应用化学专业英语 教学课件 作者 黄忠 谢普会 付颖 主编lesson 22 heat transfer.ppt

《应用化学专业英语 教学课件 作者 黄忠 谢普会 付颖 主编lesson 22 heat transfer.ppt》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

1、Lesson22HeatTransferHowistheheattransfer?MechanismofConvectionApplications.MeanfluidVelocityandBoundaryandtheireffectontherateofheattransfer.FundamentalequationofheattransferLogarithmic-meantemperaturedifference.HeattransferCoefficients.HeatfluxandNusseltcorrel

2、ationSimulationprogramforHeatExchangerHowistheheattransfer?Heatcantransferbetweenthesurfaceofasolidconductorandthesurroundingmediumwhenevertemperaturegradientexists.ConductionConvectionNaturalconvectionForcedConvectionNaturalandforcedConvectionNaturalconvection

3、occurswheneverheatflowsbetweenasolidandfluid,orbetweenfluidlayers.AsaresultofheatexchangeChangeindensityofeffectivefluidlayerstakenplace,whichcausesupwardflowofheatedfluid.Ifthismotionisassociatedwithheattransfermechanismonly,thenitiscalledNaturalConvectionForc

4、edConvectionIfthismotionisassociatedbymechanicalmeanssuchaspumps,gravityorfans,themovementofthefluidisenforced.Andinthiscase,wethenspeakofForcedconvection.HeatExchangersAdevicewhoseprimarypurposeisthetransferofenergybetweentwofluidsisnamedaHeatExchanger.Applica

5、tionsofHeatExchangersHeatExchangerspreventcarengineoverheatingandincreaseefficiencyHeatexchangersareusedinIndustryforheattransferHeatexchangersareusedinACandfurnacesTheclosed-typeexchangeristhemostpopularone.OneexampleofthistypeistheDoublepipeexchanger.Inthisty

6、pe,thehotandcoldfluidstreamsdonotcomeintodirectcontactwitheachother.Theyareseparatedbyatubewallorflatplate.PrincipleofHeatExchangerFirstLawofThermodynamic:“Energyisconserved.”0000ControlVolumeQhCrossSectionAreaHOTCOLDThermalBoundaryLayerQhotQcoldThTi,wallTo,wal

7、lTcRegionI:HotLiquid-SolidConvectionNEWTON’SLAWOFCCOLINGRegionII:ConductionAcrossCopperWallFOURIER’SLAWRegionIII:Solid–ColdLiquidConvectionNEWTON’SLAWOFCCOLINGTHERMALBOUNDARYLAYEREnergymovesfromhotfluidtoasurfacebyconvection,throughthewallbyconduction,andthenby

8、convectionfromthesurfacetothecoldfluid.VelocitydistributionandboundarylayerWhenfluidflowthroughacirculartubeofuniformcross-suctionandfullydeveloped,Thevelocitydistributionde