7 Combined Modes of Heat Transfer for Electronic Equipment文稿

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1、0082-07.fmPage343Wednesday,August23,200010:12AM7CombinedModesofHeatTransferforElectronicEquipment7.1INTRODUCTIONAlthoughwehaveexaminedthethreemodesofheattransferingreatdetail,inengineeringpracticeweusuallyseecaseswheremodesarecombinedsimultaneously.Ra

2、diationisnotusuallyasignificantfactor,buttoachievethebestresultweshouldcalculatetheimportanceofeachmode.Inacomputerchip,forexample,heatisconductedinparallelpathsfromthejunctiontothecaseandleads.Heatisthenconductedfromtheleadstothecircuitboard,andfromth

3、ecasetoaheatsink.Simultaneously,theheatintheleadsandintheheatsinkisconvectedtotheairandradiatedtotheambientenvironment.Table7.1isareminderoftheequationsusedforheattransferandthermalresistanceinthethreemodes.Thesimplestwaytosolvemostproblemsofcombinedm

4、odesistosetuparesistancenetwork.Inthismannerwecangraphicallyexaminethepathsofeachmodeforsimultaneous,parallel,andseriesheattransfer.7.2CONDUCTIONINSERIESANDINPARALLELBecausecombinedmodesofheattransferoftenoccurinparallel,reviewingthetheoryofseriesandp

5、arallelcombinationsmaybehelpfulasappliedtoconduction.Whenheatisconductedthroughasinglewallofasinglematerial,therateofheatconductionandthethermalgradientareconstant.However,whenheatisconductedinaseriespathofdifferentmaterials,thetemperaturegradientisdi

6、fferentforeachmaterial.Examiningthecompositewallofthreematerialsinseries,asshowninFigure7.1,therateofheatconductionthroughtheentireblock,qk,canbefoundbyT1T4q-----------------------------------------------------k------L------L------LkAAkA

7、BkAC©2001byCRCPRESSLLC0082-07.fmPage344Wednesday,August23,200010:12AMTABLE7.1ThermalResistancesandHeatTransferRatesHeatTransferModeRateofHeatTransfer,qThermalResistance,Conductionk------Lqk()T1T2--L-kkAConvection1qchcAT()sTc---------hcARad

8、iationT1T2qA()T4T4----------------------------------------------r112,12rA()T4T4112,12Ifwehaveanindeterminatenumber,N,ofindividualmaterials,n,inseries,therateofheatconductionisTT1TN1qk-----------L-------------------------------nN