruising.

Thethrustfromthepropulsionsystemmustexceedthedragoftheairplanefortheairplane

toaccelerate,climbandmaneuver.ThisiscalledasexcessthrustoverdragknownasThrust

Dragi.e. (TD)

PurposeoftheJetPropulsion

During straightand level flight called cruise, theenginemustproduce sufficient thrust tobalancetheaircraftdrag.

For civil or commercial engines fuel economy or specific fuel consumption is of primeimportancetogetmaximumrangeandendurance.

Specificfuelconsumptionisdefinedas:fuelflow/thrustoftheengine.

Forfighteraircraftapplicationshorttakeoff,fastacceleration,fastrateof climbandgoodmaneuverperformanceareofprimeimportanceforwhichadditionalthrustisrequired.

Excessthrust

over

drag

i.e.

(Thrust

Drag

)is

used

for

climbing

to

higher

altitudes

or

for

accelerating fromonemachnumber toanothermachnumberandalso formaneuver formilitaryaircraft.

MachNumber isdefined as the ratioof the velocityof theobject to the velocityof thesound.Itisnon dimensionalquantity.

Thrust toWeight ratio (T/W) is one of the important figures ofmerit for fighter aircraftengines.

AnadditionalfigureofmeritforaircraftisLift/Dragratioi.e.L/Dratio.

AircraftCeiling

The

absolute

ceiling

of

an

aircraft

is

that

altitude

at

which

the

rate

of

climb

is

zero.

RateofClimbisdefiedas(ThrustDrag)/WeightxVelocityoftheaircraft:(T D)/WxV

TheunitofRateofClimbisft/secormeters/sec

TheaircraftcruisesataltitudesincetheS.F.C.decreaseswithaltitude

ThemilitaryaircrafthasanincreasedRateofClimbcomparedtotheCivilaircraft

ForobtainingincreasedRateofClimbafterburnerisemployedinMilitaryaircraft

TheaircraftgenerallycruisesataRPMslightlylowerthanthemaximumRPMsay95to96%RPMforfuelefficiency

AircraftControls Pitch,RollandYaw

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AeroGasTurbineEngine Technology

Forthepast4to5decadestheaerogasturbineenginetechnologyhasgrowntremendouslyintermsofengineoverallpressure,TurbineentrytemperatureandThrust/Weightratio

TheOverallpressureratiohasgoneupby~9to10times

Turbine

entry

temperature

(TET)

has

doubled

ThrusttoWeightratiohasincreasedby~2.5times

ThishasresultedinlesserengineAssembliesandSubassemblies,enginepartcounts,majorreductioninenginelengthanddiameter

All these Technology improvements have been made possible by improvedAerothermodynamics, Computational Fluid Dynamics techniques, Advancement inmanufacturing and Fabrication technologies, Advanced Control Systems and Advancedmaterials

ClassificationofGasturbineengine

Theclassification

can

be

made:

BasedontheapplicationofGasturbineengine

BasedonthefluidunderwhichtheGasturbineengineoperates

PropulsionSystemClassification(Applicationbased)

AeroEngines(foraircraftsandhelicopters)

PowerGeneration (100KW 1000MW)

Marineengines

IndustrialApplications

CombatVehicles,

Automobiles

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PropulsionSystemClassification(Workingfluidbased)

JetandPropellerEngines(Airbreathingengines)

Jetenginegivesalargeaccelerationtoasmallweightofair

Propellerenginegivesasmallaccelerationtoalargeweightofair

GasTurbineEngines:Turbojet

Theturbojet,thesimplestandearliesttypeofgasturbine,isusedprincipallyinhighspeedaircraftwhereitsrelativelysmallfrontalareaandhighjetvelocityareadvantageous.Theturbineextractsonlysufficientenergyfromthegasstreamtodrivethecompressor,leavingtheremainingenergytoprovidethethrust.

ExamplesoftheturbojetaretheRollsRoyceOLYMPUS593intheConcordesupersonicairlinerandtheRollsRoyceVIPERwhichisusedinavarietyofmilitaryaircraft.

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GASTURBINEENGINES:TURBOFAN

Theturbofanisthemostcommontypeofgasturbineusedforaircraftpropulsiontoday.Partoftheairenteringtheengineiscompressedfullyandpassedintothecombustionchamber,whiletheremainder,compressedtoalesserextent,bypassesthecombustionsection,toprovidecoldthrust.Thisbypassflow

rejoinsthe

hot

flow

downstream

of

the

turbine,

as

in

the

AE

3007

engine.

Examplesof the turbofanare theAE3007 in theCessnaCitationXandEmbraerEMB145, theRollsRoyceRB211intheBoeing747,theRollsRoyce535intheBoeing757,theRollsRoyceTAYintheGulfstreamIVandFokker100,theRollsRoyceADOURintheJaguarandHawk,andtheRollsRoyceRB199intheTornado.

JetEnginewithhighbypassratio

Bypassratioisdefinedastheratioofthebypassair(cold air)tothecoreair(Gasgeneratorair).

ThisBypassratioisoftheorderof8to9inTurbofanenginesresultingingoodfuelefficiencynamelyGoodSpecificfuelconsumption(SFC).

Theseareofunmixedtype

TwotypesofthrustnamelyColdthrustandHotthrustareproducedandsumofthetwoisthetotalthrust

Enginesize

is

big

because

of

high

bypass

ratio

and

gives

lower

specific

thrust

and

very

low

SFC

SpecificthrustisdefinedasThrustperunitmassflowrate.

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TurbofanEngine

BypassEngine

Thiscanbeconsideredasaturbofanenginewithlow(small)bypassratiointherangeof0.2to1.

This is quite suitable for military engines where both high thrust and moderate fuelefficiency(SFC)areofprimeimportance.

Smallbypassratioresultsinsmallersize,highspecificthrustandmoderatelylowSFC.

Theseareofmixedtypei.e.boththecoldandthehotstreamsaremixed.

GE90(Turbofan)Mostpowerfulengineinaviationforthrustproduction

Thrust :115,300lbs

SFC :0.25lb/lbt/hr

Overallpressure

ratio

:42:1

MaximumTurbineinlettemperature :1750K

Bypassratio :9

Airmassflow :3,000lb/sec

Weight :18,260lbs

Thrusttoweightratio :6.3:1

Fuelburnduringtakeoff :3,750gallons/hr

Singlestagefanfollowedby04stageaxialboosterand9stageaxialflowHPcompressor

2stageaxialturbine

HighbypassratiodualshaftTurbofan

GASTURBINEENGINES:TURBOPROP

The turboprop is a turbojetwith an additional turbinewhich uses the energy remaining in the gasstream,after sufficientenergyhasbeenabsorbed todrive the compressor, todriveapropeller.Theadditionalturbine,calledthepowerturbine,drivesthepropellerthroughashaftandareductiongear.Asmallamountofresidualthrustremainsintheexhaustgasesduringnormaloperation.

Theturbopropisaveryefficientforrelativelylowspeed,lowaltitudeaircraft,

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Exampleofthe turbopropare theAE2100used in theSAAB2000and IPTNN250, theT56used inavarietyofmilitaryturboprops,theRollsRoyceDART intheBritishAerospace748and theFokkerF27,andtheRollsRoyceTYNEintheTransallC160andDassaultBreguetAtlantic.

GasTurbineEngines:TurboShaft

The turbo shaft is effectively a turbopropwithout a propeller, the power turbine in this case beingcoupledtoa reductiongearboxordirectly toanoutputshaft. In thesamewayastheturboprop,thepowerturbineabsorbsasmuchoftheremainingenergyaspossibleandtheresidualthrustisverylow.

Themostcommonapplicationoftheturboshaftisthehelicopter, inwhichtheenginedrivesboththemainandtailrotors.Turboshaftsarealsowidelyusedfor industrialandmarine installations, includingpowerandpumpingstations,hovercraftandships.

