高等工程热力学试卷和答案

11Trueorfalse(20pts)1)MasscancrossthesystemboundaryintheOpensystem.(T)2)HeatandworkcannotcrossintheClosedsystem.(F)3)Intensivepropertyisapropertythatisindependentoftheextentormassofthesystem.(T)4)Heatexchangersaredevicesthattransferenergybetweenfluidsatdifferenttemperatures.(T)5)Heatenginecanhaveathermalefficiencyof100%.(F)6)HeatEnginesareDevicesthatareusedtoconvertheattowork.(T)7)Thedecreaseinqualityisalwaysaccompaniedbyanincreaseinentropy.(T)8)Reversibleprocessesactuallycanoccurinnature.F)9)Energycanbeneithercreatedordestroyed;itcanonlychangeforms.（T）10)BothHeatandWorkareassociatedwithaprocessandastate.(F)11)Oncethetemperatureequalityisestablished,theheattransferstops.(T)12)Thedirectionofheattransferisalwaysfromthelowertemperaturebodytothehighertemperature.body.(F)13)Workisnotapropertyofasystemoritssurroundings.(T)14)ClosedFeedwaterHeatersrequireaseparatepumpforeachheater.(F)15)Thereheatprocessingeneralcansignificantlychangethecycleefficiency.(T)2Chosingacorrectanswer(30pts)1)Athermodynamicsystemgenerallyhasaboundary,thisboundary（b）a)canbepersonallydefinedfromyourowninterestsb)canonlybedefinedfollowingcertainrolesc)isdependonthephysicalboundariesd)isindependentonthephysicalaswellaspersonalwills2)Asystemiscalledaclosedsystemwhen（c）a)thereisonlyheatexchangebetweenthesystemanditssurroundingsb)thereisonlyworkinteractionsbetweenthesystemanditssurroundingsc)therearebothheatexchangeandworkinteractoinsbetweenthesystemanditssurroundingsd)thereareneitherheatexchangenorworkinteractionsbetweenthesystemanditssurroundings3)Thespecificinternalenergy,u,otherthanU,ofasystemisdeterminedbythe（c）ofthesystema)volumeb)pressurec)temperatured)mass4)Anexampleofinternalpropertyofasystemis（c？）a)massb)totalvolumec)pressured)enthalpy5)Asteadystateprocessisrelatedto（a？）a)aclosedsystemwithinvariblepropertiesb)acontrolvolumewithitspropertychangesc)anopensystemkeepingitspropertiesconstantd)aclosedsystemhavingitspropertieschanged6)Agoodapproximationforastreamflowingthroughaporousplugis（a）a)hin=houtb)Pin=Poutc)Tin=Toutd)aandbe)bandc7)ΔT=0istrue（d）a)foranadiabaticprocess.b)ifnoworkisdone.c)foranisentropicprocess.d)forareversibleprocess.8)CharacteristicsofHeatEnginesinclude（e）a)Theyreceiveheatfromahigh-temperaturesource.2b)Theyconvertpartofthisheattowork.c)Theyrejecttheremainingwasteheattolow-temperaturesink.d)Theyoperateonacycle.e)allofabove9)Quality（a）a)isdefinedbyM/M+M)liqliqvapb)isdefinedbyv=xv+(1-x)vfgc)equalszeroforasaturatedvapor.d)isonlydefinedinthetwophaseregion.e)isthefractionofmassthatisliquid.10)Agoodapproximationforastreamflowingthroughaheatexchangeris（e？）a)h=hinoutb)P=Pinoutc)T=Tinoutd)aandbe)bandc11)Inanadiabaticprocess:（b）a)temperaturecannotchange.b)thereisnoheatflow.c)internalenergycannotchange.d)aandbe)a,bandc12)Whichofthefollowingisanextensiveproperty?（b）a)temperatureb)weightc)compositiond)pressuree)noneoftheabove13)WhatistheefficiencyofanidealCarnotheatenginecycleoperatingbetween38Cand482C?（c）a)90%b)89%c)59%d)16%e)11%14)Whichofthefollowingdoesnothaveunitsofenergy?（d）a)N-mb)kw-hrc)Pa-m3d)Allhaveunitsofenergy15)Extensiveproperties（d）a)applyatapoint.b)donotdependonthesizeofthesystem.c)dependonpositionwithinasystem.d)areproportionaltothemassofasystem.e)areprintedinthetablesinthebackofyourbook.3Providedefinitionforthefollowingterms(10pts)1)Internalenergy:Internalenergyisreallythekineticandpotentialenergyoftheatomsinthemolecules.2)Heat:Energycrossingthesystemboundary,notassociatedwithmass,duetoatemperaturedifference.3)Work:Workistheenergytransferassociatedwithaforceactingthroughadistance.4)Closedsystem:Nomasscrossestheboundary.Heatandworkcancross.5)Opensystem:Masscancrossthesystemboundary.4Answeringandcalculations(40pts)1)FeedwaterHeater:Inlet1T1=200ºC,p1=700kPa,1m40kg/s3Inlet2T2=40ºC,p2=700kPa,A2=25cm2Exitsat.liquid,p3=700kPa,FindSteadyStateInlet2:compressedliquidTableA-4,v2=0.001008m3/kgExit:saturatedliquidTableA-5,v3=0.001108m3/kg=54.15–40=14.15kg/s2)Arigidtankcontains10kgofwaterat90ºC.If8kgofthewaterisintheliquidformandtherestisinthevaporform.Determinethepressureinthetankandthevolumeofthetank.TableA-4,Psat=70.14kPa,P=Psat=70.14kPaTableA-4,vf=0.001036m3/kg,vg=2.361m3/kgv=vf+x(vg-vf)=0.001036+0.2(2.361–0.001036)=0.473m3/kgV=mv=10(0.473)=4.73m33)Heatistransferredtoaheatenginefromafurnaceatarateof80MW.Iftherateofwasteheatrejectiontoanearbyriveris50MW,determinethenetpoweroutputandthethermalefficiency.4)Aheatenginereceives600kJofheatfromahigh-temperaturesourceat1000Kduringacycle.Itconverts150kJofthisheattoworkandrejectstheremaining450kJtoalow-temperaturesinkat300K.Determineifthisheatengineviolatesthe2ndlawofthermodynamicsonthebasisof(a)theClausiusinequality.(b)theCarnotprinciple.(a)Clausiusinequality33(AV)0.06m/s232m?,m?andV?iemm123mmmAVmAVv333(AV)0.06m54.15kg/sv0.001108231mmm2222mv(14.15)(0.001008)V5.7m/sA0.0025HLQ80MW,Q50MWHLWQQ805030MWHW300.37580QHLHLQQQTTT60045010003004=-0.9kJ/K0(b)Carnotprincipleηthηrev5)Aheatpumpmaintainsahouseataspecifiedtemperature.Therateofheatlossofthehouseandthep