工程熱力學(xué)(英文版)第7單元課件

Chap6Summary-11Whyweneed2ndLaw?Allprocessessatisfy1stLaw;Satisfying1stdoesnotensuretheprocesscanactuallyoccurHeatEnginesRefrigerator,HeatPumpIntroductionto2ndLawRefrigerators/heatpump:ThedevicesdriveheatQtransferfromTLtoTH,ThermalenergyReservoirReceiveheatQHfromahightemperaturesourceTheworkinputtotherefrigerator/heatpumpHeatQLabsorbedfromrefrigeratedspaceTLAprocesshasdirectionEnergyhasqualityandquantityHeatSourceHeatSinkConvertpartQHtoworkWnet,outRejectwasteheatQLtoalowtemperaturesinkHeatengine2ndlaw,Kelvin-PlanckStatement:Itisimpossibleforanydevicethatoperatesonacycletoreceiveheatfromasinglereservoirandproduceanetamountofwork.

Noheatenginecanhaveη=100%HeatQHrejectedtohightemperatureTHRefrigeratorwantsQLHeatpumpwantsQHCOP2ndlaw,ClausiusStatement:Heatdoesnot,ofitsownvolition,transferfromacoldmediumtoawarmerone.(熱不能自發(fā)地、不付代價(jià)地從低溫物體傳到高溫物體)Chap6Summary-11Whyweneed2nChap6Summary-22Aprocesscanbereversedwithoutleavinganytraceonthesurroundings.

CarnotCycleCarnotRefrigeratorCarnotHeatPumpReversibleProcessesTheheatengineoperatesonthereversibleCarnotCycleThebestknownreversiblecycle;fourreversibleprocessesCarnotheatengineCarnotPrinciple1:GivenTLandTH,ηth,irrev<ηth,revSystemSurroundingsInternalRevExternalRevWhyneedRevIrreversible:heattransferIsothermalexpansionIsothermalCompressionAdiabaticcompressionReversedCarnotCycleCarnotrefrigerator/heatpumpCarnotPrinciple2:GivenTLandTH,ηth,allrev=ηth,revCarnotHeatEngineTherefrigerator/heatpumpoperatesonareversedCarnotCycleAdiabaticexpansionChap6Summary-22AprocesscanChap7Summary3Clausiusinequality:thecyclicintegralofisalways≦zeroIncreaseofEntropyPrinciple(熵增原理)Entropy(熵)1,Entropychangeofpuresubstances:Entropychangeofaclosedsystem:Increaseofentropyprinciple(孤立系統(tǒng)熵增原理，簡(jiǎn)稱熵增原理)：theentropyofanisolatedsystemduringaprocessalwaysincreaseor,inthelimitingcaseofareversibleprocessremainsconstant.(孤立系統(tǒng)的熵可以增大，或保持不變，但不可能減少)

EntropySomeremarksIsoT>ReversibleprocessIrreversibleprocess≥0sfat0.01℃=0kJ/kg.k2,Isentropicprocess0,adibatic0,Reversible3,T-S,h-sdiagrams=4,The3rdlawofthermodaynamics:Theentropyofapurecrystallinesubstanceatabsolutezerotemperatureiszero5,Tdsrelations:6,reversibleworkoutputChap7Summary3ClausiusinequalChapter7Entropy4Chapter7Entropy47-1EntropyThe2ndlawofthermodynamicsleadstoexpressionsthatinvolveinequalities.Heatengine:ηth,irrev<ηth,revHeatpump:COPHP,irrev<COPHP,revCOPR,irrev<COPR,rev;Clausiusinequality:Thecyclicintegralofisalwayslessthanorequaltozero.StatedbyR.J.E.Clausius.Validforallcycles,reversibleorirreversibleThecyclicintegralofcanbeviewedasthesumofallthesedifferentialamountsofheattransferdividedbythetemperatureoftheboundary.57-1EntropyThe2ndlawoftheWhy