ExamplesoftheturboshaftaretheT406intheV22Osprey,theT800intheRAH66Comanche,the250used inapproximately75%oftheworld's lighthelicopters,theRollsRoyceGEM intheWestlandLynxandtheRollsRoyceGNOMEintheWestlandSeaKingHelicopters.

Ramjet

NoRotatingparts (i.e.no compressor& turbine) and consistsofaductwith adivergententry,combustionchamberandconvergentdivergentnozzleexit.

Itcannotbestartedunderstaticconditionandairhastobeforcedintotheairintake

Inotherwordsitisnotselfpropellingatzerovelocity

To initiatetheoperationtheRamjetmustbeeither launched fromairplane in flightorbegivenaninitialvelocitybysomeauxiliarymeans.

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RocketEngines(NonAirbreathingEngine)

DoesnotuseAtmosphericairasworkingfluid

Producesitsownpropellingfluidbythecombustionofliquidorchemicallydecomposedfuelwithoxygenwhichitcarries,thusenablingittooperateoutsidetheearthsatmosphere.

Hence

it

is

suitable

only

for

operation

over

short

periods

GasTurbineEnginesPrincipleofOperation

GasTurbineEngineoperatesonathermodynamiccycleknownastheBraytoncycle

Airisdrawnfromatmosphere

Pressurerise(Compression)takesplaceinthecompressor

Highpressureairismixedwithfineatomizedfuelsprayandignitedwithhighenergyspark.Combustiontakesplaceatconstantpressure

Hotgasesarisingoutofcombustion impingeontheturbineandrotate itandhencecalledgasturbine.

TheTurbine

drives

the

compressor

and

Turbine

compressor

combination

becomes

self

sustainingafterstart

SelfsustainingRPMisthatRPMatwhichtheTurbineproducessufficientpowertodrivethecompressor

Balancepressure energy is converted into velocity in the exhaustnozzle and the rateofchangeofmomentumproducesthe thrustwhich isequal totheMass flowratetimesthechangeinvelocityfromfronttotherearoftheengine.

Fortakingtheengineuptoselfsustainingspeedanexternalstartingsystemisrequired.

HowaJetEngineworks?

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During compression the work is done on the air which increases the pressure andtemperatureanddecreasesthevolumeoftheair

During combustion fuel is added to the compressed air and burnt. This increases thetemperatureand thevolumeofairwhile thepressure remainsalmostconstant since theengineoperatesonaconstantpressurecycle.

Duringexpansion

when

the

work

is

taken

from

the

gas

stream

by

the

turbine

to

drive

the

compressor,pressureandtemperaturedecreasewhilethevolumeincreases

WorkingCycleofGasTurbineEngine

Similartothatofa4strokepistonengine

InGasturbineenginecombustionoccursatconstantpressurewhereas inpistonengine itoccursatconstantvolume.

InBothcasesthecyclecomprisesofInduction,Compression,Combustionandexhaust.

InPistonenginethecycleisintermittent,pistonbeingconcernedinall4strokes

In Gas turbine engine the cycle is continuous with a separate compressor, combustor,

Turbineand

the

exhaust

system

ComparisonofWorkingCycleofaPistonandTurbojetEngines

Advantagesofgasturbineoverpistonengines

TheContinuousCycleandabsenceofreciprocatingpartsgiveasmootherengineandenablemoreenergytobereleasedforagivensize

Peakpressuresthatoccurinpistonengineareavoided

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GasTurbineengineComponentsandSubsystems

GasTurbine

Engine

Components

and

Subsystems

Thegasturbineenginecanbedividedintovariouscomponentsandsubsystems

Thesecomponentsandsubsystemsarealsocalledasenginemoduleswhentheengine isbuiltinamodularfashion

Generally those items which perform some thermodynamic process are called asComponentsandthose itemswhichaidthesecomponentstoperformthethermodynamicarecalledasSubSystems

Theaboveisonlyagenericclassificationandtheycanbeinterchangedi.e.thesubsystemscanbecalledascomponentsandviceversa

DefinitionsOf

Component

Efficiencies

Isentropicefficiencyofcompressor:

= Idealtemperaturerise/ActualTemperaturerise

Isentropicefficiencyofturbine:

=Actualtemperaturedrop/Idealtemperaturedrop

Combustionefficiency:

=Idealfueltoairratio/Actualfueltoairratio

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FunctionalConceptofSingleandTwinSpoolEngines

Theolderengineswere single spoolengineshavinga largenumberof compressor stagesresultinginlargeenginelength,operationalcomplexityandincreasedcost

Since thecompression isanadversepressuregradientprocess itwas felt that theengine

compressorcan

be

split

into

low

and

high

pressure

compressors

resulting

in

two

spool

engines

There are a few three spool engines also mainly from Rolls Royce (RR Trent series ofengines,RB199,RB211engines)

The advantage of multi spool ( two and three spool) engines is that it increases theoperationalflexibilityoftheaerogasturbineengines

FunctionalConcept(TwinSpoolEngines)

Inatwinspoolengine,typically

Highpressureturbinegeneratespowertodrivehighpressurecompressor

Lowpressureturbinegeneratespowertodrivelowpressurecompressor

Thrustisobtainedbyexpandingthegasesthroughtheexhaustnozzle

TheLowpressurecompressor,LPshaftandLowpressureturbineformstheLPspool

Thehighpressurecompressor,HPshaftandHighPressureturbineformstheHPspool

Multispooldesign

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PropulsiveEfficiency

PropulsiveEfficiency=PropulsivePower/RateofproductionofKineticenergy

IfVistheentryvelocityandVJistheexitvelocitythenPropulsiveEfficiencycanderivedas

Propulsiveefficiency=2V/(V+VJ)

whichcanbesimplifiedas

2/(1+VJ/V)

Itcan

be

seen

that

when

the

propulsive

efficiency

is

maximum

i.e.

equal

to

1,

the

propulsive

thrustiszero.HencetherelationshipbetweenVJandVisacompromisebetweenPropulsivethrustandPropulsiveefficiency

PropulsiveEfficiency

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Aircraftenginerequirements

ATypicalCompressor

ATypicalCompressor

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CompressorDesignRequirements

Highstageandoverallpressureratio

Lessnumberofstages

Highrateofmassflowperunitfrontalarea

Good

surge

margin

Optimumpressureratiosplitbetweenlowandhighpressurecompressorstages

Variablegeometry

Goodefficiency

Inletdistortiontolerancecapability

TypicalCombustor

TypicalCombustor 3Dsectionalview

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CombustorDesignRequirements

Lowpressureloss

Highheatreleaserateforagivenvolume

Goodtemperaturedistributiontopreventlocaloverheatingofturbineblades

Circumferential

and

Radial

Pattern

factors

StableoperationfromidlingtomaxRPMoftheengine

Goodrelightcharacteristics

Combustorstabilityathighaltitudes

Lessnoiseandpollutionlevel

ATypicalTurbine

TurbineDesignRequirements

Highstagepressureratio

Highstageloading

Lessnumberofstages

Minimumnumberofblades

Highefficiency

AfterBurning

Afterburnerisoneofthethrustaugmentationdevices

InthisprocesstheMomentumthrustisincreased

Theafterburneroperation increases the thrustof theenginewithout increasing the inletsizeoftheenginei.e.withoutincreasingthemassflowrateoftheengine

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Inthemaincombustoronlyabout30%oftheoxygen intheair isusedandbalanceair isused forcooling thecombustor linerso that thecombustionexit temperature i.e. turbineinlettemperatureisbroughtwithinacceptablelimitskeepingtheturbinematerialinmind

Thisbalanceoxygen /air canbemixedwith fuelagain (afterburning) in theafterburnercombustionchambertogetadditionalthrust.ThisisalsocalledasReheat.

Thisadditional

thrust

can

be

used

for

faster

takeoff,

climb

,acceleration

and

maneuvers

Afterburner combustion efficiency will be poorer compared to main combustor (sinceafterburner operation is at lower pressure levels) and hence less fuel efficient. In otherwords the SFCwithafterburnerwillbequitehigh and this restricts the timedurationofafterburnerusage.

