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1、0082-06.fmPage307Wednesday,August23,20003:54PM6HeatTransferwithPhaseChange6.1INTRODUCTIONHeattransferofaphasechangecoolantismuchmorecomplexthanthepreviousmodesofheattransferthatwehavestudied.Byphasechangewedenotethefollowingprocesses:a.Solidchangingtoaliquid—
2、fusion,ormelting,b.Liquidchangingtoavapor—evaporation,alsoboiling,c.Vaporchangingtoaliquid—condensation,e.Liquidchangingtoasolid—crystallization,orfreezing,f.Solidchangingtoavapor—sublimation,g.Vaporchangingtoasolid—deposition.Becausetheseprocessesinvolveaflui
3、dmedium,wegenerallyclassifythemasconvectionprocesses.Beyondalloftheparametersthatwepreviouslyexaminedinconvection,wemustnowaddthevariablesofsurfacetension,;ambientpressure,P;andeithertheenthalpyorlatentheatofevaporation,hfg;thelatentheatofsolidification,hfs;o
4、rthelatentheatofsublimation,hsg.Thelatentheatistheamountofheatrequiredtoconvertaunitmassofasubstancefromonephasetoanotherphase.Thereisalsoadifferenceindensitybetweentheliquidphaseandthevaporphasethatinducesabuoyancyforce,lv,whenbubblesarepresent.Also,w
5、ewillnotethatheattransferduringphasechangedoesnotalwaysoccurwithachangeintemperatureofthemedia.Infact,averylargerateofheattransfercanbeachievedwithverylittlechangeintemperature.Thisisoneoftheattractionsofphasechangeheattransfer.Furthermore,incontrasttonatural
6、orforcedconvec-tion,increasingtheTmayresultinadecreaseintheheattransfercoefficient.Becauseofthenumberofvariables,therearenoaccurategeneralequationsorcorrelationstouse.Oftheusableequations,mosthaveanempiricalvaluethatchangeswiththesurfacecharacteristicsandmust
7、beevaluatedbyexperimentation.Theaccuracyofthesecorrelationswithoutexperimentalverificationmaybe50%.Althoughheattransferbyphasechangeisnotyetwidelyusedinelectronicscooling,asthecomponentheatfluxrisesthelawsofphysicsdictatethathigh-endcoolingtechnologieswillprog
8、ressfromair-cooledtoliquid-cooledtophasechange.Ourstudiesinthissectionwillbelargelytheoretical.©2001byCRCPRESSLLC0082-06.fmPage308Wednesday,August23,20003:54PM6.1.1DEFINITIONSOFPHASECHANG
