fluent-v6.3-多相流.ppt

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 Bubblyflow 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 transport Gas LiquidLiquid Liquid Gas Solid Liquid Solid SlugFlow Bubbly Droplet orParticle LadenFlow Stratified Free SurfaceFlow PneumaticTransport Hydrotransport orSlurryFlow Sedimentation FluidizedBed VolumeandParticulateLoading Volumeloading diluteordenseReferstothevolumefractionofsecondaryphase s Fordiluteloading 10 theaverageinter particledistanceisaroundtwicetheparticlediameter Thus interactionsamongparticlescanbeneglected Particulateloading ratioofdispersedandcontinuousphaseinertias TurbulenceModelinginMultiphaseFlows Turbulencemodelingwithmultiphaseflowsischallenging Presently single phaseturbulencemodels suchask orRSM 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 volumefraction 12 ParticulateLoading LowtomoderateTurbulencemodeling WeaktostrongcouplingbetweenphasesStokesNumber AllrangesofStokesnumberApplicationexamplesCyclonesSpraydryersParticleseparationandclassificationAerosoldispersionLiquidfuelCoalcombustion DPMExample SprayDrierSimulation 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 EulerianExample 3DBubbleColumn Iso SurfaceofGasVolumeFraction 0 175 LiquidVelocityVectors z 5cm z 10cm z 15cm z 20cm EulerianExample CirculatingFluidizedBed ContoursofSolidVolumeFraction CourtesyofFullerCompany TheMixtureModel TheMixtureModel ThemixturemodelisasimplifiedEulerianapproachformodelingn phaseflows ThesimplificationisbasedontheassumptionthattheStokesnumberissmall particleandprimaryfluidvelocityisnearlyequalinbothmagnitudeanddirection Solvesthemixturemomentumequation formass averagedmixturevelocity andprescribesrelativevelocitiestodescribethedispersedphases Interphaseexchangetermsdependonrelative slip velocitieswhicharealgebraicallydeterminedbasedontheassumptionthatSt 1 Thismeansthatphaseseparationcannotbemodeledusingthemixturemodel Turbulenceandenergyequationsarealsosolvedforthemixtureifrequired Solvesavolumefractiontransportequationforeachsecondaryphase Asubmodelforcavitationisavailable seetheAppendixfordetails ApplicabilityofMixturemodel Flowregime Bubbly droplet andslurryflowsVolumeloading DilutetomoderatelydenseParticulateLoading LowtomoderateTurbulencemodeling WeakcouplingbetweenphasesStokesNumber St 1ApplicationexamplesHydrocyclonesBubblecolumnreactorsSolidsuspensionsGassparging MixtureModelExample GasSparging Thespargingofnitrogengasintoastirredtankissimulatedbythemixturemultiphasemodel Therotatingimpellerissimulatedusingthemultiplereferenceframe MRF approach FLUENTsimulationprovidedagoodpredictiononthegas holdupoftheagitationsystem ContoursofGasVolumeFractionatt 15sec WaterVelocityVectorsonaCentralPlane TheVolumeofFluidModel VOF TheVolumeofFluid VOF Model TheVOFmodelisdesignedtotrackthepositionoftheinterfacebetweentwoormoreimmisciblefluids Trackingisaccomplishedbysolutionofphasecontinuityequation resultingvolumefractionabruptchangepointsouttheinterfacelocation Amixturefluidmomentumequationissolvedusingmixturematerialproperties Thusthemixturefluidmaterialpropertiesexperiencejumpacrosstheinterface Turbulenceandenergyequationsarealsosolvedformixturefluid Surfacetensionandwalladhesioneffectscanbetakenintoaccount Phasescanbecompressibleandbemixturesofspecies InterfaceInterpolationSchemes ThestandardinterpolationschemesusedinFLUENTareusedtoobtainthefacefluxeswheneveracelliscompletelyfilledwithonephase Theschemesare GeometricReconstructionDefaultscheme unsteadyflowonly nonumericaldiffusion sensitivetogridqualityEulerExplicitUnsteadyflowonly canbeusedonskewedcellsnumericaldiffusionisinherent usehighorderVOFdiscretization HRIC CICSAM EulerImplicitCompatiblewithbothsteadyandunsteadysolvers canbeusedonskewedcellsnumericaldiffusionisinherent usehighorderVOFdiscretization HRIC CICSAM Actualinterfaceshape Geo reconstruct piecewiselinear Scheme ApplicabilityofVOFmodel FlowregimeSlugflow stratified free surfaceflowVolumeloadingDilutetodenseParticulateloadingLowtohighTurbulencemodelingWeaktomoderatecouplingbetweenphasesStokesnumberAllrangesApplicationexamplesLargeslugflowsFillingOffshoreseparatorsloshingBoilingCoating VOFExample AutomobileFuelTankSloshing Sloshing freesurfacemovement ofliquidinanautomotivefueltankundervariousacceleratingconditionsissimulatedbytheVOFmodelinFLUENT Simulationshowsthetankwithinternalbaffles atbottom willkeepthefuelintakeorificefullysubmergedatalltimes whiletheintakeorificeisoutofthefuelatcertaintimesforthetankwithoutinternalbaffles top FuelTankWithoutBaffles FuelTankWithBaffles t 1 05sec t 2 05sec VOFExample HorizontalFilmBoiling Plotsshowingtheriseofbubblesduringthefilmboilingprocess thecontoursofvaporvolumefractionareshowninred Summary Chooseanappropriatemodelforyourapplicationbasedonflowregime volumeloading particulateloading turbulence andStokesnumber UseVOFforfreesurfaceandstratifiedflows UsetheEuleriangranularmodelforhighparticleloadingflows ConsidertheStokesnumberinlowtomoderateparticleloadingflows ForSt 1 themixturemodelisnotapplicable Instead useeitherDPMorEulerian ForSt 1 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 Solidspressuretermisincludedforgranu