第七章 传热学沸腾对流换热9.ppt

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1、NineChapterBoilingAndCondensationBoilingprocessCondensationprocessSolid-liquidinterfaceChangeinphaseofaliquidCondensationBoilingConvectionheattransferWithoutinfluencingfluidtemperaturePhasechangeLatentheatSurfacetensionDensitydifferenceThreeparameters9.1Dimensio

2、nlessParametersinBoilingandCondensation(1)(2)(3)(1)Jakobnumberisratioofthesensibleenergytothelatentenergy.(2)Bondnumberistheratioofthegravitationalbodyforcetothesurfacetensionforce.(3)Unnamedparameterrepresentsthebuoyancy-inducedfluidmotiononheattransfer.9.2Boil

3、ingModesWhenevaporationoccursatasolid-liquidinterface,ittermedboiling.TheprocessoccurswhenthetemperatureofthesurfaceTsexceedsthesaturationtemperaturecorrespondingtotheliquidpressureTsat.Boingmayoccurundervariousconditions.(1)Poolboiling(byfreeconvection).(2)Forc

4、ed-convectionboiling(byexternalmeans,andfreeconvection.Boilingmayalsobeclassifiedaccordingtosubcooledorsaturated.(1)Subcooledboiling(TlTsat).(1)PoolBoilingCPDBAFreeconvectionNucleateBoilingregimestransitionFilm530120101000Criticalhe

5、atfluxLeidenfrostpoint,FreeconvectionboilingInsufficientvaporincontactwiththeliquidphase.Freeconvectionfluidmotion.NucleateboilingA-B:Isolatedbubblesformatnucleationsitesandseparatefromthesurface.Mostoftheheatexchangeisthroughdirecttransferfromsurfacetoliquidinm

6、otionatthesurface.B-C:thevaporescapesasjetsorcolumns.TransitionboilingBubbleformationissorapidthatavaporfilmorblanketbeginstoformonthesurface.Conditionsmayoscillatebetweenfilmandnucleateboiling.FilmBoilingThesurfaceiscompletelycoveredbyavaporblanket.Heattransfer

7、fromthedurfacetoliquidoccursbyconductionthroughthevapor.Radiationthroughvaporfilmbecomesignificant.9.3PoolBoilingCorrelations(1)NucleatePoolBoiling(2)CriticalHeatFluxforNucleatePoolBoiling(3)MinimumHeatFlux(4)FilmPoolBoiling9.4Forced-ConvectionBoiling9.5Condensa

8、tion:PhysicalMechanismsVaportemperatureSaturationtemperature