流体力学与传热电子双语

HeattransfertofluidsphaseConvectionHeatTransfer对HeattransfertofluidsphaseConvectionHeatTransfer对ANewton’slawofT膜模型：dt=deNewton’slawofT膜模型：dt=de+tWdt──总有效膜d──层流底层膜厚tQ= k热阻全部在h牛顿冷却定律Q=ConvectionHeatTransfer对Newton’slawof，ConvectionHeatTransfer对Newton’slawof，，A-t1q=h0A0(T1-T2q=hiAih=★复杂问题简单化表示hRegimesofRegimesofheattransferin•Afluidbeingheatedorcooledmaybeflowingindifferentflowpatterns.Also,thefluidmaybeflowinginforcedornaturalconvection.twotypesofconvectionaretwotypesofconvectionarenatural自然对force强制对---mostimportantsituationinheatSincemostequipmentisoperatedin自然对流：由于流体内温度不同造成的浮升引起的流强制对流：流体受外力作用而引起的流动AtordinaryvelocitiesAtordinaryvelocitiestheheatgeneratedfromfluidfrictionisnegligibleincomparisonwiththeheattransferredbetweenthefluids.一般情况下，由摩擦引起的发热可忽略不•Becausethesituationsofflowattheentrancetoatubediffersfromthosewelldownstreamfromtheentrance,thevelocityfieldandassociatedtemperaturefieldmaydependonthedistancefromthetubeentrance•Thepropertiesofthefluid-viscosity,Thepropertiesofthefluid-viscosity,thermalconductivity,specificheat,anddensityareimportantparametersinheat•Eachofthese,especiallyviscosity,isHeattransferHeattransferbyforcedconvectioninturbulentflow流体在圆管内作强制湍Perhapsthemostimportantsituationinheattransferistheheatflowinastreamoffluidinturbulentflow.Sincetherateofheattransferisgreaterinturbulentflowthaninlaminarflow,mostequipmentisoperatedintheturbulent流体在圆管内作强制湍fluid流体在圆管内作强制湍fluidturbulentthroughastraightBasedondimensional因次分析=c(dur)m(cpmkkm★c,m,narecontantstobedeterminedbyThethreegroup1.Nusselt努塞尔NuThethreegroup1.Nusselt努塞尔Nu----表面传热系数的特征khThefilmcoefficient膜传热系2.Reynolds雷诺Re——流动型m普朗特3.Prandtl普朗特3.PrandtlcpPr——流体物kThePrandtlnumberofagasisusuallycloseto1（0.69forair,1.06forsteam).各特征数的名称和涵义见各特征数的名称和涵义见努塞尔数雷诺数普朗特数湍强制对无相湍强制对无相管内、直、弯圆或非圆自然对h关联蒸汽冷液体沸有相Empirical经验公ForcedconvectionForturbulentEmpirical经验公ForcedconvectionForturbulentNu=0.023Re0.8Prnh=0.023k(dur)0.8(cpm)ndmln=0.4,thefluidisbeingheatedn=0.3,thefluidisbeingcooled.流体被加流体被冷Eq.(12-32)issatisfactoryforlow-viscosityfluids,suchaswater.h=0.023k(duh=0.023k(dur)0.8(cpm)ndlm强化措施 u0.8，u›hd一定u一定 1/d0.2，dflh使用h关联式的注意事（1）确定物使用h关联式的注意事（1）确定物性参数数值的温度称为定性温=t1+(b)膜温+(a)m22（2）特征尺管内径Re>6000,（3）适用范以下是对上面的公式进行以下是对上面的公式进行修正••Abetterrelationshipforturbulentisknown(1)高黏theSieder-TatePr1/3(mNu=μ---bulkμw---fluidμat适用范围定性温度特性尺寸(2)Effectoftube(2)EffectoftubeD0.7 =1+iLhi----anaveragevalueoffilm★过渡区d›a6过渡区d›a6f=1<弯曲管f=1+1.77d>R（5）Crosssections（5）Crosssectionsotherthan非圆形管强制湍d---canusetheequivalentdiameterde.toreplacethediameterinbothReynoldsandNusseltdeisdefinedas4timesthehydraulicradiusrH.de=4倍流通截面/润湿周边长度2.圆形管内强制层2.圆形管内强制层液特点导致速度分布受热流方向影层流受自然对流影响大——自然对1)3dmNu=1.86(l(RePr1)3dmNu=1.86(l(RePrd)>适用范围：GrRe<l定性温度特性尺寸1乘以系数：f0.8(10.015Gr3当GrHeatingHeatingandcoolingoffluidsinforcedconvectionoutsidetubes管外强制对流的mechanismofheatflowinforcedconvectionoutsidetubesdiffersfromthatofflowinsidetubes.Thelocalvalueofheat-transfercoefficientvariesfrompointtopointaroundcircumferenceinforcedconvectionoutsidetube.管外强制对管外强制对流的流体在管束外垂直流aaa2．流体在列2．流体在列管换热器壳程的流挡板形式••圆圆缺目的：增强壳程流体的湍动，提高af(l,cp,af(l,cp,m提高壳程a的措施u2,hfu›α但defl›3）加强湍动无折流挡板时，流体平行流过管束，a按管制对流计算，但di换为有相变时的对流传热有相变时的对流传热系数关联一、蒸汽冷••冷凝方膜状冷滴状冷工a滴>a主要热阻——液4．4．冷凝传热的影响因素和强化措（1）流体物冷凝液r›mfllr›（2）温层流（3）不凝气不凝（3）不凝气不凝气存在（4）蒸汽流速与流定期排）同向dfla；反向时d›af