ConsiderasystemconnectedtoathermalenergyreservoirataconstanttemperatureofTRthroughareversiblecyclicdevice.Thecyclicdevicereceivesheatfromreservoir;suppliesheattothesystem,andproducingworkThesystemproducesworkasaresultofheattransfer，andtheboundarytemperatureisT(variable)Wehaveacombinedsystemasthedashedlinesyields:6ForacycleKelvinPlanckstatement:nosystemcanproduceanetamountofworkwhileoperatinginacycleandexchangingheatwithasinglethermalenergyreservoir.WCcannotbeaworkoutput,cannotbeapositivequantityWhy6ForacycleKelvinPlanckForreversiblecycles:allquantitieshavethesamemagnitudebuttheoppositesign,thus:TheequalityintheClausiusinequalityholdsforreversiblecyclesTheinequalityfortheirreversiblecycles.Forreversiblecycles:allquaEntropy:Cyclicintegraliszero:W,no;Q,no;netchangeinvolume,yes.?Dependsonstateonly,nottheprocesspath.So,itisaproperty,statefunctionIn1865,ClausiusnamedthispropertyasENTROPY,S.theentropychangeisdefinedas?Entropy:?Then,theentropychangeofasystemduringaprocesscanbedeterminedbyintegrationbetweentheinitialandthefinalstates:WeconcernmoreonchangeofS,butnotSitself,sowehaveareferencestate(S=0).TherelationofQandTisoftennotavailable,sowegetentropymostlyfromtables.Entropyisaproperty,isfixedatfixedstates.Theentropychangebetweentwospecifiedstatesisthesamenomatterwhatprocesspathitisfollowed.Thecalculationisonlyvalidforreversiblepathbetweenthetwostates;andusedascomparedvalueforirreversibleprocess.9Then,theentropychangeofaSpecialcase–reversibleisothermalheattransferprocesses.QheattransferduringthereversibleprocessT0theconstanttemperatureofthesystemHeattransfertoasystemincreasestheentropyofasystemHeattransferfromasystemdecreasetheentropyofasystem10Specialcase–reversibleisoth11P=const=PsatT=const=TsatAssumptions:reversibleprocess11P=const=PsatT=const=TsatAssu7-2theincreaseofentropyprinciple(熵增原理)Foracycle:

process1-2arbitraryProcess2-1reversibleFromClausiusinequality12=forreversibleprocess>ForirreversibleprocessTheentropychangeofaclosedsystem

DuringanirreversibleprocessisgreaterthantheintegralofδQ/TDuringareversibleprocessisequaltotheintegralofδQ/T7-2theincreaseofentropypr7-2theincreaseofentropyprinciple(熵增原理)Entropychangeofthesystem:Forreversibleprocesses,theentropychange:

RepresenttheentropytransferwithheatQ—heattransferbetweensystemandsurroundings,kJT—absolutetemperatureattheboundary.KForirreversibleprocesses,theentropychange:AlwaysgreaterthantheentropytransferSomeentropyisgeneratedorcreatedTheentropygeneratedduringanirreversibleprocessiscalledentropygeneration(熵產(chǎn))Entropygeneration(熵產(chǎn))：Entropygenerationisalwaysapositiveorzeroquantity.Itsvaluedependsontheprocess,thusitisnotapropertyofasystemIntheabsenceofanyentropytransfer,theentropychangeofasystemisequaltotheentropygeneration.>≥07-2theincreaseofentropypr7-2theincreaseofentropyprinciple(熵增原理)Intheabsenceofanyentropytransfer,theentropychangeofasystemisequaltotheentropygeneration.Foranisolatedsystem,theheattransferiszeroIncreaseofentropyprinciple(孤立系統(tǒng)的熵增原理，簡(jiǎn)稱熵增原理):theentropyofanisolatedsystemduringaprocessalwaysincreaseor,inthelimitingcaseofareversibleprocessremainsconstant.（孤立系統(tǒng)的熵可以增大，或保持不變，但不可能減少）≥0≥0or7-2theincreaseofentropyprSystem+Surroundings=IsolatedsystemEntropychangeofaclosedsystemincludesEntropytransfer(熱熵流)withheattransferEntropygeneration（熵產(chǎn)）-irreversibilitiesEntropychangeofasystemcanbenegativeduringaprocess；butentropygenerationcannot.System+Surroundings=IsolatedRemarksonEntropyProcessescanoccurinacertaindirectiononly,notinanydirection.Aprocessmustproceedinthedirectionthatcomplieswiththeincreaseofentropyprinciple,thatis,Entropyisanonconservedproperty.Entropyisconservedduringtheidealizedreversibleprocessesonlyandincreasesduringallacutalprocesses.Theperformanceofengineeringsystemsisdegradedbythepresenceofirreversibilities,andentropygenerationisameasureofthemagnitudesoftheirreversibilitiespresentduringthatprocess.Thegreatertheextentofirreversibilities,thegreatertheentropygeneration.Entropygenerationcanbeusedasaquantitativemeasureofirreversibilitiesassociatedwithaprocess.Itisalsousedtoestablishcriteriafortheperformanceofengineeringdevices.RemarksonEntropya)haslargerSgen,soitismoreirreversiblea)haslargerSgen,soitis7-3Entropychangeofpuresubstances(tables)Referencepoint:Entropyofsaturatedliquidwater:sfat0.01℃=0kJ/kg.k(TableA-4)7-3EntropychangeofpuresubSuperheatedSat.MixSuperheatedSat.Mix7-4Isentropicprocesses(等熵過(guò)程)Isentropicprocessisaprocessduringwhichtheentropyremainsconstant.theentropyofafixedmasscanbechangedby1)heattransfer,(Adiabatic)2)irreversibilities(reversible)Areversibleadiabaticprocessisanisentropicprocess.(可逆絕熱過(guò)程是等熵過(guò)程)Manyengineeringsystemsordevicessuchaspumps,turbines,nozzles,anddiffusersareessentiallyadiabaticintheiroperation.

AnisentropicprocesscanserveasanappropriatemodelforactualprocessesAnisentropicprocessenableustodefinetheefficiencyforaprocesstocomparetheactualperformanceofthesedevicesasidealizedconditions.7-4Isentropicprocesses(等熵過(guò)程工程熱力學(xué)(英文版)第7單元課件7-5Propertydiagramsinvolvingentropy(T-S)(H-S)Temperature-Entropy(T-S)diagramTheareaundertheprocesscurveonaT-Sdiagramrepresentsheattransferduringareversibleprocess.TheareaundertheprocesscurveonaP-Vdiagramrepresentstheboundaryworkforareversibleprocess.

Theareahasnomeaningforirreversibleprocesses.7-5PropertydiagramsinvolvinSpecialcasesreversibleisothermalprocessIsentropicprocess:ds=0;Q=0VerticallikeonaT-sdiagram12s1s2Specialcases12s1s2Enthalpy-Entropy(h-s)diagramCommonlyusedintheanalysisofsteady-flowdevicessuchasturbines,compressors,andnozzles.Enthalpy,h:aprimarypropertyin1stlawanalysisofsteady-flowdevicesEntropy,s:thepropertyaccountsforirreversibilitiesduringadiabaticprocessesE.g:steadyflowofsteamthroughanadiabaticturbine:Verticaldistance△h:isameasureoftheworkoutputoftheturbine;Horizontaldistance△s:isameasureoftheirreversibilitiesassociatedwiththeprocessEnthalpy-Entropy(h-s)diagram工程熱力學(xué)(英文版)第7單元課件7-6whatisentropy?Thirdlawofthermodynamics:Theentropyofapurecrystallinesubstanceatabsolutezerotemperatureiszerosincethereisnouncertaintyaboutthestateofthemoleculesatthatinstant.Thereisnoentropytransferassociatedwithenergytransferaswork.Workisfreeofentropy.Thequantityofenergyisalwayspreservedduringanactualprocess(the1stlaw);Butthequalityisboundtodecrease(the2ndlaw).Thisdicreaseinqualityisalwaysaccompaniedbyanincreaseinentropy.Processescanoccuronlyinthedirectionofincreasedoverallentropy.7-6whatisentropy?Thirdlaw7-7TheTdsrelationsEnergybalanceofareversibleprocessinaclosedstationarysystem.Thesetworesults(7-23and7-24)arevalidforbothreversibleandirreversibleprocesses.(Entropyisindependentofprocess.)Theyareapplicablewhetherthechangeofstateoccursinaclosedoranopensystem.7-7TheTdsrelationsEnergyb7-7TheTdsrelationsCalculationof△S:

