多相流(multiphaseflows)fluent教程

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 IntroductoryFLUENTTrainingIntroductoryFLUENTTraining9-2©2006ANSYS,Inc.Allrightsreserved.ANSYS,Inc.ProprietaryFluentUserServicesCenter
Aphaseisaclassofmatterwithadefinableboundaryandaparticulardynamicresponsetothesurroundingflow/potentialfield.
Phasesaregenerallyidentifiedbysolid,liquidorgaseousstatesofmatterbutcanalsorefertootherforms:Materialswithdifferentchemicalpropertiesbutinthesamestateorphase(i.e.liquid-liquid,suchas,oil-water)
Thefluidsystemisdefinedbyaprimaryandmultiplesecondaryphases.Oneofthephasesisconsideredcontinuous(primary)Theothers(secondary)areconsideredtobedispersedwithinthecontinuousphase.Theremaybeseveralsecondaryphasedenotingparticleswithdifferentsizes
Incontrast,multi-componentflow(speciestransport)referstoflowthatcanbecharacterizedbyasinglevelocityandtemperaturefieldforallspecies.PrimaryPhaseSecondaryPhase9-3©2006ANSYS,Inc.Allrightsreserved.ANSYS,Inc.ProprietaryFluentUserServicesCenter
Inordertoselecttheappropriatemodel,usersmustknowapriorithecharacteristicsoftheflowintermsofthefollowing:FlowregimeParticulate(bubbles,dropletsorsolidparticlesincontinuousphase)Stratified(fluidsseparatedbyinterfacewithlengthscalecomparabletodomainlengthscale)MultiphaseturbulencemodelingForparticulateflow,onecanestimateParticlevolumeloadingStokesnumber9-4©2006ANSYS,Inc.Allrightsreserved.ANSYS,Inc.ProprietaryFluentUserServicesCenterBubblyflow–Discretegaseousbubblesinacontinuousfluid,e.g.absorbers,evaporators,spargingdevices.Dropletflow–Discretefluiddropletsinacontinuousgas,e.g.atomizers,combustorsSlugflow–LargebubblesinacontinuousliquidStratified/free-surfaceflow–Immisciblefluidsseparatedbyaclearlydefinedinterface,e.g.free-surfaceflowParticle-ladenflow–Discretesolidparticlesinacontinuousfluid,e.g.cycloneseparators,airclassifiers,dustcollectors,dust-ladenenvironmentalflowsFluidizedbeds–FluidizedbedreactorsSlurryflow–Particleflowinliquids,solidssuspension,sedimentation,andhydro-transportGas/LiquidLiquid/LiquidGas/SolidLiquid/SolidSlugFlowBubbly,Droplet,orParticle-LadenFlowStratified/Free-SurfaceFlowPneumaticTransport,Hydrotransport,orSlurryFlowSedimentationFluidizedBed9-5©2006ANSYS,Inc.Allrightsreserved.ANSYS,Inc.ProprietaryFluentUserServicesCenter
Volumeloading–diluteordenseReferstothevolumefractionofsecondaryphase(s)Fordiluteloading(10%),theaverageinter-particledistanceisaroundtwicetheparticlediameter.Thus,interactionsamongparticlescanbeneglected.
Particulateloading–ratioofdispersedandcontinuousphaseinertias
@=raracouplingwaytwo1,couplingwayone,1contcontpartpartncell/domaitheofVolumencell/domaiainphasetheofVolumeFractionVolume=a=primaryVcellVsecondaryV9-6©2006ANSYS,Inc.Allrightsreserved.ANSYS,Inc.ProprietaryFluentUserServicesCenter
Turbulencemodelingwithmultiphaseflowsischallenging.
Presently,single-phaseturbulencemodels(suchask–
orRSM)areusedtomodelturbulenceintheprimaryphaseonly.
Turbulenceequationsmaycontainadditionaltermstoaccountforturbulencemodificationbysecondaryphase(s).
Ifphasesareseparatedandthedensityratioisoforder1oriftheparticlevolumefractionislow(10%),thenasingle-phasemodelcanbeusedtorepresentthemixture.
Inothercases,eithersinglephasemodelsarestillusedor“particle-presence-modified”modelsareused.9-7©2006ANSYS,Inc.Allrightsreserved.ANSYS,Inc.ProprietaryFluentUserServicesCenter
Forsystemswithintermediateparticulateloading,theStokesnumberprovidesaguidanceforselectingthemostappropriatemodel.TheStokesnumber,St,istheratiooftheparticle(i.e.dispersedphase)relaxationtime(d)tothecharacteristictimescaleoftheflow(c).whereand.DandUarethecharacteristiclengthandvelocityscalesoftheproblem.ForSt1,theparticleswillcloselyfollowtheflowfield.ForSt1,theparticlesmoveindependentlyoftheflowfield.cdtt=Stcddddmr=t182UDc=t9-8©2006ANSYS,Inc.Allrightsreserved.ANSYS,Inc.ProprietaryFluentUserServicesCenter
InallmultiphasemodelswithinFLUENT,anyphasecanbecomposedofeitherasinglematerialoramixtureofspecies.
Materialdefinitionofphasemixturesisthesameasinsinglephaseflows.
Itispossibletomodelheterogeneousreactions(reactionswherethereactantsandproductsbelongtodifferentphases).Thismeansthatheterogeneousreactionswillleadtointerfacialmasstransfer.9-9©2006ANSYS,Inc.Allrightsreserved.ANSYS,Inc.ProprietaryFluentUserServicesCenter