参考传热传质学-convection and phase change

HeatLecture14Naturalconvection&phaseCollegeofEngineeringPekingUniversity2 Laminarflowincircular velocityboundarylayerthickensonsurfaceoftubeincreasing inviscidregionofuniformvelocityshrinksasboundary subsequenttoboundarylayermergeratthecenterline,hd illfll uhd illfll uu3Free(Natural)4FreeConvection(orNatural fluidmotioninducedbybuoyancyBuoyancyforcesarisewhentherearedensitygradientsafluidandabodyforceproportionalto Densitydue

temperaturegradient(inthislikeduetoconcentration Bodygravity(functionofotherslikecentrifugalWhynulcearpowerplantshopetohavenaturalcooling(takewarningofJapanFukushimanucleardisaster)Whywehavetokeeplowtobreathewhen fromfireaccidentplaceSolartower(hotwind Bottomdiameter>6 Internalvs. freeconvectioncanbegeneratedalongafreesurface inaduct管道、槽orenclosureinbothinternal&externalcases,freeconvectioncaninteractwithforcedconvection(mixedconvection)Boundary7BoundaryLayerdevelopmentonavertical heatedplate:ascendingflow umvelocityoccurringinboundarylayerandzerovelocityattheplateandinthefreeCompare8Transitionto similartoforcedflows,freeconvectionflowstransitionfromalaminarstatetoaturbulent infreeconvection,amplificationofdisturbancesonrelativemagnitudeofbuoyancyandviscousCriticalRayleigh Pr

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esfrictionandinduces

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vyky2 Boussinesqapproximation:a.densitychangeonlyconsideredinthebodyforceterm;b.Usingsimplifiedexpressionfordensitychange.theenergyandmomentumequationsarenowcoupledmustsolvedconcurrentlybecausesolutionsaremutuallyRefertothesupportinginformationattheendofpptfordetailed Velocity TemperatureHeatTransferOnlyempiricalrelationsexistformostImmersedVertical averageNusseltnumbeL

nC n1/n

NuLk

C n1/

Churchill&Chucorrelation:la1inarflow4L Churchill&Chucorrelation:all1low 0.387Ra NuL0.825 27 10.492Pr16 ExternalInclined&Horizontal buoyancyforceisnotparalleltohorizontal:perpendicularbuoyancyinclined:angledbuoyancy flowandheattransferdependthermalconditionofplate:heatedororientation:facingupwardorcooled

facingup

cooledfacingheated

facingup

facingdownInternalParallel similartointernalforcedconvection,buoyancy-inducedboundarylayersdeveloponbothplates boundarylayerscanmergetogeneratefullydevelopedflowconditionsinsufficientlylong(L)andcloselyspaced(S)channelsotherwisethechannelbehavesastwoisolatedplates channelsoftenconsideredvertical(θ=0°)orinclinedflowsmorecomplexandoftenresultindimensionalflowBenard-cellOnlyempiricalcorrelationsexistformost“real”MixedMixed aconditionwherebothfreeandforcedconvectioneffectsare freeconvectioncanassist,oppose,oractorthogonallytoforcedWhenismixedconvection anassessmentofscalesisconductedthroughacomparisonoffreeandnon-dimensionalbuoyancyforce~

freeconvectiondominatesmixedconvectioncondition

NuLf fRe,Grinertial

Re2L

forcedconvection

NuLfAffectonheat+- forcedandfree+-Nun

n37 L, L,7

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or4Briefintroductionaboutphase-changeheattransferPhaseLatentheatofwater*:2.26MJ/*1calory=Boilingisassociatedwithtransformationofliquidtovaporatasolid/liquidinterface(heterogeneousboiling)orinbulkliquid(homogeneousInmostcases,heterogeneousOna OnaSystempressureSystempressure

PvTv

saturationtemperatureq 均相沸腾非均相沸腾

Homogeneous需要较高过热度tl

Pl；；过程剧烈 Heterogeneous

概念和分铜表超疏水核HeterogeneousPoolLiquidmotionisduetonaturalconvectionandbubble-inducedForcedConvectionFluidmotionisinducedbyexternalmeans,aswellasbybubble-inducedSaturatedLiquidtemperatureisslightlyhigherthansaturationSubcooledLiquidtemperatureislessthansaturationBoilingcurveofsaturatedpoolRevealsrangeofconditionsassociatedwithsaturatedpoolboilingon Example:awateratatmosphericpressure,onanichromeorplatinumFreeConvectionBoilingΔTeLittlevaporLiquidmotionisdueprincipallytosingle-phasenaturalOnsetofNucleateBoiling-ONBΔTeNucleateBoiling5ΔTeCriticalHeatFlux-CriticalHeatFlux-umattainableheatfluxinnucleatePotentialPotentialBurnoutforPower-ControlledAnincreaseinqsbeyondqmaxcausesthesurfacetobeblanketedbyvapor,andthesurfacetemperaturecanspontaneouslyachieveavaluethatpotentiallyexceedsitsmeltingpointIfthesurfacesurvivesthetemperatureshock,conditionsarebyfilmFilmHeattransferisbyconductionradiationacrossthevapor AreductioninqsfollowsthecoolingcurvecontinuouslytotheLeidenfrostpointcorrespondingtotheminimumheatfluxqminforfilmboiling.TransitionBoilingforTemperature-ControlledSurfaceconditionsoscillatebetweennucleateandfilmboiling,butportionofsurfaceexperiencingfilmboilingincreaseswithΔTe.AlsotermedunstableorpartialfilmHeatCappillaryheatGravityheatHeatedendmustbeatthebottom,重力工作范围-60～80。长低温高温寒季/暖季

7-7-7-7-ChapterReviewquestionsPage(443)Supporting uv

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p 密度