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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
verticalflatDimensionless9 PertinentDimensionless GrashoffgTsT
buoyancyforceAsitincreases:Gr
esfrictionandinduces
viscous
kinematicviscosityof
Lcharacteristic
ideal1[K-1T CopmaredtoReynoldsnumberforforced
uL~inertial viscous RayleighgT
comparabletoPecletnumber(Pe)forRaLGrLPr
BoundaryLayer(Laminar,BoussinesqapproximationgTgTT Xdirectionmomentum:uxvyy2idealuv
1[K-1]T T
T cpu
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
n
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
vuF
2u y
v
2cp
y
Fx
pgx
2 11 dpg
dp动量方程变 u
就是重力场中由于密度差而产生的浮升力项
x
gy
yBoussinesq近 为了推导该近似,把密度展开 级(p,t) (tt)(pp)...
t
p 密度
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