应用化学专业英语lesson22heattransfer.ppt

Lesson 22 Heat TransferHow is the heat transfer?Mechanism of ConvectionApplications.Mean fluid Velocity and Boundary and their effect on the rate of heat transfer.Fundamental equation of heat transferLogarithmic-mean temperature difference.Heat transfer Coefficients.Heat flux and Nusselt correlation Simulation program for Heat Exchanger How is the heat transfer?Heat can transfer between the surface of a solid conductor and the surrounding medium whenever temperature gradient exists.ConductionConvectionNatural convection Forced ConvectionNatural and forced ConvectionNatural convection occurs whenever heat flows between a solid and fluid,or between fluid layers.As a result of heat exchangeChange in density of effective fluid layers taken place,which causes upward flow of heated fluid.If this motion is associated with heat transfer mechanism only,then it is called Natural ConvectionForced ConvectionIf this motion is associated by mechanical means such as pumps,gravity or fans,the movement of the fluid is enforced.And in this case,we then speak of Forced convection.Heat ExchangersA device whose primary purpose is the transfer of energy between two fluids is named a Heat Exchanger.Applications of Heat ExchangersHeat Exchangers prevent car engine overheating and increase efficiencyHeat exchangers are used in Industry for heat transferHeat exchangers are used in AC and furnacesThe closed-type exchanger is the most popular one.One example of this type is the Double pipe exchanger.In this type,the hot and cold fluid streams do not come into direct contact with each other.They are separated by a tube wall or flat plate.Principle of Heat ExchangerFirst Law of Thermodynamic:“Energy is conserved.”0000Control VolumeQhCross Section AreaHOTCOLDThermal Boundary LayerQ hot Q coldThTi,wallTo,wallTcRegion I:Hot Liquid-Solid ConvectionNEWTONS LAW OF CCOLINGRegion II:Conduction Across Copper WallFOURIERS LAWRegion III:Solid Cold Liquid ConvectionNEWTONS LAW OF CCOLINGTHERMALBOUNDARY LAYEREnergy moves from hot fluid to a surface by convection,through the wall by conduction,and then by convection from the surface to the cold fluid.Velocity distribution and boundary layerWhen fluid flow through a circular tube of uniform cross-suction and fully developed,The velocity distribution depend on the type of the flow.In laminar flow the volumetric flowrate is a function of the radius.V=volumetric flowrateu=average mean velocityIn turbulent flow,there is no such distribution.The molecule of the flowing fluid which adjacent to the surface have zero velocity because of mass-attractive forces.Other fluid particles in the vicinity of this layer,when attempting to slid over it,are slow down by viscous forces.rBoundary layerAccordingly the temperature gradient is larger at the wall and through the viscous sub-layer,and small in the turbulent core.The reason for this is 1)Heat must transfer through the boundary layer by conduction.2)Most of the fluid have a low thermal conductivity(k)3)While in the turbulent core there are a rapid moving eddies,which they are equalizing the temperature.heatingcoolingTube wallhRegion I:Hot Liquid Solid ConvectionRegion II:Conduction Across Copper WallRegion III:Solid Cold Liquid Convection+U=The Overall Heat Transfer Coefficient W/m.KroriCalculating U using Log Mean TemperatureHot Stream:Cold Stream:Log Mean TemperatureCON CURRENT FLOWCOUNTER CURRENT FLOW T1T2T4T5T3T7T8T9T10T6Counter-Current FlowT1T2T4T5T6T3T7T8T9T10Parallel FlowLog Mean Temperature evaluationT1A12T2T3T6T4T6T7T8T9T10Wall T1 T2 AA12T1A12T2T3T6T4T6T7T8T9T10Wall DIMENSIONLESS ANALYSIS TO CHARACTERIZE A HEAT EXCHANGERFurther Simplification:Can Be Obtained from 2 set of experimentsOne set,run for constant Pr And second set,run for constant RehFor laminar flowNu=1.62(Re*Pr*L/D)Empirical CorrelationGood To Predict within 20%Conditions:L/D 100.6 Pr 20,000For turbulent flowExperimentalApparatus Two copper concentric pipesInner pipe(ID=7.9 mm,OD=9.5 mm,L=1.05 m)Outer pipe(ID=11.1 mm,OD=12.7 mm)Thermocouples placed at 10 locations along exchanger,T1 through T10Hot Flow RotametersTemperature IndicatorCold Flow rotameterHeat ControllerSwitch for concurrent and countercurrent flowTemperature ControllerExamples of Exp.ResultsTheoretical trendy=0.8002x 3.0841Experimental trendy=0.7966x 3.5415Theoretical trendy=0.3317x+4.2533Experimental trendy=0.4622x 3.8097Theoretical trend y=0.026x Experimental trendy=0.0175x 4.049Experimental Nu=0.0175Re0.7966Pr0.4622Theoretical Nu=0.026Re0.8Pr0.33Effect of core tube velocity on the local and over all Heat Transfer coefficients