All military engines employ afterburning for short burst additional thrust application asmentionedabove.

R&Deffortsareon to increase themaincombustorexit temperature itself toamaximumvalueoftheorderof2100Ksothattheusageofafterburnercanbedispensedwith.

TypicalAfterburner

PrincipleofAfterBurning

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AfterburnerDesignRequirements

Lowpressureloss

Highthrustboost

Goodcombustionefficiency

Good

Flame

Stabilisation

using

V

Gutters

EfficientFuelinjection

Goodrelightcharacteristics

Stablereheatoperation

Reheatstaging

SelectionofoptimumblockageratioandL/Dratio

Linerdesigntoallowforthermalexpansion,Antiscreechandtoreducebucklingeffects

GASTURBINESECTIONALVIEW(AEROENGINE)

Inlet Guide vanes

FanGuide vanes

Fan CasingHP Turbine

Annular combustor

IP shaft

Shaft Coupling

Turbine Discs

HP Compressor Discs

Bleed Air Cooling

HP Compressor

LP Turbine

LP Compressor Blades

FuelSystem FunctionsoftheFuelsystem

Toprovidetheenginewithfuelinaformsuitableforcombustion

To control the flow to the requiredquantitynecessary foreasystarting,accelerationandstablerunningatallengineoperatingconditions

Theturndownratiobetweenlightupfuelflowandmaximumfuelflowcouldbeabout50

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Inotherwords the fuel control system should cater for awide rangeof fuel flows fromenginelightuptomaximumRPM/Maximumfuelflow

Fuelisfedtothespraynozzlesoratomizersofthecombustorswhichinjectthefuelintothecombustionchamberintheformofanatomizedspray

The flow rate must very according to the amount of air passing through the engine to

maintainthe

selected

engine

speed

To achieve this the controlling devices are fully automatic with the exception of enginepowerselectionwhichisachievedbyamanualthrottleorpowerlever

Ashutoffcockisusedtostoptheengine(alsotostarttheengine)

It is also necessary to have automatic safety controls to prevent the exhaust gastemperature(EGT),compressordeliverypressureandtheengineRPMfromexceedingtheirmaximumlimits

TheseareintheformofRPMlimiters,EGTlimiter,CompressordeliverypressurelimiterandPressureRatioLimiter(PRL)

Typical

Fuel

Control

System

FullAuthorityDigitalEngineControl(FADEC)system

Overthepast20to25yearsuseofFADECsystemhasbecomeastandardfeature

FADECsystemhasmajorbenefitsintermsof:

Engineperformance

Reducedpilotworkload

Easeofmaintenance

Improvedenginehandling

Improvedfaultdetection

EvolutionofGasTurbineControls

Fullauthorityhydromechanicalorpneumaticcontrolregulatingspeed

Fullauthority analog electronic controls regulating speed and temperature andprovidingsomeBIT

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Hybrid controls (fullauthority hydro mechanical or pneumatic controls with supervisoryanalogordigitalelectroniccontrolsforfinetuning

Hybridcontrols (fullauthorityanalogordigitalelectroniccontrolwith fullauthorityhydromechanicalbackup)

SinglechannelFADECs

Dualchannel

FADECs

Diagrammaticarrangementofenginecontrolandinstrumentation

Startingsystem Necessity

Two separate systems are required to ensure that a gas turbine engine will startsatisfactorily

First,provisionmustbemadeforthecompressorandtheturbineassemblytoberotateduptoaspeedatwhichadequateairpasses intothecombustionsystemtomixwiththe fuelfromthefuelspraynozzles

Secondly,provisionmustbemadeforIgnitionofthefuelairmixtureinthecombustor.

Duringenginestarting,thetwosystemsmustoperatesimultaneously.

ThereareoccasionswhentheStartingandIgnitionSystemsmayoperateindependently

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The Starter alone will have to operate when the engine is undergoing motoring runsespeciallyforenginesunderdevelopment.DuringmotoringrunsIgnitionsystemisisolated

Themotoringrun(withoutignition)consistsofthefollowing:

Dryrun

Wetrun

Duringrun

the

engine

is

allowed

to

to

rotate

up

to

aparticular

RPM

say

about

30

to

32%

onlywiththeStarteron(withtheshutoffcockclosed;i.e.theenginethrottleisshut)

Thisrunisgiventocheckthefreenessoftherotorsandalsotochecktheswingbackoftheengine

Thewetrunisgiventocheckthelightupfuelflowbeforetheactualpowerrunisgivenwiththeignitionon

TheIgnitionsystemaloneoperateswhentheenginerelightattemptismadebythepilotataltitudesi.e.windmillstartsoftheengine

TypesofStarters

ElectricstarterSectionalview

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IgnitionSystem

HighEnergyIgnitionsystemisusedforstartingallenginesandadualsystemisalwaysfitted.

Eachsystemhasanignitionunitconnectedtoitsownigniterplugs.

TwoPlugsarelocatedindifferentpositionsinthecombustionchamber

Each

Ignition

Unit

receives

alow

voltage

supply

from

the

aircraft

electrical

systems

The Electrical energy is stored in the unit until a predetermined value the energy isdissipatedasahighvoltage,highamperagedischargeacrosstheigniterplug

DCIgnitionUnit

PerformanceTestingandAnalysis Preamble

Capabilitiesofanaircraftsystemsaredefinedbytheprescribedneedthattheaircraftmustmeet

Acceptable

levels

of

these

capabilities

are

substantiated,

demonstrated

and

qualified

Thisisthroughacomprehensivedevelopmentprocessencompassing:

Design,Testinganddevelopment

Deployment

Maintenance&Logisticssupportplans

Propulsionsystemisoneofthemajorsubsystemsoftheaircraft.

Normallyenginedesignprecedestheaircraftdesign.

Thedevelopmentaircraftisnotflownwithadevelopmentengine.

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Normallythedevelopmentaircraft isflownwithaprovenengineandaircraftperformanceandhandlingcapabilitiesareestablished.

Thenthedevelopmentengineistestedinthedevelopmentaircraft.

EngineTesting

Twotypesofenginetestingnamelyproductionenginetestinganddevelopment/prototypeenginetesting

Productionenginetestinginvolvesonlylimitedmeasurementsandengineacceptance

Important parameters are RPM, Thrust, Fuel flow, Compressor delivery pressure, Typicalvibration,engineexhausttemperature

Development/Prototype engine testing involves detailed instrumentation and dataprocessing

About800parameterslikepressures,temperatures,vibration,strainsignals,coolingflows,secondaryflowsetcaremeasured.

Engine testing is conducted in a test cell fully equipped to measure all the desired

parameters.

New facilitieshavebeenbuilt tosimulateconditionsencounteredathighMACHnumbersandhighaltitudesintheflightspectrum.

Engineperformanceisgenerallydefinedintermsofthrust,fuelflowandairmassflow.

Gasturbineengineperformanceisconsiderablyinfluencedbychangesinambientpressureandtemperature

Increaseininletpressureisadvantageoustotheenginewhileincreaseininlettemperatureisdisadvantageoustotheengine.

In order to compare the performance of the engine on different dates and at differentplacesitisnecessarytocorrecttheperformanceofagivenenginetostandarddaycondition

knownas

International

Standard

Atmosphere

Sea

Level

Static

Conditions(ISA

SLS

).

ThisCorrectionisessentialforcomparingtheperformanceofdifferentengines

Inorder to correct theengineperformance to ISASLSconditions thereare two importantcorrection factors known as Pressure correction factor delta and temperature correctionfactorknownastheta

Delta=AmbientPressure(absolute)underenginetest/ISASLSreferencepressure

Theta=AmbienttemperatureinKelvin/ISASLSreferencepressurenamely288K

The testperformanceof the engine is corrected to ISA SLS conditions and the engine isacceptedbasedonthecorrectedperformance;thisiscarriedoutforallproductionengines

ISASLScorrectionhastobecarriedoutfordevelopmentenginesalso

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EngineDevelopmentProcess Anoverview

GasTurbineMaterials

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AdvantagesandDisadvantagesofGasTurbines

AdditionalReferences

TheJetEngine RollsRoyceplc

Aircraft

Gas

Turbine

Engine

Technology

by

Irwin

E.

