自然对流jie.ppt

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1、Chapter7Naturalconvectionsystems7-1Introductionnaturalorfreeconvection:themotionofthefluidduetodensitychangesarisingfromheatingprocessBuoyancyforcesthatgivenrisetothefree-convectioncurrentsarebodyforcesExamples:radiator,refrigeratorandelectroniccomponentsIninf

2、initespaces(大空间自然对流)Nobodyhaveinfluenceonthenaturalconvectioninenclosedspaces(有限空间自然对流)Thenaturalconvectionisaffectedbysomebody7-2Free-convectionheattransferonaverticalflatplateVelocityprofileisdifferentfromforcedconvectionTemperatureprofileissimilartoforcedco

3、nvectionOutofboundarylayer,theT=const.u=0Theflowmaybelaminarflow,orturbulentflowSolutionsDifferentialequationIntegralequationWhere7-3Empiricalrelationsforfree-convectionvolumecoefficientofexpansionForideal(perfect)gaspv=RTReferencetemperatureCharacteristicdim

4、ensionsThevaluesoftheconstantsCandmarelistedintableRayleighnumberthelengthLforverticalsurfaceDiameterdforhorizontalcylinderAverticalcylindermaybetreatedasaverticalflatplatewhen7-4Freeconvectionfromverticalplanesandcylinders7-5Freeconvectionfromhorizontalcylind

5、ers7-6Freeconvectionfromhorizontalplates7-7Freeconvectionfrominclinedsurfaces7-8Nonnewtonianfluids7-9Simplifiedequationsforair7-10Freeconvectionfromspherescanceled7-11FreeconvectioninenclosedspacesGrashofnumberbasedonthedistanceCorrelations(7-54),(7-55)and(7-

6、60)MomentumequationCriterionforfreeorforcedconvectionFreeconvectioneffectsshouldbeconsideredFreeconvectionisofprimaryimportance7-12CombinedfreeandforcedconvectionThankyou!