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ModelingMultiphaseFlows,IntroductoryFLUENTTraining,Introduction,Aphaseisaclassofmatterwithadefinableboundaryandaparticulardynamicresponsetothesurroundingflow/potentialfield.Phasesaregenerallyidentifiedbysolid,liquidorgaseousstatesofmatterbutcanalsorefertootherforms:Materialswithdifferentchemicalpropertiesbutinthesamestateorphase(i.e.liquid-liquid,suchas,oil-water)Thefluidsystemisdefinedbyaprimaryandmultiplesecondaryphases.Oneofthephasesisconsideredcontinuous(primary)Theothers(secondary)areconsideredtobedispersedwithinthecontinuousphase.TheremaybeseveralsecondaryphasedenotingparticleswithdifferentsizesIncontrast,multi-componentflow(speciestransport)referstoflowthatcanbecharacterizedbyasinglevelocityandtemperaturefieldforallthespecies.,ChoosingaMultiphaseModel,Inordertoselecttheappropriatemodel,usersmustknowapriorithecharacteristicsoftheflowintermsofthefollowing:FlowregimeParticulate(bubbles,dropletsorsolidparticlesincontinuousphase)Stratified(fluidsseparatedbyinterfacewithlengthscalecomparabletodomainlengthscale)Forparticulateflow,onecanestimateParticlevolumeloadingStokesnumberMultiphaseturbulencemodeling,MultiphaseFlowRegimes,BubblyflowDiscretegaseousbubblesinacontinuousfluid,e.g.absorbers,evaporators,spargingdevices.DropletflowDiscretefluiddropletsinacontinuousgas,e.g.atomizers,combustorsSlugflowLargebubblesinacontinuousliquidStratified/free-surfaceflowImmisciblefluidsseparatedbyaclearlydefinedinterface,e.g.free-surfaceflowParticle-ladenflowDiscretesolidparticlesinacontinuousfluid,e.g.cycloneseparators,airclassifiers,dustcollectors,dust-ladenenvironmentalflowsFluidizedbedsFluidizedbedreactorsSlurryflowParticleflowinliquids,solidssuspension,sedimentation,andhydro-transport,Gas/LiquidLiquid/Liquid,Gas/Solid,Liquid/Solid,SlugFlow,Bubbly,Droplet,orParticle-LadenFlow,Stratified/Free-SurfaceFlow,PneumaticTransport,Hydrotransport,orSlurryFlow,Sedimentation,FluidizedBed,VolumeandParticulateLoading,VolumeloadingdiluteordenseReferstothevolumefractionofsecondaryphase(s)Fordiluteloading(10%),theaverageinter-particledistanceisaroundtwicetheparticlediameter.Thus,interactionsamongparticlescanbeneglected.,Particulateloadingratioofdispersedandcontinuousphaseinertias,TurbulenceModelinginMultiphaseFlows,Turbulencemodelingwithmultiphaseflowsischallenging.Presently,single-phaseturbulencemodels(suchaskorRSM)areusedtomodelturbulenceintheprimaryphaseonly.Turbulenceequationsmaycontainadditionaltermstoaccountforturbulencemodificationbysecondaryphase.Ifphasesareseparatedandthedensityratioisoforder1oriftheparticlevolumefractionislow(10%),thenasingle-phasemodelcanbeusedtorepresentthemixture.Inothercases,eithersinglephasemodelsarestillusedor“particle-presence-modified”modelsareused.,StokesNumber,Forsystemswithintermediateparticulateloading,theStokesnumberprovidesaguidanceforselectingthemostappropriatemodel.TheStokesnumber,St,istheratiooftheparticle(i.e.dispersedphase)relaxationtime(d)tothecharacteristictimescaleoftheflow(c).whereand.DandUarethecharacteristiclengthandvelocityscalesoftheproblem.ForSt1,theparticlesmoveindependentlyoftheflowfield.,PhasesasMixturesofSpecies,InallmultiphasemodelswithinFLUENT,anyphasecanbecomposedofeitherasinglematerialoramixtureofspecies.Materialdefinitionofphasemixturesisthesameasinsinglephaseflows.Itispossibletomodelheterogeneousreactions(reactionswherethereactantsandproductsbelongtodifferentphases).Thismeansthatheterogeneousreactionswillleadtointerfacialmasstransfer.,MultiphaseModelsinFLUENT,ModelssuitedforparticulateflowsDiscretePhaseModel(DPM)MixtureModelEulerianMultiphaseFlowModelModelssuitedforstratifiedflowsVolumeofFluidModel(VOF),Define,Models,Multiphase