Treager

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COMPRESSOR

CONTENTS

Typesofcompressors.

Advantagesanddisadvantagesofdifferenttypesofcompressors.

Application.

Principleofoperation(centrifugalandaxialtypes).

Flowcontrolandsurgephenomenon.

DesignconsiderationsandTradeoff

Balancing

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Reviewofbasicprinciples

System:Afixedidentitywithanarbitrarycollectionofmatterisknownasasystem

Boundary: The boundary is an imaginary surface which separates the system from itssurroundings

Surroundingsare

those

which

are

outside

the

system

Systemcanbeclassifiedaseitheranopensystemoraclosedsystem

Opensystem:Whenthereisacontinuousflowofmatteritiscalledanopensystem.Suchasystem isusuallydepictedbyacontrolvolume. Ithasa fixed spacebutdoesnot containfixedmassofmatter;insteadthereiscontinuousflowofmassthroughit.Thepropertiesofmatter occupying the control volume can vary with time. The surface which encloses acontrolvolumeiscalledcontrolsurface.

Closedsystem:Whenthere isa fixedquantityofmatter (fluidorgas), it iscalledaclosedsystem.However,aclosedsystemcaninteractwithitssurroundingsthroughworkandheattransfer.Theboundariesofaclosedsystemcontainingthefixedmassofmattercanchange.

State:Condition

of

asystem,

defined

by

its

properties,

is

known

as

the

state

of

asystem.

Process:Achangeoraseriesofchangesinthestateofasystemisknownasaprocess.

Pressure: It is the force per unit area, that is pressure at a point surrounded by aninfinitesimal area. Pressure is usually designated by Pascal I SI units. It may also beexpressedinN/m2orbar.

Density:Thedensityofamediumismassofthematter(gas)perunitvolume.

Temperature: When two systems are in contact with each other and are in thermalequilibrium, the property common to both the systems having the same value is calledtemperature.Thustemperatureisameasureofthethermalpotentialofasystem

COMPRESSORS

Compressioniseffectedbyoneortwotypesofcompressors.

One gives centrifugal flowand theotheraxial flow knownas centrifugal compressorandaxialcompressorrespectively.

Bothtypesaredrivenbytheengineturbine.

Compressorisdirectlycoupledtotheturbineshaft.

In thecompressorwork isdoneon theairwhich increases thepressureand temperatureanddecreasesthevolumeoftheair.

Centrifugalcompressoremploysan impellertoacceleratetheairandadiffusertoprovidetherequiredpressurerise.

Theaxial

compressor

is

amulti

stage

unit

employing

alternate

rows

of

rotating

(rotor)

blades and stationary (stator) vanes to accelerate and diffuse the air until the requiredpressureriseisobtained.

Insomesmallengineapplicationsanaxialcompressorisusedtoboosttheinletpressuretothecentrifugalcompressor.

ADVANTAGES AND DISADVANTAGES OF CENTRIFUGAL AND AXIAL

COMPRESSORS

Centrifugalcompressorisusuallymorerobustthantheaxialcompressorandisalsoeasiertomanufacture.

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Axialcompressorhoweverconsumesfarmoreairthanacentrifugalcompressorofthesamefrontalarea.

Axial compressor can be designed to attain much higher pressure ratio compared tocentrifugalcompressor.

Axial compressorwillgivemore thrust for the same frontalarea. Inotherworlds specific

thrust(thrust

per

unit

frontal

area)

will

be

much

higher

in

the

case

of

axial

compressor.

COMPRESSORAPPLICATION

Because of the ability to increase the pressure ratio by addition of extra stages, axialcompressorsareemployedinmostoftheengineapplications.

The trend to high pressure ratiowhich has favored the addition of axial compressors isbecauseoftheimprovedefficiencythatresultswhichinturnleadstoimprovedspecificfuelconsumptionforagiventhrust.

Howevercentrifugalcompressorisstillfavoredforsmallerengineswhereitssimplicityandruggednessoutweighanyotherdisadvantages.

Specificfuelconsumptionandpressureratio

CENTRIFUGALCOMPRESSOR(PRINCIPLEOFOPERATION)

TheCentrifugalCompressorconsistsessentiallyofastationarycasingcontainingarotatingimpellerwhichimpartsahighvelocitytotheairandanumberoffixeddivergingpassagesinwhichtheairisdeceleratedwithaconsequentriseinstaticpressure.

Thelatterprocessisoneofdiffusion(increaseinpressure)andconsequentlythepartofthecompressorcontainingthedivergingpassagesisknownasdiffuser.

Impeller is rotated athigh speedby the turbineandair is continuously induced into thecenteroftheimpeller.

Centrifugalactioncausestoflowradiallyoutwardsalongwiththevanestotheimpellertipthusacceleratingtheairandalsocausingariseinpressuretooccur.

Atanypointintheflowofairthroughtheimpeller,thecentripetalaccelerationisobtainedbyapressurehead,sothatthestaticpressureoftheairincreasesfromtheeyetothetipoftheimpeller.

Airleavingthe impellerpasses intothediffusersectionwherethepassagesformdivergentnozzlesthatconvertmostofthekineticenergyintopressureenergy.

Theremainderofthepressureriseisobtainedinthediffuser,wherethehighvelocityoftheairleavingtheimpellertipisreducedtosomewhereintheregionofthevelocitywithwhichtheairenterstheimpellereye

Thenormalpracticeistodesignthecompressorsothathalfthepressureriseoccursintheimpellerandtheotherhalfinthediffuser

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Inordertomaximizetheairflowandpressureratiothecentrifugalcompressorrequirestoberotatedathighspeedandhencetheimpellersaredesignedtooperateattipspeedsupto1,600ft.persecondandthecorrespondingRPMcouldrangefrom60,000to1,00,000plus.

Byoperatingatsuchhightipspeeds/RPMstheairvelocityfromtheimpellerisincreasedso

that

greater

energy

is

available

for

conversion

to

pressure.

Inordertomaintaintheefficiencyofthecompressor,itisnecessarytopreventexcessiveairleakagebetweentheimpellerandthecasing.

Thisisachievedbykeepingtheclearancesassmallaspossible.

ConstructionFeatures(CentrifugalCompressor)

Theconstructionofthecentrifugalcompressorcentresaroundtheimpeller,diffuserandairintakesystem.

The impeller shaft rotates inballand rollerbearingsand iseithercommon to the turbineshaftorsplitinthecentreandconnectedbyacoupling,whichisusuallydesignedforeaseof

detachment.

Theimpellerconsistsofaforgeddiscwithintegral,radiallydisposedvanesononeorbothsidesformingconvergentpassagesinconjunctionwiththecompressorcasing.

Inorder toease theair fromaxial flow in theentryducton to the rotating impeller, thevanesarecurvedinthedirectionofrotation.

Thecurvedsectionsmaybe integralwith the radialvanesor formedseparately foreasierandmoreaccuratemanufacture.

Diffuser:

The diffuser assemblymay be an integral part of the compressor casing or a separatelyattachedassembly.

Ineachcaseitconsistsofanumberofvanesformedtangentialtotheimpeller.

The vane passages aredivergent to convert kinetic energy intopressure energy and theinner edges of the vanes are in line with the direction of the resultant flow from theimpeller.

Theclearancebetweenthe impellerandthediffuser isan importantfactor,astoosmallaclearance will set up aerodynamic buffeting impulses that could be transferred to theimpellerandcreateanunsteadyflowandassociatedvibration.

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Atypicalcentrifugalcompressor

Typicalimpellersforcentrifugalcompressors

AirflowatentrytodiffuserCentrifugalcompressor

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AXIALFLOWCOMPRESSOR(PRINCIPLEOFOPERATION)

Inaxialflowcompressor,astageconsistsofarowofrotatingblades(rotor)followedbyarowofstator(stationary)vanes.

