《传热学综合传热》PPT课件

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1、ChapterTwelveMultimodeHeatAndMassTransferThatmosttechnicallyimportantproblemsinvolvemultimodeheattransfereffects.qcondqradqconvqradqcondqconvTosolvesuchproblems,itmaybenecessarytosimultaneouslyworkwithheatequation,surfaceenergybalance,overallenergybalances,andappropriter

2、aterateequationsforconduction,convectionandradiationheattransfer.Example12.1TherearwindowofanautomobileisofthicknessL=8mmandheightH=0.5mandcontainsfine-meshedheatingwiresthatcaninducenearlyvolumetricheating.Considersteady-stateconditionsforwhichtheinteriorsurfaceofthewin

3、dowisexposedtoairat10℃,whiletheexteriorsurfaceisexposedtoairat-10℃andaverycoldskywithaneffectivetemperatureof-33℃.Theairmovesinparallelflowoverthesurfacewithavelocityof20m/s.DeterminethevolumetricheatingrateneededmaintaintheinteriorwindowsurfaceatTs,i=15℃.solutionKnown:B

4、oundaryconditionsassociatedwitharearwindowexperiencinguniformvolumetricheating.Find:VolumetricheatingrateneededtomaintaininnersurfacetemperatureatTs,i=15℃.Schematic:Tsky=240KGlassAssumptions:steady-state,one-dimensionalconditions.Constantproperties.Uniformvolumetricheati

5、nginwindow.Convectionheattransferfrominteriorsurfaceofwindowtointeriorairmaybeapproximatedasfreeconvectionfromaverticalplate.Radiationexchangewithautomobileinteriorisnegligible.Heattransferfromoutersurfaceisduetoforcedconvectionoveraflatplateandradiationexchangewiththesk

6、y.Boundarylayerseparationontheoutersurfaceinnegligible.Outersurfaceisdiffuseandgray.Properties:Glass;Air;k,v,Pr,,,(lookattables)Analysis:Thetemperaturedistributionintheglassisgovernedbytheappropriateformoftheheatequation.BoundaryconditionsApplyinganenergybalancetothei

7、nnersurfaceHenceApplyinganenergybalancetotheoutersurface.(1)(2)(3)TheinsideconvectioncoefficientmaybeobtainedfromEquationTheoutsideconvectioncoefficientmaybeobtainedbyfirstevaluatingtheReynoldsnumber.Mixedboundarylayerconditions(4)(5)Equation5maybeexpressedas(6)Substitut

8、ingintoequation4Example12.2Thecompositeinsulationshownisbeingconsideredasaroofingmaterial.Known:Dimensi