流體力學(xué)與傳熱習(xí)題參考解答英文

1.Waterispumpedataconstantvelocity1m/sfromlargereservoirrestingonthefloortotheopentopofanabsorptiontower.Thepointofdischargeis4meterabovethefloor,andthefrictionlossesfromthereservoirtothetoweramountto30J/kg.Atwhatheightinthereservoirmustthewaterlevelbekeptifthepumpcandeveloponly60J/kg?U1=0W=60j/kg2.Thefluid(density1200kg/m3)ispumpedataconstantrate20m3/hfromthelargereservoirtotheevaporator.Thepressureabovethereservoirmaintainsatmospherepressureandthepressureoftheevaporatorkeeps200mmHg(vacuum).Thedistancebetweenthelevelofliquidinthereservoirandtheexitofevaporatoris15meterandfrictionallossinthepipeis120J/kgnotincludingtheexitofevaporator,whatisthepumpeffectiveworkandpowerifthediameterofpipeis60mm3.Watercomesoutofthepipe(Φ108x4mm),asshowninFig.Thefrictionlossofthepipelinewhichdoesnotcoverthelossattheexitofpipecanbecalculatedbythefollowingequation:hf=6.5U2whereUisthevelocityinthepipe,finda.watervelocityatsectionA-A'.b.waterflowrate,inm3/h.4.Waterpassesthroughthevariablepipe.Thevelocityinthesmallpipeis2.5m/s.TheverticalglasstubesareinsertedrespectivelyatthesectionAandBtomeasurethepressure(seefig.)Ifthefrictionlossbetweentwosectionis15J/kg,whatisthewatercolumndifferencebetweentwoglasstubes?Bytheway,drawtherelativeliquidcolumnheightoftwotubesintheFig.5.Acentrifugalpumptakesbrine(density1180kg/m3,viscosity1.2cp)fromthebottomofasupplytankanddeliversitintoanothertank.Thelinebetweenthetanksis300mof25mmdiameterpipe(innerdiameter).Theflowrateis2m3/h.Inthisline,therearetwogatevalves,fourelbows(90o)andonereturnbend,whatisthefrictionlossiftheroughnessofpipeis0.025mm6.Theorificemeter(diameteroforifice0.0001m)isinstalledformeasuringtheflowrate.TheindicatingliquidoforificeismercuryifUshapepressuregaugereadingis0.6meterandorificecoefficientcanbetakenas0.61,whatistheflowrateofwater?7.Waterflowsthroughapipewithadiameterdi100mmasshowninfigure.a.whenthevalveisclosed,Ris600mmandhequals1500mm.Whilethevalveopenspartially,R=400mmandh=1400mm,f=0.00625(Finningfactor)andkc=0.5(contractioncoefficient),whatistheflowrateofwater,inm3/h?b.Ifthevalveopensfully,whatisthepressureofsection2-2',inN/m2?Theequivalentlengthofthevalveis1.5mandtheFanningfactorfkeepsthesame?(H2O=1000kg/m3,Hg=13600kg/m3)(1)thevalveopenspartially,forselection1-1’and2-2WecangetZ1fromthevalveclosed(2)whenthevalveopensfully,forsection1-1’and3-3Forsection1-1’and28.Therotameterisinstalledtomeasurethewaterflowrate,asshowninfigure.IfthetotallengthincludingequivalentlengthofpipelineAis10mandthereadingofrotameteris2.72m3/h,whatistheflowrateforpipelineB?(fA=0.0075,fBForparallelpipeline10.Aflatfurnacewallisconstructedof120mmlayerofsil-o-celbrick,withathermalconductivity0.08w/(moC),backedbya150mmofcommonbrick,ofconductivity0.8w/(moC),thetemperatureofinnerfaceofthewallis1400o,andthatoftheouterfaceis200oC.a.Whatistheheatlossthroughthewallinwpersquaremeter.b.Toreducetheheatlossto600w/m2byaddingalayerofcorkwithk0.2w/(moC)ontheoutsideofcommonbrick,howmanymetersofcorkarerequied?a.b.600=(1400-200)/(0.12/0.08+0.15/0.8+x/0.2)x=0.0625m13.Airatthenormalpressurepassesthroughthepipe(di20mm)andisheatedfrom20oCto100oC.Whatisthefilmheattransfercoefficientbetweentheairandpipewalliftheaveragevelocityofairis10m/s?Thepropertiesofairat60oCareasfollows:density1.06kg/m3,viscosity0.02cp,conductivity0.0289w/(moC),andheatcapacity1kJ/kg-K14.Ahotfluidwithamassflowrate2250kg/hpassesthrougha25x2.5mmtube.Thephysicalpropertiesoffluidareasfollows:k=0.5w/(moC),Cp=4kJ/kg-K,viscosity10-3N-s/m2,density1000kg/m3Find:a.Heattransferfilmcoefficienthi,inw/(m2-K).b.Iftheflowratedecreasesto1125