Rotor is turnedathigh speedby the turbine so thatair is continuously induced into thecompressorwhichisthenacceleratedbytherotatingbladesandsweptrearwardsontotheadjacent

row

of

stator

vanes.

Pressureriseresultsfromtheenergyimpartedtotheairintherotorwhichincreasestheairvelocity.

Theairisthendecelerated(diffused)inthefollowingstatorpassageandthekineticenergytranslatedintopressure.

Theprocess is repeated inasmany stagesasarenecessary to yield the requiredoverallpressureratio.

Inthecompressionprocesstheflow isalwayssubjecttoanadversepressuregradientandthehigherthepressureratiothemoredifficultbecomesthedesignofthecompressor.

Theprocessconsistsofaseriesofdiffusionbothintherotorandstatorbladepassages.

Asingle

spool

compressor

consists

of

one

rotor

assembly

and

stators

with

as

many

stages

as

necessary toachieve thedesiredpressure ratioandall theairflow from the intakepassesthroughthecompressor.

Multispoolcompressorconsistsoftwoormoreassemblies,eachdrivenbyitsownturbineat an optimum speed to achieve higher pressure ratios and to give better operatingflexibility.

Atwinspoolcompressor ismoresuitable forabypasstypeenginethanapurejetenginewherethefrontorlowpressurecompressorisdesignedtohandlealargerairflowthanthehighpressurecompressor.

Onlyapercentageoftheairfromthelowpressurecompressorpassesintothehighpressure

compressor,

the

reminder

of

the

air,

the

by

pass

flow

is

ducted

around

the

high

pressure

compressor.

Both flowsmix in theexhaust system (lowbypass ratioengines)beforepassing into thepropellingnozzle.

Thisarrangementmatches thevelocityof thejetnearer to theoptimum requirementsoftheaircraftandresultsinhigherpropulsiveefficiencyandhencelowerfuelconsumption.

Forthisreason,thepurejetenginewherethebypassratio iszero isnowobsolete forallaircraftbutforthehighestspeedaircraft.

ConstructionFeaturesAxialCompressor

The

construction

of

the

compressor

centers

around

the

rotor

assembly

and

casings.

Therotorshaftissupportedinballandrollerbearingsandcoupledtotheturbineshaftinamannerthatallowsforanyslightvariationofalignment.

Incompressordesigns the rotational speeds is such thatadisc is required to support thecentrifugalbladeload.

Where a number of discs are fitted onto one shaft they may be coupled and securedtogetherbyamechanicalfixingbutgenerallythediscsareassembledandweldedtogether,closetotheirperiphery,thusforminganintegraldrum.

Therotorbladesareofairfoilsectionandusuallydesignedtogiveapressuregradientalongtheirlengthtoensurethattheairmaintainsareasonablyuniformaxialvelocity.

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Thehigherpressuretowardsthetipbalancesoutthecentrifugalactionoftherotorontheairstream.Inordertoobtainthisitisnecessarytotwistthebladefromroottotiptogivethecorrectangleateachpoint.

Thestatorvanesareagainofairfoilsectionandaresecuredintothecompressororintothestatorvaneretainingringswhicharethemselvessecuredtothecasing.

Thevanes

are

often

assembled

in

segments

in

the

front

stages

and

may

be

shrouded

at

their

innerendstominimizethevibrationaleffectofflowvariationsonthelongervanes.

Itisalsonecessarytolockthestatorvanesinsuchamannerthattheywillnotrotatearoundthecasing.

Methodofsecuringbladestodisc

Atypicalrotorbladeshowingtwistedcontour

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Methodsofsecuringvanestocompressorcasing

Singlespool

compressor

Twinspoolcompressor

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AIRFLOWCONTROL

Wherehighpressureratiosonasinglespoolisrequired,itbecomesnecessarytointroduceairflowcontrolintothecompressordesign.

Thismay take the formofvariable inletguidevanes for the first stageplusanumberofstages incorporatingvariablestatorvanes for thesucceedingstagesas thespoolpressureratio

is

increased.

As the compressor speed is reduced from its design value these static vanes areprogressivelyclosed inorder tomaintainanacceptableairanglevalueonto the followingrotorblades.

Also interstagebleedmaybeprovidedbut itsuse indesign isnowusually limited to theprovisionofextramarginwhile theengine isbeingacceleratedbecauseuseof interstagebleedatsteadyoperatingconditionsisinefficientandwastefuloffuel.

Typicalvariablestatorvanes

EffectofVariablegeometryoperationonCompressorCharacteristics

SURGE

Surging isassociatedwithasuddendrop indeliverypressureof thecompressorandwithviolentaerodynamicpulsationwhichistransmittedthroughoutthemachine.

Unstable flow inaxialcompressorscouldbedue totheseparationof flow fromthebladesurfacescalledstalling.

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Unstable flow could also be due to complete breakdown of steady through flow calledsurging.

SurgeMarginisdefinedas:

(SurgePressureRatio/OperatingPressureRatio)1x100

CentrifugalCompressorswillhavemoresurgemarginthantheaxialcompressors.s

Surgingincompressors

N3curveoperatingpointA(PA,mdotA)

PointB(PBmdotB)andPointC(PCmdotC)

Increasedpressureandreducedmassflowresultinginveslopeindicatingstableoperation

ForpointsB&C, (abovemdots) thepressuredeveloped by the compressor matches withtheincreaseddeliverypressureinthepipe.

For points D & E (below mdots) lowerpressures are developed by the compressor.

Butthe

pipe

pressure

will

be

higher

than

these;+veslopeindicatingunstableoperation

EBCSDE is the surge cycle that is repeatedagainandagain

Surging leads to vibrationof the engine thatcanultimatelyleadtomechanicalfailure

Compressor operation to the left of S isinjurioustotheengineandshouldbeavoided(+vesloperegion)

StableoperationofthecompressoristotherightofpointS(vesloperegion)

DesignConsiderations

CentrifugalcompressorswereusedinearlyBritishandAmericanfighteraircraftandalsointheoriginalCometairlineswhichwerethefirstgasturbinepoweredcivilaircraft inregularservice.

Aspowerrequirementsgrew,however,itbecameclearthattheaxialflowcompressorwasmoresuitableforlargerengines.

Hencetheresultwasthataveryhighproportionofdevelopment fundingwasdivertedtothe axial type, leading to theavailabilityofaxial compressorswithanappreciablyhigherisentropicefficiencythanthatcouldbeachievedbytheircentrifugalcounterparts.

Later

it

became

clear

that

smaller

gas

turbines

would

have

to

use

centrifugal

compressors

andseriousresearchanddevelopmentworkstartedagain.

Small turboprops, turboshafts andAuxiliary PowerUnits (APUs) havebeenmade in verylargenumbersandhavenearlyallusedcentrifugalcompressors.

NotableexamplesareP&W,CanadaPT6enginesandHoneywellsmallenginesanda largenumberofAPUs.

CentrifugalCompressorsarealsousedforhighpressurespoolsinsmallturbofanengines.

Centrifugal compressorsareprimarilyused for their suitability forhandling small volumeflowswithhighstagepressureratio.

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Other advantages include a shorter length than an equivalent axial compressor, betterresistance to ForeignObjectDamage (FOD), less susceptibility to lossofperformance bybuildupofdepositsonthebladesurfacesandtheabilitytooperateoverawiderrangeofmassflowatahighparticularrotationalspeed

Bettersurgemarginthanitsaxialcounterpart.

Centrifugalcompressors

are

widely

used

on

natural

gas

pipe

lines,

directly

driven

by

the

free

powerturbineofthepromemover.

Thesamedesignmethodsareapplicablebutthesemachineswouldnormallyoperateatlowpressureratiosandatveryhighinletpressures.

Multistagecentrifugalcompressorsmayalsobeusedinhighpressureratioprocessesuptofive stages with intercooling between stages. This will not be suitable for aircraftapplications.