IntegrationsoftheequationsTabulateddata7-7TheTdsrelationsCalculat7-10reversiblesteady-flowworkEnergybalanceforasteadyflowdeviceundergoinganreversibleprocessForincompressiblefluid,specificvolumeisconstant7-10reversiblesteady-flowwoNeglectchangesinKEandPE,wegetthereversibleworkoutputassociatedwithreversibleprocessinsteadyflowdeviceFromthisequation,wecanseethat:thelargerthespecificvolume,thelargerthereversibleworkproducedorconsumedbythesteady-flowdevice.Forcompressionprocess:keepthespecificvolumeassmallaspossibleForexpansionprocess,keepthespecificvolumeaslargeraspossible.

NeglectchangesinKEandPE,Insteampowerplants,thepressureriseinthepumporcompressorisequaltothepressuredropintheturbineifwedisregardthepressurelossesinvariousothercomponents.thepumphandlesliquid,whichhasaverysmallspecificvolumetheturbinehandlesvapor,whosespecificvolumeismanytimeslarger.Therefore,theworkoutputoftheturbineismuchlargerthantheworkinputtothepump.Thisisoneofthereasonsforthewide-spreaduseofsteampowerplantsinelectricpowergeneration.Insteampowerplants,thepre工程熱力學(xué)(英文版)第7單元課件7–8ENTROPYCHANGEOFLIQUIDSANDSOLIDS自學(xué)7–9THEENTROPYCHANGEOFIDEALGASES7–11MINIMIZINGTHECOMPRESSORWORK7–12ISENTROPICEFFICIENCIESoFSTEADY-FLOWDEVICES7-13Entropybalance7–8ENTROPYCHANGEOFLIQUIDSChap7Summary34Clausiusinequality:thecyclicintegralofisalways≦zeroIncreaseofEntropyPrinciple(熵增原理)Entropy(熵)1,Entropychangeofpuresubstances:Entropychangeofaclosedsystem:Increaseofentropyprinciple(孤立系統(tǒng)熵增原理，簡(jiǎn)稱熵增原理)：theentropyofanisolatedsystemduringaprocessalwaysincreaseor,inthelimitingcaseofareversibleprocessremainsconstant.(孤立系統(tǒng)的熵可以增大，或保持不變，但不可能減少)

EntropySomeremarksIsoT>ReversibleprocessIrreversibleprocess≥0sfat0.01℃=0kJ/kg.k2,Isentropicprocess0,adibatic0,Reversible3,T-S,h-sdiagrams=4,The3rdlawofthermodaynamics:Theentropyofapurecrystallinesubstanceatabsolutezerotemperatureiszero5,Tdsrelations:6,reversibleworkoutputChap7Summary34ClausiusinequaClausiusinequalityEntropyChange≥0EntropychangeofclosedsystemIncreaseofEntropyPrinciple(isolatedsys)TdsrelationsreversibleworkoutputAreversibleadiabaticprocessisanisentropicprocess熱力學(xué)第二定律的數(shù)學(xué)表達(dá)式ClausiusinequalityEntropyCha367-59C7-60C7-267-31367-59C7-26Chap6Summary-137Whyweneed2ndLaw?Allprocessessatisfy1stLaw;Satisfying1stdoesnotensuretheprocesscanactuallyoccurHeatEnginesRefrigerator,HeatPumpIntroductionto2ndLawRefrigerators/heatpump:ThedevicesdriveheatQtransferfromTLtoTH,ThermalenergyReservoirReceiveheatQHfromahightemperaturesourceTheworkinputtotherefrigerator/heatpumpHeatQLabsorbedfromrefrigeratedspaceTLAprocesshasdirectionEnergyhasqualityandquantityHeatSourceHeatSinkConvertpartQHtoworkWnet,outRejectwasteheatQLtoalowtemperaturesinkHeatengine2ndlaw,Kelvin-PlanckStatement:Itisimpossibleforanydevicethatoperatesonacycletoreceiveheatfromasinglereservoirandproduceanetamountofwork.

