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1、ConvectionAsmentionedearlier,therearethreemechanismsofheattransfer:conduction,convection,andradiation.Conductionandconvectionaresimilarinthatbothmechanismsrequirematerialmedium,butthedifferenceisthatconvectionrequiresthepresenceoffluidmotion.Andheattransferthroughaliquidorgascan
2、bebyconvectionorconduction,dependingonifthereispresenceofbulkfluidmotion.Inotherwords,ifthereexistsbulkfluidmotion,itisconvection;ifthereisnone,thenitisconduction.Convectioniscomplicatedbecauseitinvolvesfluidmotionandheatconduction.Thus,rateofheattransferbyconvectionishigherthan
3、conduction.Andthehigherthefluidvelocity,thehighertheheattransferrate.FurtherreadingaboutconvectionisavailableatAlthough,convectioniscomplex,therateofconvectionheattransferisobservedtobeproportionaltothetemperaturedifferenceandcanbeconvenientlyexpressedbyNewton’slawofcoolingasqco
4、nv=h(Ts-T∞)(W/m2)(x.1)orQconv=hAs(Ts-T∞)(W)(x.2)Whereh=convectionheattransfercoefficient,W/m2·oCAs=heattransfersurfacearea,m2Ts=temperatureofthesurface,˚CT∞=temperatureofthefluidsufficientlyfarfromthesurface,˚CConvectionheattransfercoefficienthcanbedefinedastherateofheattransfer
5、betweenasolidsurfaceandafluidpersurfaceareaperunittemperaturedifference.Theconvectioncoefficientisdecidedbyvariablesinfluencingconvectionsuchassurfacegeometry,thenatureoffluidmotion,thepropertiesofthefluid,andthebulkfluidvelocity.TypicalvaluesofharegiveninTablex.1.Tablex.1Nussel
6、tNumberInconvectionstudies,tonondimensionalisethegoverningequations,dimensionlessnumbersareintroducedtoreducethenumberoftotalvariables.Nusseltnumber,viewedasthedimensionlessconvectionheattransfercoefficientisdefinedas:Nu=hLckWherekisthethermalconductivityofthefluidandLcisthechar
7、acteristiclength.Thephysicalsignificancemeanstheheattransferratioofconvectiontoconduction.ForNu=1,theheattransferispureconduction.FluidflowsHeattransferbetweenmovingfluidandsolidsurfaceorbetweenmovingfluidandinterface(to/fromairtofallingdrop).Itiscommonlyassumedthatallresistance
8、toheat/momentumtransferoccursinboundarylayerdef