Thesemayfindapplications inairseparationplantsandthecompressormaybedrivenbysteamturbinesorelectricmotorsviaaspeedincreasinggearboxes

DesignConsiderations(Centrifugal)

CentrifugalCompressorsarealsousedforhighpressurespoolsinsmallturbofanengines.

Centrifugal compressorsareprimarilyused for their suitability forhandling small volumeflowswithhighstagepressureratio.

Other advantages include a shorter length than an equivalent axial compressor, betterresistance to ForeignObjectDamage (FOD), less susceptibility to lossofperformance bybuildupofdepositsonthebladesurfacesandtheabilitytooperateoverawiderrangeofmassflowatahighparticularrotationalspeed

Bettersurgemarginthanitsaxialcounterpart.

Centrifugalcompressorsarewidelyusedonnaturalgaspipelines,directlydrivenbythefreepowerturbineofthepromemover.

Thesamedesignmethodsareapplicablebutthesemachineswouldnormallyoperateatlowpressureratiosandatveryhighinletpressures.

Multistagecentrifugalcompressorsmayalsobeusedinhighpressureratioprocessesuptofive stages with intercooling between stages. This will not be suitable for aircraftapplications.

Thesemay findapplications inairseparationplantsandthecompressormaybedrivenbysteamturbinesorelectricmotorsviaaspeedincreasinggearboxes

With increase in overall pressure the specific fuel consumption reduces and in aircraftapplicationstheendeavouristogetashighapressureratioaspossible.

Butthemechanicalcomplexitiesassociatedwithahighnumberofaxialcompressorstages

mayrestrict

the

pressure

ratio

to

about

40

with

amutispool

compressor.

Itisdifficulttogethighpressureratiowithacentrifugalcompressor.

Axialcompressorhasthepotentialforhigherpressureratioandhigherisentropicefficiencythan thecentrifugalcompressor.But thesurgemargin is lesscompared to thecentrifugalcompressor.

Anothermajoradvantageofanaxialcompressor is thehighmass flow ratepossible foragivenfrontalarea.Inotherwordstheaxialcompressorcanswallowmuchhighermassflowforagivenfrontalareathanitscentrifugalcounterpartofthesamefrontalarea.

Hencetheaxialcompressorsarebestsuitedfor largecivilengineswhichrequirehighmassflowandhighpressureratio.

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Thesepotentialgainshavenowbeen fullyrealizedasthe resultof intensiveresearch intotheaerodynamicsofaxialcompressors.

Theaxialflowcompressordominatesthefieldforlargethrust/powerrequirementsandthecentrifugalcompressor isrestrictedtothe lowerendofthethrust/powerspectrumwheretheflowistoosmalltobehandledefficientlybyaxialbalding.

Inthe

early

days

the

pressure

ratio

of

the

axial

compressor

was

5:1

and

this

required

about

10stages.

Over the years the overall pressure ratios have risen dramatically and some turbofanengineshavepressureratiosexceeding40:1.

Continued aerodynamic developmenthas resulted in a steady increase in stagepressureratiowiththeresultthatthenumberofstagesrequiredforagivenpressureratiohasbeengreatlyreduced.

As a consequence there has been a reduction in engine weight for a specified level ofperformance,whichisparticularlyimportantforaircraftengines.

However itshouldbenotedthathighstagepressureratios implyhighMachnumbersandlargedeflectionsinthebladingwhichwouldnotgenerallybejustifiable inan industrialgasturbineenginewhereweightisnotcritical.

Industrialunitsbuiltonmuchmorerestrictedbudgetthananaircraftenginewillinvariablyusemoreconservativedesigntechniquesresultinginmorestages.

DesignTradeoff

LargeCivilengines:TradeoffbetweenverylowSFCandlow/mediumSpecificThrust:

Axialcompressorswithlargemassflowsandveryhighpressureratios

Multispool(LPandHP)andlargenumberofstages

Highbypassratio(resultinginverylowSFCandlow/mediumspecificthrust)

HighThrust

Goodsurgemargin

Longrangeandendurance

Longlife

BusinessJets:TradeoffbetweenmoderateSFCandlow/mediumspecificthrust

Axialcompressors followedbyCentrifugalboostercompressors;Normalconfiguration isaFanstagefollowedbyaxialcompressorandacentrifugalcompressor.Theaxialcompressorprecedes the centrifugal compressor since the mass flow per unit frontal area of axialcompressorishighcomparedtocentrifugalcompressorresultinginhighpressureratiowithmediummassflowwhichischaracteristicofBusinessjets(Smallpassengeraircraft)

Multispoolconfiguration

Highbypassratio(resultinginlowSFCandlow/mediumspecificthrust)

Mediumthrustlevel

Goodsurgemargin

Mediumrangeandendurance

Reasonablelife

MilitaryEngines:Tradeoffmedium/highSFCandhighspecificthrust

Axialcompressorswithmedium/highSFCandhighSpecificthrust

Multispoolwithlargenumberofstages

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Lowbypassratiooftheorderof0.3to0.5resultinginhighspecificthrustandmediumSFC

Sincethemassflowsarecomparatively lowcomparedtothe largecivilenginesthethrustlevelsarebetweenBusinessjetsandlargecivilengines

Shortradiusofaction(Twicetherange)

Shortendurance

Reasonablelife

SmallGasTurbineEngine

Small Gas Turbine Engines invariably employ Centrifugal Compressors with high stagepressureratio

MainlyintendedforUAVsandGasTurbineStarter(JetFuelStarter)

Lowmassflows

MediumSFC

Mainly straightjets and some small engines will have axial compressor in front of theCentrifugalcompressorandmayalsohavebypassconfigurationforfuelefficiency(LowSFC)

Shortlife

BALANCING

Thebalancingofacompressorrotoror impeller isanextremely importantoperation in itsmanufacturing.

Inviewofthehighrotationalspeedsandthemassofmaterialsanyunbalancewouldaffecttherotatingassemblybearingsandengineoperation.

Balancingofthesepartsiseffectedonaspecialbalancingmachine.

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TURBO PROP & TURBO SHAFT ENGINES

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Turbopropengine

Aswithallgasturbineengines,thebasicpowerproductionintheturbopropisaccomplishedin thegas generatoror coreof theengine,wherea steady streamofairdrawn into theengineinletiscompressedbyaturbocompressor.Thehighpressureairisnextheatedinacombustionchamberbyburninga steady streamofhydrocarbon fuel injected insprayor

vaporform.

The

hot,

high

pressure

air

is

then

expanded

in

aturbine

that

is

mounted

on

the

samerotatingshaftasthecompressorandsuppliestheenergytodrivethecompressor.Byvirtueoftheairhavingbeenheatedathigherpressure,there isasurplusofenergy intheturbinethatmaybeextractedinadditionalturbinestagestodriveausefulload,inthiscaseapropeller.

VariationsinTurbopropengines

Alargevarietyofdetailedvariationsarepossiblewithinthecore.Thecompressormaybeanaxialflow type,acentrifugal (that is,radialflow) type,oracombinationofstagesofbothtypes(thatis,anaxicentrifugalcompressor).Inmodernmachines,thecompressormaybe

split

in

two

sections

(a

low

pressure

unit

followed

by

a

high

pressure

unit),

each

driven

by

its own turbine through concentric shafting, in order to achieve very high compressionratiosotherwiseimpossibleinasinglespool.

Hybridenginethatprovidesjetthrustandalsodrivesapropeller.Itissimilartotheturbojetexceptthatanaddedturbine,behindthecombustionchamber,worksthroughashaftandspeedreducing gears to turn a propeller at the front of the engine. Because ofimprovements inturbojetdesign,theturboprop,which is lessefficientathighspeeds, lostmuchofitsimportanceinthe1960s,thoughitisstillusedforrelativelyshortrangeaircraft.

TypicalDiagram

ShaftHorsePower

A turbopropengine isa typeofgas turbineengineused inaircraft.Mostofa turbopropengine'spower isusedtodriveapropeller,andthepropellersusedareverysimilartothepropellers used in piston or reciprocating enginedriven aircraft (with the exception thatturbopropsusuallyuseaconstantvelocitypropeller).