Noheatenginecanhaveη=100%HeatQHrejectedtohightemperatureTHRefrigeratorwantsQLHeatpumpwantsQHCOP2ndlaw,ClausiusStatement:Heatdoesnot,ofitsownvolition,transferfromacoldmediumtoawarmerone.(熱不能自發(fā)地、不付代價(jià)地從低溫物體傳到高溫物體)Chap6Summary-11Whyweneed2nChap6Summary-238Aprocesscanbereversedwithoutleavinganytraceonthesurroundings.

CarnotCycleCarnotRefrigeratorCarnotHeatPumpReversibleProcessesTheheatengineoperatesonthereversibleCarnotCycleThebestknownreversiblecycle;fourreversibleprocessesCarnotheatengineCarnotPrinciple1:GivenTLandTH,ηth,irrev<ηth,revSystemSurroundingsInternalRevExternalRevWhyneedRevIrreversible:heattransferIsothermalexpansionIsothermalCompressionAdiabaticcompressionReversedCarnotCycleCarnotrefrigerator/heatpumpCarnotPrinciple2:GivenTLandTH,ηth,allrev=ηth,revCarnotHeatEngineTherefrigerator/heatpumpoperatesonareversedCarnotCycleAdiabaticexpansionChap6Summary-22AprocesscanChap7Summary39Clausiusinequality:thecyclicintegralofisalways≦zeroIncreaseofEntropyPrinciple(熵增原理)Entropy(熵)1,Entropychangeofpuresubstances:Entropychangeofaclosedsystem:Increaseofentropyprinciple(孤立系統(tǒng)熵增原理，簡(jiǎn)稱熵增原理)：theentropyofanisolatedsystemduringaprocessalwaysincreaseor,inthelimitingcaseofareversibleprocessremainsconstant.(孤立系統(tǒng)的熵可以增大，或保持不變，但不可能減少)

EntropySomeremarksIsoT>ReversibleprocessIrreversibleprocess≥0sfat0.01℃=0kJ/kg.k2,Isentropicprocess0,adibatic0,Reversible3,T-S,h-sdiagrams=4,The3rdlawofthermodaynamics:Theentropyofapurecrystallinesubstanceatabsolutezerotemperatureiszero5,Tdsrelations:6,reversibleworkoutputChap7Summary3ClausiusinequalChapter7Entropy40Chapter7Entropy47-1EntropyThe2ndlawofthermodynamicsleadstoexpressionsthatinvolveinequalities.Heatengine:ηth,irrev<ηth,revHeatpump:COPHP,irrev<COPHP,revCOPR,irrev<COPR,rev;Clausiusinequality:Thecyclicintegralofisalwayslessthanorequaltozero.StatedbyR.J.E.Clausius.Validforallcycles,reversibleorirreversibleThecyclicintegralofcanbeviewedasthesumofallthesedifferentialamountsofheattransferdividedbythetemperatureoftheboundary.417-1EntropyThe2ndlawoftheWhy

