化工原理课程(全英文)教学课件-14

©2015YanweiWangBasicPrinciplesofChemicalEngineeringProcessesLecture14FallSemester,2014YanweiWang(王衍伟)Email:[e-mail]©2015YanweiWang2PreviousLecturesWeek1:UnitSystemsandDimensionalAnalysisWeek2-5:BasicFluidMechanics−Hydrostatics,Manometers−FluidFlowPhenomenaandBasicEquations−IncompressibleFlow(Laminarvs.Turbulent)inPipes−PipeFlowSystemsandMeteringofFluidsWeek6:TransportationofFluids(Pumps)Week7:HeterogeneousFlowandSeparation−DragandDragCoefficients,ParticleReynoldsNumber−Motionofparticlesthroughfluids−FlowthroughbedsofsolidsWeeks1-7©2015YanweiWang3UpcomingLecturesWeeks8-11Chapter4HeatTransferandItsApplications−Introduction:Conduction,Convection,andRadiation−HeatTransferbyConduction−PrinciplesofHeatFlowinFluids−HeatTransfertoFluidswithoutPhaseChange−HeatTransfertoFluidswithPhaseChange−RadiationHeatTransfer(Week10)−HeatExchangeEquipmentChapter5Evaporation−Single-Effectvs.MultipleEffectEvaporation©2015YanweiWang4HeatTransferIsEverywhere©2015YanweiWang5EngineeringHeatTransferSolutionsShellandTubeHeatExchangersOilandAirCoolersPlateHeatExchangers©2015YanweiWang6EngineeringHeatTransferSolutions©2015YanweiWang7NatureofHeatFlowHeatisenergythatflowsfromahighertemperatureobjecttoalowertemperatureobjectbecauseofadifferenceintemperatures.Theheatthatflowsfromhottocoldoriginatesintheinternalenergyofthehotsubstance.SIUnitofHeat:joule(J)1J=1N∙m=1kg∙m2∙s−21kcal=4184.0J,1Btu=1055JThermochemicalcalorievs.Internationalsteamtablecalorie(1calth=4.1840J,1calIT=4.1868J)©2015YanweiWang8Heat&TemperatureChangeTheheatthatmustbesuppliedorremovedtochangethetemperatureofasubstanceis𝑄=𝑚𝑐𝑝∆𝑇𝑄=Heat,J𝑚=Massofthesubstance,kg∆𝑇=Changeintemperature,Kor℃𝒄𝒑=Specificheatcapacity,oftensimplycalledspecificheat（比热）,istheheatcapacityperunitmassofamaterial，unitJ/(kgK)orJ/(kg℃)©2015YanweiWang9SpecificHeatCapacityExerciseWhenyoudrinkcoldwater,yourbodymustexpendmetabolicenergytomaintainnormalbodytemperatureof37℃bywarmingupthewaterinyourstomach.Coulddrinkingicewatersubstituteforexerciseasawayto“burncalories?”Supposeyouexpend430kilocaloriesduringabriskone-hourwalk.Howmanylitersoficewaterwouldyouhavetodrinkinordertouse430kilocaloriesofmetabolicenergy?Asacomparison,thestomachcanholdaboutoneliter.©2015YanweiWang10SpecificHeatCapacitySolution.𝑄=430kcal=430∗4184J∆𝑇=37℃𝑐𝑝at15℃=4186Jkg∙℃𝑄=𝑚𝑐𝑝∆𝑇𝑚=𝑄𝑐𝑝∆𝑇=430∗4184J37℃×4186Jkg∙℃𝑚=11.6kgDensityofliquidwaterat0℃,ρ=999.8kgm3=0.9998kgL𝑉=𝑚ρ=11.6L©2015YanweiWang11ThreeModesofHeatTransferHeatConductionHeatConvectionRadiation热传导对流传热辐射传热©2015YanweiWang12HeatConduction(1/3)Heatflowsfromtheregionofhighertemperaturetothatatalowertemperatureunaccompaniedbyanyobservablemacroscopicmotion.Conductionisamicroscopic–levelmechanism,resultingfromtheexchangeoftranslational,rotational,andvibrationalenergyamongthemoleculescomprisingthemedium.©2015YanweiWang13HeatConduction(2/3)Inmetals,heatconductionresultsfromthemotionoffreeelectrons.Thus,thereisclosecorrespondencebetweenthermalconductivityandelectricconductivity.Insolidsthatarepoorconductorsofelectricityandinmostliquids,heatconductionresultsfrommomentumtransferbetweenadjacentvibratingmoleculesoratoms.Ingases,heatconductionoccursbytherandommotionofmolecules,soheatisdiffusedfromhotterregionstocolderones.©2015YanweiWang14HeatConduction(3/3)Themostcommonexampleofpureconductionisheatflowinopaquesolidssuchasthebrickwallofafurnace（熔炉）orthemetalwallofaheatexchangertube.Conductionofheatinliquidsorgasesisofteninfluencedbyflowofthefluids,andbothconductiveandconvectiveprocessesareoftenlumpedtogetherunderthetermconvectionorconvectiveheattransfer.©2015YanweiWang15ConvectiveHeatTransfer(1/2)Convectionreferstotheflowofheatassociatedwiththemovementofafluid,suchaswhenhotairfromafurnaceentersaroom,ortothetransferofheatfromahotsurfacetoaflowingfluid.©2015YanweiWang16ConvectiveHeatTransfer(2/2)Whencurrentsinafluidresultfrombuoyancyforcescreatedbydensitydifferences,andthedensitydifferencesarecreatedbytemperaturegradientsinthefluid,theactioniscallednaturalconvection（自然对流）.Whenthecurrentsareduetoamechanicaldevicesuchasapumporagitator（搅拌器）,theflowisindependentofdensitydifferencesandiscalledforcedconvection（强制对流）.©2015YanweiWang17RadiationHeatTransfer(1/3)Radiationisatermappliedtomanyprocesseswhichinvolveenergytransferbyelectromagneticwavephenomena.Allbodiesconstantlyemitenergybyaprocessofelectromagneticradiation.Theintensityofsuchenergyfluxdependsuponthetemperatureofbodyandthenatureofitssurface.Mostoftheheatthatreachesyouwhenyousitinfrontofafireisradiationenergy.Radiantenergyalsowarmsyouwhenyouwalkinthesun.©2015YanweiWang18RadiationHeatTransfer(2/3)Theradiativemodeofheattransferdiffersintwoimportantrespectsfromtheconductiveandconvectivemodes:(1)Nomediumisrequired;(2)Theemissionofenergyisproportionaltothefourthpoweroftheabsolutetemperaturesofthebodiesinvolved.Stefan-Boltzmannlaw:Theenergyemittedbyablackbodyisproportionaltothefourthpoweroftheabsolutetemperatureofthebody:𝑊𝑏=𝜎𝑇4𝑾𝒃=Thefluxofenergyradiatingfromabody,unit???Stefan-Boltzmannconstant,𝜎=5.67×