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1、17.1.1OverviewandLimitationsNOxemissionconsistsofmostlynitricoxide(NO).Lesssignificantarenitrogenoxide(NO2)andnitrousoxide(N2O).NOxisaprecursorforphotochemicalsmog,contributestoacidrain,andcausesozonedepletion.Thus,NOxisapollutant.TheFLUENTNOxmodelprovidesatooltoun
2、derstandthesourcesofNOxproductionandtoaidinthedesignofNOxcontrolmeasures.NOxModelinginFLUENTTheFLUENTNOxmodelprovidesthecapabilitytomodelthermal,prompt,andfuelNOxformationaswellasNOxconsumptionduetoreburningincombustionsystems.ItusesratemodelsdevelopedattheDepartme
3、ntofFuelandEnergy,TheUniversityofLeeds,Englandaswellasfromtheopenliterature.TopredictNOxemission,FLUENTsolvesatransportequationfornitricoxide(NO)concentration.WithfuelNOxsources,FLUENTsolvesanadditionaltransportequationforanintermediatespecies(HCNorNH3).TheNOxtrans
4、portequationsaresolvedbasedonagivenflowfieldandcombustionsolution.Inotherwords,NOxispostprocessedfromacombustionsimulation.ItisthusevidentthatanaccuratecombustionsolutionbecomesaprerequisiteofNOxprediction.Forexample,thermalNOxproductiondoublesforevery90 Ktemperatu
5、reincreasewhentheflametemperatureisabout2200 K.Greatcaremustbeexercisedtoprovideaccuratethermophysicaldataandboundaryconditioninputsforthecombustionmodel.Appropriateturbulence,chemistry,radiationandothersubmodelsmustbeapplied.Toberealistic,onecanonlyexpectresultsto
6、beasaccurateastheinputdataandtheselectedphysicalmodels.Undermostcircumstances,NOxvariationtrendscanbeaccuratelypredictedbuttheNOxquantityitselfcannotbepinpointed.AccuratepredictionofNOxparametrictrendscancutdownonthenumberoflaboratorytests,allowmoredesignvariations
7、tobestudied,shortenthedesigncycle,andreduceproductdevelopmentcost.ThatistrulythepoweroftheFLUENTNOxmodeland,infact,thepowerofCFDingeneral.TheFormationofNOxinFlamesInlaminarflames,andatthemolecularlevelwithinturbulentflames,theformationofNOxcanbeattributedtofourdist
8、inctchemicalkineticprocesses:thermalNOxformation,promptNOxformation,fuelNOxformation,andreburning.ThermalNOxisformedbytheoxidationofatmosphericni
