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1、HeatTransferConceptsDraft2,10/01/06Part1IntroductionTherearethreedifferenttypesofheattransfer:conduction,convection,andradiation.Atemperaturedifferencemustexistforheattransfertooccur.Heatisalwaystransferredinthedirectionofdecreasingtemperature.Tempera
2、tureisascalar,butheatfluxisavectorquantity.Conductiontakesplacewithintheboundariesofabodybythediffusionofitsinternalenergy.Thetemperaturewithinthebody,T,isgiveninunitsofdegreesCelsius[C],Fahrenheit[F],Kelvin[K],orRankin[R].Itsvariationinspacedefinesth
3、etemperaturegradientvector,gradT,withunitsof[K/m]say.Theheatfluxvector,q,isdefinebyFourier’sConductionLaw,asthethermalconductivity,k,timesthenegativeofthetemperaturegradient,q=-kgradT.Thermalconductivityhastheunitsof[W/m-K]whiletheheatfluxhasunitsof[W
4、/m^2].Theconductivity,k,isusuallyonlyknowntotwoorthreesignificantfigures.Forsolidsitrangesfromabout417[W/m-K]forsilverdownto0.76[W/m-K]forglass.Aperfectinsulatormaterial(k≡0)willnotconductheat;thereforetheheatfluxvectormustbeparalleltotheinsulatorsurf
5、ace.Aplaneofsymmetry(wherethegeometry,kvalues,andheatsourcesaremirrorimages)actsasaperfectinsulator.Infiniteelementanalysis,allsurfacesdefaulttoperfectinsulatorsunlessyougiveaspecifiedtemperature,aknownheatinflux,aconvectioncondition,oraradiationcondi
6、tion.Convectionoccursinafluidbymixing.Herewewillconsideronlyfreeconvectionfromthesurfaceofabodytothesurroundingfluid.Forcedconvection,whichrequiresacoupledmasstransfer,willnotbeconsidered.Themagnitudeoftheheatfluxnormaltoasolidsurfacebyfreeconvectioni
7、sqn=hAh(Th–Tf)wherehistheconvectioncoefficient,Ahisthesurfaceareacontactingthefluid,Thistheconvectingsurfacetemperature,andTfisthesurroundingfluidtemperature,respectively.Theunitsofhare[W/m^2-K].Itsvaluevarieswidelyandisusuallyknownonlyfromonetofoursi
8、gnificantfigures.Typicalvaluesforconvectiontoairandwaterare5-25and500-1000[W/m^2-K],respectively.Radiationheattransferoccursbyelectromagneticradiationbetweenthesurfacesofabodyandthesurroundingmedium.Itisahighlynonlinearfunctionoftheabsolutetem