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Applications

A turboprop engine is similar to a turbojet, but has additional stages in the turbine torecovermorepowerfromtheenginetoturnthepropeller.Turbopropenginesaregenerallyusedon smallor slow subsonicaircraft,but someaircraftoutfittedwith turbopropshavecruisingspeedsinexcessof500km(926km/h,575mph).

TypicalComponents

Initssimplestform,aturbopropconsistsofanintake,compressor,combustor,turbineandapropellingnozzle.Airisdrawnintotheintakeandcompressedbythecompressor.Fuelisthen added to the compressed air in the combustor. The hot combustion gases expandthrough the turbine. Part of the power generated by the turbine is used to drive thecompressor. The rest goes through the reduction gearing to the propeller. Furtherexpansion of the gases occurs in the propelling nozzle, where the gases exhaust toatmosphericpressure. Thepropellingnozzleprovides a relatively smallproportionof thethrustgeneratedbyaturboprop,theremaindercomesfromtheconversionofshaftpowertothrustinthepropeller.

Businessjets

Turboprops are very efficient atmodest flight speeds (below 450mph), because thejetvelocityofthepropeller(andexhaust) isrelatively low.Duetothehighpriceofturbopropengines, theyaremostlyusedwherehighperformance ShortTakeoffandLanding (STOL)capabilityandefficiencyatmodest flight speeds is required. Inacivilianaviationcontext,themostcommonapplicationofturbopropenginesaresmallcommuteraircraft.

Technologicalaspects

Inaturbopropmuchofthejetthrustissacrificedinfavorofshaftpower,whichisobtained

byextracting

additional

power

(to

that

necessary

to

drive

the

compressor)

from

the

turbine

expansionprocess.Whilethepowerturbinemaybeintegralwiththegasgeneratorsection,many turboprops today feature a Free Power Turbine, on a separate coaxial shaft. Thisenables the propeller to rotate freely, independent of compressor speed. Owing to theadditionalexpansion in the turbinesystem, the residualenergy in theexhaustjet is fairlylow.Consequently, theexhaustjetproduces (typically) less than10%of the total thrust,includingthatfromthepropeller.

Turbopropengine

Theactualpercentageofthrustwillvarywithahostoffactorssuchasspeed,altitude,andtemperature.

The turbopropwilldelivermore thrust,up tomediumspeeds, thaneither the turbojetorturbofan.

Also,astheturbopropclimbstohigheraltitudes,themassofairbeingacceleratedbythepropellerdecreasesduetothedecreaseinairdensity.

Components

PropellerAssembly

Majorityofthrust(90%)isaresultofthelargemassbeingacceleratedbythepropeller

Bladesareinstalledintothehub

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Thehub(barrelassembly)isthenattachedtothepropellershaft

Thepitch change/dome assembly is themechanism that changes the blade angle of thepropeller

Turboshaftengine

Itis

aGas

turbine

engine

which

powers

arotating

acylindrical

shaft

to

rotate

the

Helicopter

rotor

A turbo shaftengine isa formofgas turbinewhich isoptimized toproduce shaftpower,ratherthanjetthrust.

In principle a turbo shaft engine is similar to a turbojet, except the former featuresadditional turbineexpansion toextractheatenergy from theexhaustand convert it intooutputshaftpower.

Ideally there shouldbe little residual thrustenergy in theexhaustand thepower turbineshouldbefreetorunatwhateverspeedtheloaddemands.

The general layoutofa turbo shaft is similar to thatofa turboprop, themaindifference

beingthe

latter

produces

some

residual

propulsion

thrust

to

supplement

that

produced

by

theshaftdrivenpropeller.

Anotherdifference isthatwithaturboshaftthemaingearbox ispartofthevehicle (e.g.helicopterrotorreductiongearbox),nottheengine.

Virtuallyallturboshaftshavea"free"powerturbine,althoughthisisalsogenerallytrueformodern turbopropengines.Ata givenpoweroutput, compared to theequivalentpistonengine,aturboshaftisextremelycompactand,consequently,lightweight.

Thenameturboshaftismostcommonlyappliedtoenginesdrivingships,helicopters,tanks,locomotivesandhovercraftorthoseusedasstationarypowersources

Todayalmostallenginesarebuiltso thatpowertakeoff is independentofenginespeed,usingthefreeturbinestage.Thishastwoadvantages:

Itallowsahelicopterrotororpropellertospinatanyspeedinsteadofbeinggeareddirectlytothecompressorturbine.

Itallowstheenginetobesplitintotwosections,the"hotsection"containingthemajorityoftheengine,and the separatepowertakeoff,allowing thehotsection tobe removed foreasiermaintenance.

This leadstoslightly largerengines,butforthespeedrangesservedbytheseengines it isconsideredtobeunimportant.

Today practically all smaller turbine engines come in both turboprop and turbo shaftversions,differingprimarilyintheiraccessorysystems.

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SchematicDiagram

Thecompressorspoolisshowningreenandthefree/powerspoolisinblue.

GearedFan

Asbypassratioincreases,themeanradiusratioofthefanandLPturbineincreases.

Consequently, ifthefanistorotateat itsoptimumbladespeedtheLPturbinebladingwillrunslow,soadditionalLPTstageswillberequired,toextractsufficientenergytodrivethefan.

Introducinga reductiongearbox,withasuitablegear ratio,between theLP shaftand thefan,enablesboththefanandLPturbinetooperateattheiroptimumspeeds.Typicalofthisconfigurationare the longestablishedHoneywellTFE731and the recentPratt&WhitneyAdvancedTechnologyFanIntegrator(ATFI)demonstratorengine(nowtheGearedTurbofan

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COMBUSTION CHAMBERS

CONTENTS

INTRODUCTION

COMBUSTIONPROCESS

FUELSUPPLY

VARIOUSTYPESOFCOMBUSTIONCHAMBERS

COMBUSTIONCHAMBERPERFORMANCE

EFFECTOFOPERATINGVARIABLESONCOMBUSTORPERFORMANCE

MATERIALS

COMBUSTORCFD

RIGTESTINGOFTHECOMBUSTOR

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COMBUSTIONCHAMBER

Has thedifficult task ofburning large quantities of fuel, supplied through the fuel spraynozzles,withtheextensivevolumesofairsuppliedbythecompressor.

Releasetheheatinsuchamannerthattheairisexpandedandacceleratedtogivesmooth

streamof

uniformly

heated

gas

at

all

conditions

required

by

the

turbine.

Thistaskshouldbeaccomplishedwiththeminimumpressure lossandwiththemaximumheatreleaseforthelimitedspaceavailable

Amount of fuel added in the combustion chamber depends on the temperature riserequiredacrossthecombustionchamber.

However maximum temperature is limited by the materials of turbine rotor and nozzleguidevanes.

Airhasalreadybeenheatedbytheworkdoneduringcompression.

A temperature rise across the combustion chamber is required since the thrust or shaftpowerproducedbytheengine isafunctionofturbineentrytemperature.Thecombustion

chambershould

also

be

capable

of

maintaining

stable

and

efficient

combustion

over

awide

rangeofengineoperatingconditions.

COMBUSTIONPROCESS

Air from the engine compressor enters the combustion chamber at a typical velocity ofabout150meters/sec.

Since the velocity [orMach number] is too high for combustion, there is a necessity todiffusetheairi.etodecelerateitandraiseitsstaticpressure.

Ifthevelocityisnotreducedanyfuellitwillbeblownaway.

Hencea regionof lowvelocityhas tobecreated in thecombustionchamber,so that the

flamewill

remain

alight

throughout

the

range

of

engine

operating

conditions.

Innormaloperationtheoverallfuel/airratioofacombustionchambervariesfrom0.01to0.025.

Howeverthefuel(aviationturbinefuel,aparticularformofkerosene)willburneffectivelyatfuel/airratioofabout0.067(Stoichiometricratio).

