BasicsofHeatTransfer
Inthesimplestofterms,thedisciplineofheattransferisconcernedwithonlytwothings:temperature,andtheflowofheat.Temperaturerepresentstheamountofthermalenergyavailable,whereasheatflowrepresentsthemovementofthermalenergyfromplacetoplace.
Onamicroscopicscale,thermalenergyisrelatedtothekineticenergyofmolecules.Thegreateramaterial'stemperature,thegreaterthethermalagitationofitsconstituentmolecules(manifestedbothinlinearmotionandvibrationalmodes).Itisnaturalforregionscontaininggreatermolecularkineticenergytopassthisenergytoregionswithlesskineticenergy.
Severalmaterialpropertiesservetomodulatetheheattranferedbetweentworegionsatdifferingtemperatures.Examplesincludethermalconductivities,specificheats,materialdensities,fluidvelocities,fluidviscosities,surfaceemissivities,andmore.Takentogether,thesepropertiesservetomakethesolutionofmanyheattransferproblemsaninvolvedprocess.
HeatTransferMechanisms
Heattransfermechanismscanbegroupedinto3broadcategories:
Conduction:Regionswithgreatermolecularkineticenergywillpasstheirthermalenergytoregionswithlessmolecularenergythroughdirectmolecularcollisions,aprocessknownasconduction.Inmetals,asignificantportionofthetransportedthermalenergyisalsocarriedbyconduction-bandelectrons.
Convection:Whenheatconductsintoastaticfluiditleadstoalocalvolumetricexpansion.Asaresultofgravity-inducedpressuregradients,theexpandedfluidparcelbecomesbuoyantanddisplaces,therebytransportingheatbyfluidmotion(i.e.convection)inadditiontoconduction.Suchheat-inducedfluidmotionininitiallystaticfluidsisknownasfreeconvection.
Forcaseswherethefluidisalreadyinmotion,heatconductedintothefluidwillbetransportedawaychieflybyfluidconvection.Thesecases,knownasforcedconvection,requireapressuregradienttodrivethefluidmotion,asopposedtoagravitygradienttoinducemotionthroughbuoyancy.
Radiation:Allmaterialsradiatethermalenergyinamountsdeterminedbytheirtemperature,wheretheenergyiscarriedbyphotonsoflightintheinfraredandvisibleportionsoftheelectromagneticspectrum.Whentemperaturesareuniform,theradiativefluxbetweenobjectsisinequilibriumandnonetthermalenergyisexchanged.Thebalanceisupsetwhentemperaturesarenotuniform,andthermalenergyistransportedfromsurfacesofhighertosurfacesoflowertemperature