kg/handotherconditionsarethesame,whatisthehi?c.Ifthediameteroftube(insidediameter)decreasesto10mm,andthevelocityukeepsthesameasthatofcasea,calculatehi.d.Whentheaveragetemperatureoffluidandquantityofheatflowpermeteroftubeare40oCand400w/m,respectively,whatistheaveragetemperatureofpipewallforcasea?e.Fromthisproblem,inordertoincreasetheheattransferfilmcoefficientandenhanceheattransfer,whatkindsofmethodscanyouuseandwhichisbetter,explain?Hint:forlaminarflow,Nu=1.86[RePr]1/3forturbulentflowNu=0.023Re0.8Pr1/3(1)(2)(3)(4)(5)theremethods:increaseuorhiordecreasedThefirstisbetter15.Inadoublepipeexchange(Φ23x2mm),thecoldfluid(Cp=1kJ/kg,flowrate500kg/h)passesthroughthepipeandthehotfluidgoesthroughtheoutside.Theinletandoutlettemperaturesofcoldfluidare20and80o,andtheinletandoutlettemperaturesofhotfluidare150and90o,respectively.Thehi(filmcoefficientinsidepipe)is700w/(m2oC)andoverallheattransfercoefficientUo(basedontheoutsidesurfaceofpipe)is300w/(m2oC),respectively.Iftheheatlossisignoredandtheconductivityofpipewall(steel)istakenas45w/(moC),find:(1)heattransferfilmcoefficientoutsidethepipeho?(2)thepipelengthrequiredforcounterflow,inm?(3)whatisthepipelengthrequirediftheheatingmediumchangestosaturatedvapor(140oC)anditcondensestosaturatedliquidandotherconditionskeepunchanged?(4)Whentheexchangerisusedforayear,itisfoundthatitcannotmeettheneedofproduction(theoutlettemperatureofcoldfluidcannotreach80oC),explainwhy?(a)1/h0=1/U0-(do/hidi+bdo/kdm)=1/300-23/700*19-0.002*23/45*21Q=UoAoTm=mcCp(Tcb-Tca)300*2*0.023*70L=500/3600*1000*(80-20)L=5.4m(c)(d)scaleisformedontheoutside,V0isdecreased16.WaterflowsturbulentlyinthepipeofΦ25x2.5mmshelltubeexchanger.Whenthevelocityofwateruis1m/s,overallheattransfercoefficientUo(basedontheoutersurfaceareaofpipe)is2115w/(m2oC).Iftheubecomes1.5m/sandotherconditionskeepunchanged,Uois2660w/(m2oC).Whatisthefilmcoefficienthooutsidethepipe?(Heatresistancesofpipewallandscaleareignored)(1)(2)(1)-(2)=C=2859ho=8127W/(m2K)17.Waterandoilpassparallellythroughanexchangerwhichis1mlong.Theinletandoutlettemperaturesofwaterare15and40oC,andthoseofoilare150and100oC,respectively.Iftheoutlettemperatureofoildecreasesto80oC,andtheflowratesandphysicalpropertiesandinlettemperaturesofwaterandoilmaintainthesame,whatisthepipelengthofnewexchanger?(Heatlossandpipewallresistanceareneglected)18.Airwhichpassesthroughthepipeinturbulentflowisheatedfrom20to80oC.Thesaturatedvaporat116.3oCcondensestosaturatedwateroutsidethepipe.Ifairflowrateincreasesto120%oftheoriginandinletandoutlettemperaturesofairstayconstant,whatkindofmethodcanyouemployinordertodothat?(Heatresistanceofpipewallandscalecanbeignored)Th=118.5oC19.WaterflowsthroughthepipeofaΦ25x2.5mmshell-tubeexchangerfrom20to50oC.Thehotfluid(Cp1.9kJ/kgoC,flowrate1.25kg/s)goesalongtheshellandthetemperatureschangefrom80to30oC.Filmcoefficientsofwaterandhotfluidare0.85kw/(m2oC)and1.7kw/(m2oC).WhatistheoverallheattransfercoefficientUoandheattransferareaifthescaleresistancecanbeignored?(theconductivityofsteelis45w/(moC).W=1.25Kg/sCp=1.9Kj/kg℃20.Asphericalparticle(density2650kg/m3)settlesfreelyinairat20oC(densityofair1.205kg/m3,viscosity1x10-5Pa.s).CalculatethemaximumdiameterofparticleifthesettleobeystheStokes’Law?Re≤1=3.85x10-521.Afilterpress(A=0.1m2)isusedforfilteringslurry.Thevacuuminsidethefilteris500mmforfilterpress5min(1)10min(2)From(1)(2),wecanseeVe=0.7K=48 15minV=2.07m3/h22.Thefollowingdataareobtainedforafilterpress(A=0.0093m2------------------------------------------------------------------------------------------------pressuredifference(kgf/cm2)filteringtime(s)filtratevolume(m3)1.0550