电子设备热设计(第四章c)Thermal Design of Electronic Equipment(Ralph Remsburg).pdf

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1、0082-04Page246Wednesday,August23,200010:06AMFIGURE4.28Geometriesandnomenclatureusedtodescribe(a)asingleroundjet,(b)anin-linearrayofroundjets,(c)astaggeredarrayofroundjets,(d)asingleslotjet,and(e)anarrayofslotjets.denserfluidtofall.Withinanenclosedspac

2、e,thefluidwillformcircularflowpathsastheheatedfluidrises,beginstocoolandfall,andisthenheatedtoriseagain.Inforcedconvectionthevelocityorthemassofcoolantflowingcanbeeasilymeasuredorcalculated.Innaturalconvectiontheimposedflowresultsfromaninteractionoftheden

3、sitydifferencewiththegravitationalfieldandisthereforeinvariablylinkedwithanddependentuponthetemperatureandconcentrationfields.Thus,thefluidmotionmustbedeterminedfromaconsiderationoftheheatandmasstransferprocesscoupledwithfluidflowmechanisms.Thesedifferenc

4、esmakeheattransferinnaturalconvectionmuchmorecompli-catedthanthatofforcedconvection.ForfluidswithaPrandtlnumber1.0,theNusseltnumberdependsontheproductoftheGrashofnumberandthePrandtlnumber,whichiscalledtheRayleighnumber:32--------------------------

5、LBgTc--------p-RaGrPrkForfluidswithaPrandtlnumber1.0,theNusseltnumberisfoundtodependontheproductoftheGrashofnumberandthePrandtlnumbersquared,whichwe©2001byCRCPRESSLLC0082-04Page247Wednesday,August23,200010:06AMknowastheBoussinesq52number,B

6、o:322--------------------------LBgTc--------p-2BoGrPrkBecausethefluidmotionissmallandthevelocitiesarelow,resultinginmuchlessfluidmixing,theheattransfercoefficientinnaturalconvectionismuchsmallerthaninforcedconvection.Sometimesthefluidmotionca

7、nbelowenoughthatconductioneffectsdominate.TheNusseltnumbercansuggestthesecases,whereifNu1.0:convectionconductionifNu1.0:convectiondominatesThepointwhereforcedconvectiondominatesnaturalconvectioncanbefoundbycomparingtheGrashofnumberwiththeReynolds

8、numbersquared:ifGrRe2:naturalconvectiondominatesifGrRe2:mixedconvectionifGrRe2:forcedconvectiondominatesThebuoyancyforceofthenaturalconvectionfluidmotioncanbefoundas(e)g,whereisthelocalfluiddensity,eistheambientfluiddensity,andgisaccelerationdu