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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、LBgTc--------p-RaGrPrkForfluidswithaPrandtlnumber1.0,theNusseltnumberisfoundtodependontheproductoftheGrashofnumberandthePrandtlnumbersquared,whichwe©2001byCRCPRESSLLC0082-04Page247Wednesday,August23,200010:06AMknowastheBoussinesq52number,B
6、o:322--------------------------LBgTc--------p-2BoGrPrkBecausethefluidmotionissmallandthevelocitiesarelow,resultinginmuchlessfluidmixing,theheattransfercoefficientinnaturalconvectionismuchsmallerthaninforcedconvection.Sometimesthefluidmotionca
7、nbelowenoughthatconductioneffectsdominate.TheNusseltnumbercansuggestthesecases,whereifNu1.0:convectionconductionifNu1.0:convectiondominatesThepointwhereforcedconvectiondominatesnaturalconvectioncanbefoundbycomparingtheGrashofnumberwiththeReynolds
8、numbersquared:ifGrRe2:naturalconvectiondominatesifGrRe2:mixedconvectionifGrRe2:forcedconvectiondominatesThebuoyancyforceofthenaturalconvectionfluidmotioncanbefoundas(e)g,whereisthelocalfluiddensity,eistheambientfluiddensity,andgisaccelerationdu