,Define,Phases,DiscretePhaseModel,DiscretePhaseModel(DPM),Trajectoriesofparticles/droplets/bubblesarecomputedinaLagrangianframe.Particlescanexchangeheat,mass,andmomentumwiththecontinuousgasphase.Eachtrajectoryrepresentsagroupofparticlesofthesameinitialproperties.Particle-particleinteractionsareneglected.Turbulentdispersioncanbemodeledusingeitherstochastictrackingora“particlecloud”model.Numeroussub-modelingcapabilitiesareavailable:Heating/coolingofthediscretephaseVaporizationandboilingofliquiddropletsVolatileevolutionandcharcombustionforcombustingparticlesDropletbreakupandcoalescenceusingspraymodelsErosion/Accretion,ApplicabilityofDPM,Flowregime:Bubblyflow,dropletflow,particle-ladenflowVolumeloading:Mustbedilute(volumefraction12%)ParticulateLoading:LowtomoderateTurbulencemodeling:WeaktostrongcouplingbetweenphasesStokesNumber:AllrangesofStokesnumberApplicationexamplesCyclonesSpraydryersParticleseparationandclassificationAerosoldispersionLiquidfuelCoalcombustion,DPMExampleSprayDrierSimulation,Spraydryinginvolvesthetransformationofaliquidsprayintodrypowderinaheatedchamber.Theflow,heat,andmasstransferaresimulatedusingtheFLUENTDPM.CFDsimulationplaysaveryimportantroleinoptimizingthevariousparametersforthespraydryer.,PathLinesIndicatingtheGasFlowField,Airandmethaneinlets,CenterlineforParticleInjections,Outlet,SprayDryerSimulation(2),ContoursofEvaporatedWater,StochasticParticleTrajectoriesforDifferentInitialDiameters,InitialparticleDiameter:2mm,1.1mm,0.2mm,TheEulerianMultiphaseModel,TheEulerianMultiphaseModel,TheEulerianmultiphasemodelisaresultofaveragingofNSequationoverthevolumeincludingarbitraryparticles+continuousphaseTheresultisasetofconservationequationsforeachphase(continuousphase+Nparticle“medias”)Bothphasescoexistsimultaneously:conservationequationsforeachphasecontainsinglephaseterms(pressuregradient,thermalconductionetc.)+interfacialtermsInterfacialtermsexpressinterfacialmomentum(drag),heatandmassexchange.Thesearenonlinearlyproportionaltodegreeofmechanical(velocitydifferencebetweenphases),thermal(temperaturedifference).HenceequationsarehardertoconvergeAddonmodels(turbulenceetc.)areavailable,TheGranularOptionintheEulerianModel,GranularflowshappenwhenhighconcentrationofsolidparticlesleadstohighfrequencyofinterparticlecollisionsParticlesareassumedtobehavesimilartothedensegasofcollidingmoleculesandtheoryofsuchmoleculargasisappliedtoparticlephaseApplicationofthistheoryleadstoappearanceofadditionalstressesinmomentumequationsforcontinuousandparticlephasesThesestresses(granular“viscosity”,“pressure”etc.)aredeterminedbyintensityofparticlevelocityfluctuationsKineticenergyassociatedwithparticlevelocityfluctuationsisrepresentedbya“pseudo-thermal”orgranulartemperatureInelasticityofthegranularphaseistakenintoaccount,ApplicabilityofEulerianmodel,FlowregimeBubblyflow,dropletflow,slurryflow,fluidizedbeds,particle-ladenflowVolumeloadingDilutetodenseParticulateloadingLowtohighTurbulencemodelingWeaktostrongcouplingbetweenphasesStokesnumberAllrangesApplicationexamplesHighparticleloadingflowsSlurryflowsSedimentationHydrotransportFluidizedbedsRisersPackedbedreactors,EulerianExample3DBubbleColumn,Iso-SurfaceofGasVolumeFraction=0.175,LiquidVelocityVectors,z=5cm,z=10cm,z=15cm,z=20cm,EulerianExampleCirculatingFluidizedBed,ContoursofSolidVolumeFraction,CourtesyofFullerCompany,TheMixtureModel,TheMixtureModel,ThemixturemodelisasimplifiedEulerianapproachformodelingn-phaseflows.ThesimplificationisbasedontheassumptionthattheStokesnumberissmall(particleandprimaryfluidvelocityisnearlyequalinbothmagnitudeanddirection).Solvesthemixturemomentumequation(formass-averagedmixturevelocity)andprescribesrelativevelocitiestodescribethedispersedphases.Interphaseexchangetermsdependonrelative(slip)velocitieswhicharealgebraicallydeterminedbasedontheassumptionthatSt1.Thismeansthatphaseseparationcannotbemodeledusingthemixturemodel.Turbulenceandenergyequationsarealsosolvedforthemixtureifrequired.Solvesavolumefractiontransportequationforeachsecondaryphase.Asubmodelforcavitationisavailable(seetheAppendixfordetails).,ApplicabilityofMixturemodel,Flowregime:Bubbly,droplet,andslurryflowsVolumeloading:DilutetomoderatelydenseParticulateLoading:LowtomoderateTurbulencemodeling:WeakcouplingbetweenphasesStokesNumber:St1,themixturemodelisnotapplicable.Instead,useeitherDPMorEulerian.ForSt1,allmodelsareapplicable.UsetheleastCPUdemandingmodelbasedonotherrequirements.Strongcouplingamongphaseequationssolvebetterwithreducedunder-relaxationfactors.Usersshouldunderstandthelimitationsandapplicabilityofeachmodel.,Appendix,DiscretePhaseModel(DPM)Setup,Define,Models,DiscretePhase,Define,Injections,Display,ParticleTracks,DPMBoundaryConditions,EscapeTrapReflect,Wall-jet,MixtureModelEquations,SolvesoneequationforcontinuityofthemixtureSolvesforthetransportofvolumefractionofeachsecondaryphaseSolvesoneequationforthemomentumofthemixtureThemixturepropertiesaredefinedas:,Driftvelocity,MixtureModelSetup(1),Define,Models,Multiphase,Define,Phases,BoundaryConditionsVolumefractiondefinedforeachsecondaryphase.Todefineinitialphaselocation,patchvolumefractionsaftersolutioninitialization.,MixtureModelSetup(2),CavitationSubmodel,TheCavitationmodelmodelstheformationofbubbleswhenthelocalliquidpressureisbelowthevaporpressure.Theeffectofnon-condensablegasesisincluded.Massconservationequationforthevaporphaseincludesvaporgenerationandcondensationtermswhichdependonthesignofthedifferencebetweenlocalpressureandvaporsaturationpressure(correctedforon-condensablegaspresence).Generallyusedwiththemixturemodel,incompatiblewithVOF.Tutorialisavailableforlearningthein-depthsetupprocedure.,EulerianMultiphaseModelEquations,Continuity:Momentumforqthphase:Theinter-phaseexchangeforcesareexpressedas:Ingeneral:Energyequationfortheqthphasecanbesimilarlyformulated.,transient,convection,pressure,shear,interphaseforcesexchange,interphasemassexchange,body,external,lift,andvirtualmassforces,Volumefractionfortheqthphase,Solidspressuretermisincludedforgranularmodel.,Exchangecoefficient,EulerianMultiphaseModelEquations,MultiphasespeciestransportforspeciesibelongingtomixtureofqthphaseHomogeneousandheterogeneousreactionsaresetupthesameasinsinglephaseAnsysThesamespeciesmaybelongtodifferentphaseswithoutanyrelationbetweenthemselves,transient,convective,diffusion,homogeneousreaction,homogeneousproduction,heterogeneousreaction,Massfractionofspeciesiinqthphase,EulerianModelSetup,Define,Models,Viscous,Define,Phases,Eulerian-GranularModelSetup,Granularoptionmustbeenabledwhendefiningthesecondaryphases.Granularpropertiesrequiredefinition.Phaseinteractionmodelsappropriateforgranularflowsmustbeselected.,VOFModelSetup,Define,Models,Multiphase,Define,Phases,Define,OperatingConditions,OperatingDensityshouldbesettothatoflightestphasewithbodyforcesenabled.,HeterogeneousReactionSetup,Define,Phases,UDFsforMultiphaseApplications,Whenamultiphasemodelisenabled,storageforpropertiesandvariablesissetasideformixtureaswellasforindividualphases.Additionalthreadanddomaindatastructuresrequired.IngeneralthetypeofDEFINEmacrodetermineswhichthreadordomain(mixtureorphase)getspassedtoyourUDF.C_R(cell,thread)willreturnthemixturedensityifthreadisthemixturethreadorthephasedensitiesifitisthephasethread.Numerousmacrosexistfordataretrieval.,MixtureThread,Phase1Domain,Phase3Domain,PhaseThread,InteractionDomain,DomainID=,2,3,4,1,5,DomainID,
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