Solns to 2005级流体力学与传热考试题(A卷)

诚信应考,考试作弊将带来严重后果!华南理工大学期末考试流体力学与传热（双语）试卷A注意事项：1.考前请将密封线内各项信息填写清楚；所有答案请直接答在试卷上；考试形式：闭卷；本试卷共六大题，满分100分， 考试时间120分钟。题号123456总分得分评卷人(10points)Trueorfalse(F)Inaroughpipe,thefrictionallossisproportionaltovelocityincompletelyturbulentflowzone.(T)Centrifugalpumpsandreciprocatingpumpsareallsubjecttothecavitation.题答不内线封密（(F)Itisespeciallyimportantthatupstreamlengthlongerthan5timesthepipediameteranddownstreamlengthlongerthan5timesthepipediameteraswellaboveandbelowtheorificerespectivelytoensureaflowpatternthatisnormalandundisturbedbyfittings,valves,orotherequipment.题答不内线封密（(F)Atheattransferbyconduction,heattransferrateisalwaysconstant.(F)Iftheinletandoutlettemperaturesoffluidsarefixed,theLMTDofcountercurrentflowisalwayslargerthanthatofparallel-currentflow.(20points)FillintheblanksorchoosethecorrectanswerForgivendustsystemthecapacityofgravitysettlingchamberdependson(theheightandlengthofchamberheightofchamberthebottomareaofchambervolumeofchamberTherearemanyindustrialfilters,givethreeofthem:1)Cakefilters(pressurefilters,vacuumfilters);2)Clarifyingfilters:3)crossflowfilters(ultrafilter).TheorificemeterbelongstoB, C flowmeter;whiletherotameterisA,Dflowmeter.constantpressuredrop;B)constantarea;C)variablepressuredrop;D)variableareaWhenthefiltercakeisincompressible,acakeresistanceisnotinfluencedby(D).thecakelayerthicknessthespecificcakeresistancetheporosity(voidfraction)thefiltrationrateAirflowsalongthetubeandsaturatedvaporpassesthroughtheshellinashell-tubeexchanger.Inordertoenhanceheattransfer,whichwayisfeasibleinpracticeasfollows?(C)A)increasevaporvelocity;B)employsuperheatedvapor;C)increaseairvelocity;D)setupthebafflesintheshell.Theremovalofnon-condensablegasinthecondensingvaporwill(A)increasecondensingfilmcoefficient.decreasecondensingfilmcoefficient.notchangecondensingfilmcoefficient.beuncertain.Basedonthetemperaturedistributionasshownintheattachedfigure,whichlayerhasthelargestheatresistance?t"A ,andwhichhasthelargestthermalconductivity?B ,ifthethicknessofeachlayeristhesame.A)layerA;B)layerB;C)layerC7>Whenfouling(scale)andpipewallresistancesareignoredandthedifferencebetweenh.andhoisveryhuge,overallheattransfercoefficientUiscloseto(B);andwalltemperatureiscloseto(A)side.A)largerh; B)smallerh; C)averagevaluebetweenhiandh。;D)uncertainForlaminarflow,theaveragevelocityinapipeis0.5timesthemaximumvelocity,forturbulentflowaveragevelocityis 0.82〜0.87times themaximumvelocity.Forlaminarflowinapipe,iftheflowrateisconstant,whendiameterDiincreases,thefrictioncoefficientwillincrease,andfrictionlosswilldecrease.