Dynamics of Premixed H2CH4 Flames Under Near Blowoff ConditionsWord格式.docx
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Dynamics of Premixed H2CH4 Flames Under Near Blowoff ConditionsWord格式.docx
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ASMETurboExpo2009:
PowerforLand,SeaandAir
Orlando,Florida,USA
GT2009-59981
DYNAMICSOFPREMIXEDH2/CH4FLAMESUNDERNEARBLOWOFFCONDITIONS
QingguoZhang,SantoshJ.Shanbhogue,TimLieuwen
SchoolofAerospaceEngineering
GeorgiaInstituteofTechnology
Atlanta,GA30332-0150
ABSTRACT
Swirlingflowsarewidelyusedinindustrialburnersandgasturbinecombustorsforflamestabilization.Severalpriorstudieshaveshownthattheseflamesexhibitcomplexdynamicsundernear-blowoffconditions,associatedwithlocalflameletextinctionandalterationinthevortexbreakdownflowstructure.Theseextinctioneventsareapparentlyduetothelocalstrainrateirregularlyoscillatingaboveandbelowtheextinctionstrainratevaluesneartheattachmentpoint.Inthiswork,global,temporallyresolvedanddetailedspatialmeasurementswereobtainedofhydrogen/methaneflames.Supportingcalculationsofextinctionstrainrateswerealsoperformedusingdetailedkinetics.Itisshownthatflamesbecomeunsteady(orlocalextinctionshappen)atanearlyconstantextinctionstrainratefordifferenthydrogen/methanemixtures.Baseduponanalysisoftheseresults,itissuggestedthatclassicDamkohlernumbercorrelationsofblowoffare,infact,correlationsfortheonsetoflocal-extinctionevents,notblowoffitself.CorrespondingMiescatteringimagingofnear-blowoffflamesalsowasusedtocharacterizethespatio-temporaldynamicsofholesalongtheflamethatareassociatedwithlocalextinction.
Keywords:
Swirling,Hydrogen,LeanBlowout
INTRODUCTION
Swirlingflowsarewidelyusedinindustrialburnersandgasturbinecombustorsforflamestabilization.Anumberofoperationalandperformanceissuesareofconcerninmodern,highlytuneddrylowNOx(DLN)combustionsystemsutilizingswirlstabilizedflames,includingblowout,flashback,combustioninstabilities,andemissions[1,2].Theissueofleanblowoffisthefocusofthisstudy,aconcernduetotherequirementtooperateDLNsystemsunderveryfuelleanconditionsand,therefore,closetotheirblowofflimits.Flamescanonlybestabilizedinhighvelocityreactantstreamsoveracertainrangeofconditions.Determiningtheseconditions,andthosewheretheflamecannotbestabilized–referredtohereasblowoff–isanimportantissueinanypracticalcombustiondevice.
Alargeliteratureonpredicting,measuring,andcorrelatingblowofflimitsalreadyexists[3-6].Severaldifferenttheoriesorphysicalconsiderationshavebeenusedinpastblowoutcorrelationstudies,suchasthoseofZukoskiandMarble[7],Spalding[8],Longwell[9,10]andothers.AsnotedbyGlassman[4],however,theyleadtoessentiallythesameformofcorrelationthatrelatestheblowofflimitstoaDamkö
hlernumber,i.e.,ratioofafluidmechanicandchemicalkinetictime,τres/τchem.TheworkofHoffmanetal.[3]isofparticularinteresthere,asitdemonstratedthatrelatedscalingcouldbeusedtocorrelateblowofflimitsinswirling,premixedflames.
Wenextbrieflyreviewseveralpriorphenomenologicalcharacterizationsofblowoff.Longwelletal.[9]suggestedthatblowoffoccurswhenitisnotpossibletobalancetherateofentrainmentofreactantsintotherecirculationzone,viewedasawellstirredreactor,andtherateofburningofthesegases.SinceentrainmentratesscaleasD/U[11],thiscriterionreducestoaDamkö
hlernumberblowoffcriterion,usingachemicaltimethatisderivedfromthewellstirredreactor,τPSR.Asimilaridearelatestoanenergybalancebetweenheatsuppliedbythehotrecirculatingflowtothefreshgasesandthatreleasedbyreaction[12-15].Inthisview,blowoffoccurswhentheheatrequiredbythecombustiblestreamexceedsthatreceivedfromtherecirculationzone.Thisleadstothesameentrainmentbased,fluidmechanicaltimescalingasabove,andtheresultantsimilarDamkö
hlernumberblowoffcriterion.
Adifferentviewisthatthecontacttimebetweenthecombustiblemixtureandhotgasesintheshearlayermustexceedachemicalignitiontime[8,15-17].Forexample,Zukoski[16,18]suggestedthatignitionoftheincomingfreshunburntmixtureoccursintheshearlayerasitmixeswithcombustionproductsfromthere-circulationzonebehindthebluffbody.Therelevantchemicaltimeinthisdescriptionisanignitiontime.Thefluidmechanictimescalingassociatesthecharacteristicdimensionbytherecirculationzonelength,whichalsobeingproportionaltothebluffbodysize,leadstoasimilarDamkö
hlernumbercorrelation.
Finally,severalstudieshaveproposedaflameletbaseddescriptionbaseduponlocalextinctionbyexcessiveflamestretch[19,20].
Itisdifficulttodeterminewhichoftheabovedescriptionsmostaccuratelydescribesthecontrollingprocessesbaseduponanal
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