ConsiderasystemconnectedtoathermalenergyreservoirataconstanttemperatureofTRthroughareversiblecyclicdevice.Thecyclicdevicereceivesheatfromreservoir;suppliesheattothesystem,andproducingworkThesystemproducesworkasaresultofheattransfer，andtheboundarytemperatureisT(variable)Wehaveacombinedsystemasthedashedlinesyields:42ForacycleKelvinPlanckstatement:nosystemcanproduceanetamountofworkwhileoperatinginacycleandexchangingheatwithasinglethermalenergyreservoir.WCcannotbeaworkoutput,cannotbeapositivequantityWhy6ForacycleKelvinPlanckForreversiblecycles:allquantitieshavethesamemagnitudebuttheoppositesign,thus:TheequalityintheClausiusinequalityholdsforreversiblecyclesTheinequalityfortheirreversiblecycles.Forreversiblecycles:allquaEntropy:Cyclicintegraliszero:W,no;Q,no;netchangeinvolume,yes.?Dependsonstateonly,nottheprocesspath.So,itisaproperty,statefunctionIn1865,ClausiusnamedthispropertyasENTROPY,S.theentropychangeisdefinedas?Entropy:?Then,theentropychangeofasystemduringaprocesscanbedeterminedbyintegrationbetweentheinitialandthefinalstates:WeconcernmoreonchangeofS,butnotSitself,sowehaveareferencestate(S=0).TherelationofQandTisoftennotavailable,sowegetentropymostlyfromtables.Entropyisaproperty,isfixedatfixedstates.Theentropychangebetweentwospecifiedstatesisthesamenomatterwhatprocesspathitisfollowed.Thecalculationisonlyvalidforreversiblepathbetweenthetwostates;andusedascomparedvalueforirreversibleprocess.45Then,theentropychangeofaSpecialcase–reversibleisothermalheattransferprocesses.QheattransferduringthereversibleprocessT0theconstanttemperatureofthesystemHeattransfertoasystemincreasestheentropyofasystemHeattransferfromasystemdecreasetheentropyofasystem46Specialcase–reversibleisoth47P=const=PsatT=const=TsatAssumptions:reversibleprocess11P=const=PsatT=const=TsatAssu7-2theincreaseofentropyprinciple(熵增原理)Foracycle:

process1-2arbitraryProcess2-1reversibleFromClausiusinequality48=forreversibleprocess>ForirreversibleprocessTheentropychangeofaclosedsystem

DuringanirreversibleprocessisgreaterthantheintegralofδQ/TDuringareversibleprocessisequaltotheintegralofδQ/T7-2theincreaseofentropypr7-2theincreaseofentropyprinciple(熵增原理)Entropychangeofthesystem:Forreversibleprocesses,theentropychange:

RepresenttheentropytransferwithheatQ—heattransferbetweensystemandsurroundings,kJT—absolutetemperatureattheboundary.KForirreversibleprocesses,theentropychange:AlwaysgreaterthantheentropytransferSomeentropyisgeneratedorcreatedTheentropygeneratedduringanirreversibleprocessiscalledentropygeneration(熵產(chǎn))Entropygeneration(熵產(chǎn))：Entropygenerationisalwaysapositiveorzeroquantity.Itsvaluedependsontheprocess,thusitisnotapropertyofasystemIntheabsenceofanyentropytransfer,theentropychangeofasystemisequaltotheentropygeneration.>≥07-2theincreaseofentropypr7-2theincreaseofentropyprinciple(熵增原理)Intheabsenceofanyentropytransfer,theentropychangeofasystemisequaltotheentropygeneration.Foranisolatedsystem,theheattransferiszeroIncreaseofentropyprinciple(孤立系統(tǒng)的熵增原理，簡(jiǎn)稱熵增原理):theentropyofanisolatedsystemduringaprocessalwaysincreaseor,inthelimitingcaseofareversibleprocessremainsconstant.（孤立系統(tǒng)的熵可以增大，或保持不變，但不可能減少）≥0≥0or7-2theincreaseofentropyprSystem+Surroundings=IsolatedsystemEntropychangeofaclosedsystemincludesEntropytransfer(熱熵流)withheattransferEntropygeneration（熵產(chǎn)）-irreversibilitiesEntropychangeofasystemcanbenegativeduringaprocess；butentropygenerationcannot.System+Surroundings=IsolatedRemarksonEntropyProcessescanoccurinacertaindirectiononly,notinanydirection.Aprocessmustproceedinthedirectionthatcomplieswiththeincreaseofentropyprinciple,thatis,Entropyisanonconservedproperty.Entropyisconservedduringtheidealizedreversibleprocessesonlyandincreasesduringallacutalprocesses.Theperformanceofengineeringsystemsisdegradedbythepresenceofirreversibilities,andentropygenerationisameasureofthemagnitudesoftheirreversibilitiespresentduringthatprocess.Thegreatertheextentofirreversibilities,thegreatertheentropygeneration.Entropygenerationcanbeusedasaquantitativemeasureofirreversibilitiesassociatedwithaprocess.Itisalsousedtoestablishcriteriafortheperformanceofengineeringdevices.RemarksonEntropya)haslargerSgen,soitismoreirreversiblea)haslargerSgen,soitis7-3Entropychangeofpuresubstances(tables)Referencepoint:Entropyofsaturatedliquidwater:sfat0.01℃=0kJ/kg.k(TableA-4)7-3EntropychangeofpuresubSuperheatedSat.MixSuperheatedSat.Mix7-4Isentropicprocesses(等熵過(guò)程)Isentropicprocessisaprocessduringwhichtheentropyremainsconstant.theentropyofafixedmasscanbechangedby1)heattransfer,(Adiabatic)2)irreversibilities(reversible)Areversibleadiabaticprocessisanisentropicprocess.(可逆絕熱過(guò)程是等熵過(guò)程)Manyengineeringsystemsordevicessuchaspumps,turbines,nozzles,anddiffusersareessentiallyadiabaticintheiroperation.