(15points)Itisproposedtopump10000kg/hoftolueneat114°Cand1.1atmabspressurefromthereboilerofadistillationtowertoaseconddistillationunitwithoutcoolingthetoluenebeforeitentersthepump.Ifthefrictionlossinthelinebetweenthereboilerandpumpis7kN/m2andthedensityoftolueneis866kg/m3,howfarabovethepumpmusttheliquidlevelinthereboilerbemaintainedtogiveanetpositivesuctionheadof2.5m?Solution.UsetheequationforthesuctionliftHg.TheallowableNPSHforthepumpis2.5m.Alsop；=pv=1*1atm,sc =0-Fiictionlosshf3equalsthefrictionalpressuredropApfdividedbydensityp,isgivenas7000N/mz;hencehfs=7000/866=8.083J/kgh 8.803—-f-NPSH=— 2.5=—3.32mg 9.80665Thustheliquidlevelinthereboilermustbeatleast3.32mabovethepumpsuction.(20pointA)centrifugalpumptakesbrinefromthebottomofasupplytankanddeliversitintothebottomofanothertank.Thebrinelevelinthedischargetankis45mabovethatinthesupplytank,andbothbrinelevelsareopentotheatmosphere.Thelinebetweenthetanksis180mlongof100mminnerdiameterpipe.Theflowrateis90m3/h.Inthelineare2gatevalves,4standardtees,and4elbows.Whatisthepowerofthemotorrequiredforrunningthispump?Thedensityofbrineis1180kg/m3,andtheviscosityofbrineis1.2cP.Theoverallefficiencyofpumpandmotoris60percent.Hint:Losscoefficientforgatevalveis0.17,forelbowis0.75,forteeis1.0Forlaminarflow,f=16/Reor人=4f=64/ReForturbulentflow,X=0.3145/Re0-25Solution.Bernoulliequationbetweenstationaandbbecomesp V2 p V2—a+gZ+-a—+门^^——b+gZ+-bFhpa2ppb2 fa~bAccordingtothegivenconditions,Az—z一z=45mV=V=0,p=pb，门—60%••・W=1(Az-g+h)=1(45x9.81+h)p门 fa-b 0.6 fa-b, L 〜、V2七人—(4fx—+K+K+£K$)x项Here,fromthegivenconditions,

L=180m,D=0.1m,90*4——=3.183m/sx0.12x3600=3.13x1050.1=3.13x1051.2x10-3Soitisturbulentflow,and4f=X=一03145一=0.0133(3.13x105)0.25K=0.4,K=1.0£^=2x6.17+4x0.75+4x1.0=7.34, L 〜、72七人=(4fx万+K+K+£七)x--180 31832=(0.0133x苔+0.4+1.0+7.34)x-—^-=165.5J/kg.•.W=上(&-g+h)=上(45x9.81+h)P门 fa-b 0.6 fa-b=1(45x9.81+165.5)=1012J/kg0.6Powerofmotor=mWpPowerofmotor=mWp90x11803600x1012=29.9x103W(20points)theshell-and-tubeheatexchangerwiththeparallel-currentflow,waterisusedtocooltheoil.Thetemperatureofenteringwaterandleavingwaterare15oCand40oC,respectively.Whilethetemperatureofenteringandleavingoilare150oCand100oC,respectively.Becauseofproductionconditionchanges,theleavingtemperatureoftheoilistobedecreasedto80oC.Supposethatthemassflowrate,theenteringtemperaturesandthephysicalpropertiesoffluidsarenotchanged,ifthetubelengthoftheoriginalheatexchangeris1m,howlonghasthetubetobeincreasedtomeettheproductionrequirement?Theheatlossoftheheatexchangercanbeignored.Solution.Thelogarithmicmeantemperaturedifferenceis△仃(15。-⑸襟00-4°)=92.5°c/布FromtheheatbalanceW£_(七_<)_40-15_05(^-勺一150-100—.Whenthetemperatureoftheoildropsto80°C,fromtheheatbalanceQ_WC^(150—80)_WC«—15)or‘广15_WS_0.5150-80WCpFromwhicht'=50°CAndAt=(15°-15)凄0-50)=70°Cm 135n——。30Fromtheheattransferrateequation,Theformerheatexchanger:WC.(150-100)_KSAt_KmdL(92.5)Thenewheatexchanger:WC(150—80)=KS'At'=KndLf(70)hence,L=70x925_1.85m50 70(15points)Aslurryisfilteredbyafilterpressof0.1m2filteringareaatconstantpressure,th