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1、ME440ConvectiveHeatandMassTransferNameQuiz#1ID#Saturday12-10-02Duration30minutesOnacooldayinDecember,amanisrunningandheislosingheatatarateof500Wbecauseofconvectionheattransfertothesurroundingairat10oC.Hisskinremainsdryatatemperatureof30oC.InAugust,themanismovingatthesamespeed,butthedayiswarmandh
2、umidwithanairtemperatureof30oCandrelativehumidityoff=60%.Heisnowdrenchedinsweatandhasauniformtemperatureof35oC.Underbothconditionsconstantairpropertiesmaybeassumedwithn=1.6*10-5m2/s,k=0.026W/moC,Pr=0.7andDAB(watervapor-air)=2.3*10-5m2/s.Forsaturatedwatervapor(at303K),rA,sat=0.03kg/m3andat35oC,rA
3、,sat=0.039andhfg=2419kJ/kgkg/m3.1.IsthevalueofRethesameforbothcases?2.WhatisthevalueofSc?3.AssumingthemanisapproximatesasacylinderwhosesurfaceareaisAs,whatisthevalueofthemasstransfercoefficienthm?4.Inhumidsummer,thevalueofrA,sat=0.039atthesurfaceofhisskin(duetosweat)is0.03kg/m3,showthatthevalueo
4、f=0.018forthishumidday.5.Whatistherateofwaterlossduetoevaporationonthesummerday?6.Whatisthetotalconvectiveheatlossonthissummerday?7.Whatisyourcommentontheeffectofevaporativecooling?ME440ConvectiveHeatandMassTransferNameQuiz#2ID#Saturday19-10-02Duration30minutesAnobjectofanirregularshapehasachara
5、cteristiclengthofL=1mandismaintainedatauniformsurfacetemperatureofTs=325K.Itissuspendedinanairstreamthatisatatmosphericpressure(p=1atm)andhasavelocityofV=100m/sandatemperatureof=275K.Theaverageheatfluxfromthesurfacetotheairis12000W/m2.ConsiderthiscaseascaseI,nowdeterminewhetherthefollowingcondit
6、ionsareanalogoustothatcaseandstatethereason(assumingthesameshapesuspendedinairinthesamemanner).Ifanyofthefollowingcasesisanalogous;determinethecorrespondingaverageconvectioncoefficient.Note,forairat300K,n1=15.8910-6m2/s,Pr1=0.71,k1=0.0263W/mKa)ThevaluesofTsandandpremainthesame,butL=2mandV=50m/s.
7、CaseIIb)ThevaluesofTsandarethesameandL=2m,V=50m/sbutp=0.2atm.(n3=79.4510-6m2/s).CaseIIIc)Thesurfaceiscoatedwithaliquidfilmthatevaporatesintotheair.Theentiresystemisat300K,andthediffusioncoefficientfortheair-vapormixtureisDAB