AnisentropicprocesscanserveasanappropriatemodelforactualprocessesAnisentropicprocessenableustodefinetheefficiencyforaprocesstocomparetheactualperformanceofthesedevicesasidealizedconditions.7-4Isentropicprocesses(等熵過(guò)程工程熱力學(xué)(英文版)第7單元課件7-5Propertydiagramsinvolvingentropy(T-S)(H-S)Temperature-Entropy(T-S)diagramTheareaundertheprocesscurveonaT-Sdiagramrepresentsheattransferduringareversibleprocess.TheareaundertheprocesscurveonaP-Vdiagramrepresentstheboundaryworkforareversibleprocess.

Theareahasnomeaningforirreversibleprocesses.7-5PropertydiagramsinvolvinSpecialcasesreversibleisothermalprocessIsentropicprocess:ds=0;Q=0VerticallikeonaT-sdiagram12s1s2Specialcases12s1s2Enthalpy-Entropy(h-s)diagramCommonlyusedintheanalysisofsteady-flowdevicessuchasturbines,compressors,andnozzles.Enthalpy,h:aprimarypropertyin1stlawanalysisofsteady-flowdevicesEntropy,s:thepropertyaccountsforirreversibilitiesduringadiabaticprocessesE.g:steadyflowofsteamthroughanadiabaticturbine:Verticaldistance△h:isameasureoftheworkoutputoftheturbine;Horizontaldistance△s:isameasureoftheirreversibilitiesassociatedwiththeprocessEnthalpy-Entropy(h-s)diagram工程熱力學(xué)(英文版)第7單元課件7-6whatisentropy?Thirdlawofthermodynamics:Theentropyofapurecrystallinesubstanceatabsolutezerotemperatureiszerosincethereisnouncertaintyaboutthestateofthemoleculesatthatinstant.Thereisnoentropytransferassociatedwithenergytransferaswork.Workisfreeofentropy.Thequantityofenergyisalwayspreservedduringanactualprocess(the1stlaw);Butthequalityisboundtodecrease(the2ndlaw).Thisdicreaseinqualityisalwaysaccompaniedbyanincreaseinentropy.Processescanoccuronlyinthedirectionofincreasedoverallentropy.7-6whatisentropy?Thirdlaw7-7TheTdsrelationsEnergybalanceofareversibleprocessinaclosedstationarysystem.Thesetworesults(7-23and7-24)arevalidforbothreversibleandirreversibleprocesses.(