Gas Turbine Combustion Chamber

1 Gas Turbine Combustion Chamber (Source: Gas Turbine Theory , Cohen, Rogers)Akash JamesAsst. Professor, Dept. of Mechanical Engineering, RSETS imple Open Cycle Gas Turbine Schematic DiagramAJ2 Contents Combustion Process Types of Combustion Chambers Performance Parameters Pressure Loss Combustion Efficiency Stability Loop Combustion IntensityAJ3 Process Combustioninthenormal,opencycle,gasturbi neisacontinuousprocessinwhichfuelisburne dintheairsuppliedbythecompressor;anelect ricsparkisrequiredonlyforinitiatingtheco mbustionprocess,andthereaftertheflamemus tbeself-sustaining. Combustionofaliquidfuelinvolvesthemixing ofafinesprayofdropletswithair,vaporizati onofthedroplets,thebreakingdownofheavyhy drocarbonsintolighterfractions,theintima temixingofmoleculesofthesehydrocarbonswi thoxygenmolecules,andfinallythechemicalr eactionsthemselves.

2 Ahightemperature,suchasisprovidedbytheco mbustionofanapproximatelystoichiometricm ixture,isnecessaryifalltheseprocessesare tooccursufficientlyrapidlyforcombustioni namovingairstreamtobecompletedinasmallsp ace ButinactualpracticeA/Fratioisintherangeo f100:1,whilestoichiometricratioisaround1 5 ProcessAJ4 Factorsinfluencingdesign Lowturbineinlettemperature Uniformtemperaturedistributionatturbinei nlet( ,toavoidlocaloverheatingofturbineblades) Stableoperationevenwhenfactorslikeairvel ocity,A/Fratio&chamberpressurevariesgrea tly,especiallyforaircraftengines(thelimi tisthe flame-out ofthecombustionchamber)&attheeventofafla me-outthecombustorshouldbeabletorelightq uickly. Theformationofcarbondeposits('coking') Aircraftenginesshouldavoidvisiblesmokeas ithindersvisibilityinairports Finally,PollutantslikeNOx,CO,UnburnedHyd rocarbons(UHCs) method of introducing air Primaryzone(15-20%air) Airisintroducedaroundthejetoffuel burnsatapproximatelytheStoichiometricRat io Therefore,Hightemperature Andthus,RapidCombustion SecondaryZone(30%air) Introducedthroughholesintheflame-tubeint hesecondaryzonetocompletethecombustion Forhighcombustionefficiency,airmustbeinj ectedcarefullyattherightpointsintheproce ss,toavoidchillingtheflamelocallyanddras ticallyreducingthereactionrateinthatneig hbourhood TertiaryZone(remainingair)

3 DilutionZone Cooling Sufficientturbulencemustbepromotedsothat thehotandcoldstreamsarethoroughlymixedto givethedesiredoutlettemperaturedistribut ion, stability flow pattern is achieved by, vanes [fig ] downstream of a hemispherical Baffle [fig (a)] injection [fig (b)] system (walking stick/T-shaped tubes) [fig (c)] Type (Tubular) (Tubo-annular) TypeTypes of Combustion (Can)Theearliestaircraftenginesmadeuseof can(ortubular) , , ,wheretheflowisdividedintoseparatestream sinthediffuser. Easierdevelopment(couldbecarriedoutonasi nglecanusingonlyafractionoftheoverallair flowandfuelflow)But IncreasedVolume,weight&frontalarea IncreasedPressuredrop(moresurfaceareainc ontactwithair/gas)AJ9 Arrangementofthecombustors,lookingaxison , (outsidetheflametube),theorangeindicates thecombustionproductflowpath(insidethefl ametube) (Cannular) , Reducedshaftlength EasymaintenanceAlsolikeCanCombustor, Easierdevelopment,But IncreasedVolume,weight&frontalarea ,inwhichmaximumuseismadeofthespaceavaila blewithinaspecifieddiameter.

4 , disadvantages of Can type, reduces the pressure loss (less surface exposed to air/gas flow) Compact sizeBut, Less structural integrity Difficult to obtain even temperature distribution Difficult development (larger test facilities required)AJ13AJ14 Annular combustor on aPratt & Whitney JT9 Dturbofan( )AJ154. Silo typeLargeindustrialgasturbines,wherethes pacerequiredbythecombustionsystemislessc ritical, , , Lowpressureloss CanburnlowqualityfuelBut, BulkyAJ16 Performance ParametersPressure Loss1. Fundamental LossPressurelossdueto, (pxA) , Skinfriction TurbulenceThepressurelossduetofrictionis foundtobeverymuchhigherthanthatduetocomb ustion,mainlyduetoturbulence, (withoutcombustion)isduetoskinfrictionan dturbulencealone(Coldloss).[Withoutcombu stionthereisnotemperatureratioandhencefr omtheaboveequationitisevidentthatK1repre sentsthecoldloss.]

5 ]Ahotrun(withcombustion) ,K1andK2aredeterminedfromacombustioncham beronatestrigfromacoldrunandahotrunTheov erallpressurelosscanoftenbeexpressedbyan equationoftheform,AJ19 Combustion EfficiencyOverall Combustion Efficiency is defined as,Where f(Fuel/Air ratio) & Inlet and outlet mean stagnation temperatures are measured , , Intensity Lowerthecombustionintensityeasiertodesig nasystemwithdesiredrequirements Cannotcomparetosystemsbasedonefficiency& pressurelossiftheyvarywidelyinthecombust ionintensity Aircraftsystems 2-5x104kW/m3atm Industrialsystems muchlessduetolargecombustionsystemsAJ22