fluent-13.0中的自然对流问题

CustomerTrainingMaterialLt4Lecture4NaturalConvectionHeatTransferHeatTransferModelingusingANSYSFLUENTL4-1ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010ANSYSFLUENTCustomerTrainingMaterialHeatTransferModelingusingANSYSFLUENTOutline1.Theory/Definitiona.PhenomenabTransitiontoturbulentflow4.ModelsetupinFLUENTa.Referencedensityandtemperatureb.Transitiontoturbulentflow2.Turbulencea.Dynamicvs.thermalboundarytemperatureb.Boussinesqvs.incompressibleidealgas5Elyylayerb.Fullbuoyancyeffect3Mdlii5.Examplesa.Validationcase–Tallcavityb.Baselinecasesi.CloseddomainwithhighRayleigh3.Modelingtipsa.Pressurediscretizationb.TimesteppingforunsteadysimulationCoseddoatgayegnumberii.PlumewithhighRayleighnumberc.Industrialcase–GlasstankTC21simulation6.References7.AppendixL4-2ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010ppCustomerTrainingMaterialNaturalconvection:Theory&DefinitionsTheory&DefinitionsL4-3ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010CustomerTrainingMaterialHeatTransferModelingusingANSYSFLUENTPhenomena•Innaturalconvection,fluidmotionisgeneratedduetoditdiff(b)iTT=dxxPP∂∂+densitydifference(buoyancy)inthefluidcausedbytemperaturegradients.wTT=yxτxΓdyxyxyx∂τ∂+τ•Bodyforces–TypicallygravitationalPyfTT=–Centrifugal(rotatingmachinery)–Coriolis(atmosphericandoceanicvorticalmotion)•Forthisclassofproblems,flowandenergyarestronglycoupled.Forcesactingonafluidparticleinnaturalconvection.L4-4ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010CustomerTrainingMaterialHeatTransferModelingusingANSYSFLUENTLaminartoTurbulentTransition•Innaturalconvection,theReynoldsnumbernolongercharacterizestheflow.•Withanappropriatereferencevelocity,itispossibletodetermineacriticalvalueoftheRayleighnumber(RaL).–ExperimentsshowthatthecriticalRayleighnumber,Rac,isaround109.–ThetransitionzoneisquitelargeasRavariesbetween106and1010.Buoyancyforce3KinematicKinematicviscosityανΔβ==TLgLL3PrGrRaαν=PrwhereviscosityThermalKinematicviscosityThermaldiffusivityThermaldiffusivityL4-5ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010CustomerTrainingMaterialTurbulentBoundaryLayersL4-6ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010CustomerTrainingMaterialHeatTransferModelingusingANSYSFLUENTBoundaryLayers•ImpactonnumericalmodelingforturbulentflowEnergyandmomentumequationsare–Energyandmomentumequationsarestronglycoupled.•Itisrecommendedtoconstructthemeshsuchthaty+1inordertocorrectlyThermalBdVerticalWallresolveboththemomentumANDthermalviscoussublayers.•ThisisstraightforwardforPr~1orPr1.•WhenPr1thethermalsublayeriswTLBoundaryLayerFreeStream•WhenPr1,thethermalsublayerismuchthinnerthanthemomentumviscoussublayer.•Thisbehaviorisrelativelyinsensitivetogridresolutionprovidedthatthe∞Txgridresolution,providedthatthemomentumboundarylayerstructureisaccurate(y+≤1forthefirstcelllayerandatleast10cellsbetween1y+30).L4-7ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010CustomerTrainingMaterialHeatTransferModelingusingANSYSFLUENTBoundaryLayerGridGeneration•Startwitha2Dtestcase–Thisisagoodwaytoconfirmwhatarethecharacteristicintegrationtimestepsandmeshsizerequiredforthdidhithedesiredphysics.TemperaturefieldVelocitfieldInitialtemperature(K)TemperaturefieldVelocityfieldExpectedflowpatternduringcooldowninpipecrosssectionduetobuoyancyforcesL4-8ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010CustomerTrainingMaterialTurbulentFlowHeatTransferTheFullBuoyancyTheFullBuoyancyEffectsOptionL4-9ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010CustomerTrainingMaterialHeatTransferModelingusingANSYSFLUENTTurbulenceGenerationduetoBuoyancy•Theimportanceofthebuoyancyterm(Gb)canbeseeninamixinglayerexampleusingthestandardk–εturbulencemodel.()()kMbkjkTjiiSYGGxkxxkutk+−ερ−++⎥⎥⎦⎤⎢⎢⎣⎡∂∂⎟⎟⎠⎞⎜⎜⎝⎛σμ+μ∂∂=∂ρ∂+∂ρ∂T=100°CV=1m/sGbsetto0inthekEquationDefaulttreatmentgGisasinktermforstablestratificationTurbulentmixingdecreasesT=2°CV=1.2m/sGbisasinktermforstablestratification.TurbulentmixingdecreasesL4-10ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010CustomerTrainingMaterialHeatTransferModelingusingANSYSFLUENTTheFullBuoyancyEffectsOption•Toincludebuoyancyeffectsonε,youmustenabletheFullBEffttidBuoyancyEffectsoptionunderOptionsintheViscousModelpanel.•Thisoptionisavailableforthethreek–εmodels(SKE,RKE,RNG)andfortheReynoldstdl(RSM)stressmodel(RSM).•Availablefork–ωmodelsviaUDF(shownintheAppendix).L4-11ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010CustomerTrainingMaterialTipsandTricksNumericalDiscretizationNumericalDiscretizationUnsteadySimulationsL4-12ANSYS,Inc.Proprietary©2010ANSYS,Inc.Allrightsreserved.Release13.0December2010CustomerTrainingMaterialHeatTransferModelingusingANSYSFLUENTNumericalDiscretization•Pressureinterpolationscheme–RecommendedtouseeitherBodyForceWeightedorPRESTO!–Standardpressurediscretization(the