Hencethereisarequirementofintroducingtheairinthecombustionchamberinstages.

Threestagescanbedistinguished.

Around20%ofthecompressedairisintroducedaroundthejetoffuelknownastheprimaryzonetoprovidethenecessaryhightemperatureforrapidcombustion.

About30%ofthecompressedairisintroducedthroughtheholes intheflametubeinthe

secondary

zone

to

complete

the

combustion

process.

Finally inthe tertiaryor thedilutionzone the remainingair ismixedwith theproductsofcombustiontocoolthemdowntothetemperaturerequiredatinlettotheturbine(turbineinlet temperature).Thistemperatureacceptable to theturbineNozzleGuideVanes (NGV)dependsontheturbinematerialaswellasthebladecoolingtechnique.

Sufficient turbulencemustbepromoted so that thehotand cold streamsare thoroughlymixedtogivethedesiredoutlettemperaturedistributionwithnohotstreakswhichwoulddamagetheturbineblades.

An electric spark from an igniter plug initiates combustion and the flame is then selfsustained.

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Combustionoccurspracticallyatconstantpressureexcept forasmallpressure loss(about5% 6%).

Though thedesignof combustion chamber and themethodofadding the fuelmay varyconsiderably the airflow distribution used to effect and maintain combustion is alwayssimilartowhatisdescribedabove

Anearlycombustionchamber

Flamestabilizingandgeneralairflowpattern

FUELSUPPLY

Fuelsupplytothecompressedairstream isthroughthe injectionofafineatomizedsprayintotherecirculatingstreamthroughspraynozzles

Fuelnozzledesignplaysamajorpartincombustionchamberperformance.

Notonlymust the fuelnozzleatomizeanddistribute the fuel,but itmustalsobeable tohandleawiderangeoffuelflows.

Therearetwotypesoffuelatomizersnamelypressurejetatomizerandairblastatomizer.

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In a pressure jet atomizer there are two stages of fuel injection namely primary andsecondary.

Primary stage is used for light up and up to reaching idling speed. Then at a particularpressureknownasthecrackingpressure,thesecondarystagecracksopenwhichtakescareof the requirementofwide rangeof fuel flowsdependingon the engineRPMand flight

condition.

Alloftheoperatinganddesignvariablesmustbetaken intoaccountwhentheatomizer isdesignedandmanufactured.

Final configuration of the combustion chamber at best is a compromise to achieve thedesiredoperating characteristics since it is impossible todesignandmanufactureagivencombustion chamber that will have 100% combustion efficiency, zero pressure loss,maximumlife,minimumweight,minimumfrontalarea,allatthesametime

TYPESOFCOMBUSTIONCHAMBERS

CANNULARCOMBUSTOR

CAN

ANNULAR

COMBUSTOR

ANNULARCOMBUSTOR

Multiplecombustionchamber

Chambersaredisposedaroundtheengineandcompresseddeliveryairisdirectedbyductstopassintotheindividualchambers.

Eachchamberhasaninnerflametubearoundwhichthereisanaircasing.

Separate flame tubes are interconnected to allow each tube to operate at the samepressureandallowcombustiontopropagate.

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Can annularcombustionchamber

Bridges the gapbetween cannularandannularcombustionchamber.

Anumberofflametubesarefittedinsideacommonaircasing.

Annularcombustionchamber

Consist of a single flametube completely annular inform which is contained inaninnerandoutercasing.

Widely used combustionchamber.

Main advantage is that forthe samepoweroutput the

lengthis

short

(only

75%

of

canannular).

Verygoodheat release ratewithcompactsize.

Minimumpressureloss.

Elimination of combustionpropagation problems fromchambertochamber.

Results in considerablesaving of weight andproductioncost.

Advantagesanddisadvantagesofdifferenttypesofcombustionchambers

Inthecantype,individualcansaremountedinacirclearoundtheengineaxis

Oneofthemaindisadvantagesofcannularcombustoristhattheydonotmakethebestuseoftheavailablespaceandthisresultsinalargediameterengine

Ontheotherhandtheburnersareindividuallyremovableforinspectionandfuel/airratiosareeasiertocontrolthaninannulardesigns

The annular combustor is essentially a single chamber made up of concentric cylindersmountedcoaxiallyabouttheengineaxis

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Thelatestcombustorscombinethebestfeaturesofannularandcanannularconfigurations

Annular combustors have less surfacetovolume ratio than comparable cannularcombustorsandhencelesscoolingairisrequired

Annular combustor weight is less, while at the same time there is an improvement incombustorperformance

Thisarrangement

makes

more

complete

use

of

available

space,

has

low

pressure

loss,

fits

wellwiththeaxialcompressorandturbineandfromatechnicalviewpointhasthehighestefficiency

Theannularcombustorhasadisadvantagebecause structuralproblemsmayarisedue tothelargediameter,thinwallcylinderrequiredwiththistypeofcombustor

Theproblemismoresevereforlargerengines

There isalsosomedisadvantage in that theentirecombustormustbe removed from theengineforinspectionandrepair

The canannulardesignalsomakes gooduseofavailable spacebutemploysanumberofindividually replaceablecylindrical inner liners that receiveair throughacommonannular

housing

for

good

control

of

fuel

and

airflow

patterns

The canannulararrangementhas theaddedadvantageofgreater structural stabilityandlowerpressurelossthanthatofthecantype

COMBUSTIONCHAMBERPERFORMANCE

Should be capable of allowing fuel to burn efficiently over a wide range of operatingconditionswithoutincurringalargepressureloss.

Incaseofflameextinctionitshouldbepossibletorelight.

The flame tube and fuel spray nozzles should be mechanically reliable and have goodstructuralintegrity.

Shouldhave

low

pressure

loss

of

the

order

of

5%

6%.

Shouldhavehighheatintensityrateforagivenvolume.

IMPORTANTFACTORSAFFECTINGCOMBUSTORDESIGN

Acceptablecombustoroutlettemperaturetotheturbinenozzleguidevanes.

Goodtemperaturedistributionsoastopreventlocaloverheatingofturbineblades.

Stableoperationoverawiderangefuel/airratiosfromfullloadtoidlingconditions.

Formationofcarbondeposits(coking)shouldbeavoided.

Avoidanceofsmokeintheexhaustisofmajorimportance.

Lesspollution levelnamelyproductionofoxidesofnitrogen (NOx)carbonmonoxide (CO)and

unburnt

hydrocarbons

(UHC).

Effectofoperatingvariablesoncombustorperformance

Theoperatingvariablesare:

Pressure

Inletairtemperature

Fuel/airratio

Flowvelocity/Machnumber

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Combustionefficiency

Asthepressureoftheairenteringthecombustorincreasesthecombustionefficiencyrisesandlevelsofftoarelativelyconstantvalue

Thepressureatwhichthislevelingoffoccursisusuallyabout1atmosphere(atm),butthismayvarysomewhatwithdifferentcombustorconfigurations

Astheinlettemperatureisincreased,combustionefficiencyrisesuntilitreachesavalueofsubstantially100percent

Withincreaseinfuel/airratiocombustionefficiencyfirstincreases,thenlevelsoffwhenthemixtureinthecombustionzoneisclosetotheidealvalueandthendecreasesasthefuel/airratiobecomestoorich

Anincreaseinfuel/airratiowillresultinincreasedpressurelossbecauseincreasingfuel/airratioscausehighertemperatureswithacorrespondingdecreaseingasdensity

In order to maintain continuous flow the gasesmust travel at higher velocities and theenergyneededtocreatehighervelocitiesmustcomefromanincreaseinpressureloss

Increasingtheflowvelocitybeyondacertainpointreducescombustionefficiencybecause

itreduces

the

time

available

for

mixing

and

burning

Stableoperatingrange

Thestableoperatingrangeofacombustoralsochangeswithvariationsinpressureandflowvelocity

As thepressuredecreases, the stableoperating rangebecomesnarroweruntilapoint isreachedbelowwhichburningwillno