工程热力学(英文版)第六单元课件

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

Noheatenginecanhaveη=100%HeatQHrejectedtohightemperatureTHRefrigeratorwantsQLHeatpumpwantsQHCOP2ndlaw,ClausiusStatement:Heatdoesnot,ofitsownvolition,transferfromacoldmediumtoawarmerone.(热不能自发地、不付代价地从低温物体传到高温物体)Chap6Summary-11Whyweneed2n1Chap6Summary-22Aprocesscanbereversedwithoutleavinganytraceonthesurroundings.

CarnotCycleCarnotRefrigeratorCarnotHeatPumpReversibleProcessesTheheatengineoperatesonthereversibleCarnotCycleThebestknownreversiblecycle;fourreversibleprocessesCarnotheatengineCarnotPrinciple1:GivenTLandTH,ηth,irrev<ηth,revSystemSurroundingsInternalRevExternalRevWhyneedRevIrreversible:heattransferIsothermalexpansionIsothermalCompressionAdiabaticcompressionReversedCarnotCycleCarnotrefrigerator/heatpumpCarnotPrinciple2:GivenTLandTH,ηth,allrev=ηth,revCarnotHeatEngineTherefrigerator/heatpumpoperatesonareversibleCarnotCycleAdiabaticexpansionChap6Summary-22Aprocesscan26-6reversibleandirreversibleprocessesThesecondlawofthermodynamicsstatesthat:noheatenginecanhaveanefficiencyof100percent.???Whatisthehighestefficiencythataheatenginecanhave???Toanswerthisquestion,weneedtodefineanidealizedprocessReversibleprocess.36-6reversibleandirreversibl36-6reversibleandirreversibleprocessesAreversibleprocess(可逆过程)isdefinedasaprocessthatcanbereversedwithoutleavinganytraceonthesurroundings.

SystemreturnedtoinitialstatesSurroundingsreturnedtoinitialstatesThisispossibleonlyifthenetheatandnetworkexchangebetweenthesystemandthesurroundingsiszeroforthecombined(originalandreverse)processProcessesthatarenotreversiblecalledirreversibleprocesses.46-6reversibleandirreversibl46-6reversibleandirreversibleprocessesItshouldbepointedoutthat:Asystemcanberestoredtoitsinitialstatefollowingaprocess,regardlessofwhethertheprocessisreversibleorirreversible.Butforreversibleprocesses:thisrestorationismadewithoutleavinganynetchangeonthesurroundings.Whileforirreversibleprocesses:thesurroundingsusuallydosomeworkonthesystemandthereforedoesnotreturntotheiroriginalstate.

56-6reversibleandirreversibl56-6reversibleandirreversibleprocessesReversibleprocessesInternallyreversible(内部可逆):noirreversibilitiesoccurwithintheboundaryofthesystemduringtheprocess.Likethequasi-equilibriumprocessisanexample.

Externallyreversible(外部可逆):noirreversibilitiesoccuroutsidethesystemboundaryduringtheprocess.66-6reversibleandirreversibl66-6reversibleandirreversibleprocessesReversibleprocessesdonotoccurinnature.Theyareidealizationsofactualprocesses.Alltheprocessesoccurringinnatureareirreversible.Thepossibilityoffindingaperfectmateisnohigherthanthepossibilityoffindingaperfect(reversible)process.Butweneedreversibleprocesses:Easytoanalyze:aseriousofequilibriumstatesduringaprocessActasidealizedmodels(theoreticallimits)tobecomparedbyactualprocesses.76-6reversibleandirreversibl76-7TheCarnotCycle(卡诺循环)Carnotcycle(卡诺循环)isthebestknownreversiblecycle.Composedoffourreversibleprocesses:TwoisothermalprocessesTwoadiabaticprocessesFirstproposedin1824bySadiCarnotCarnotcyclecanbeexecutedeitherinaclosedorasteadyflowsystem.ThetheoreticalheatengineoperatesonaCarnotcycleisCarnotheatengine（卡诺热机）.86-7TheCarnotCycle(卡诺循环)Car86-7TheCarnotCycle(卡诺循环)Consideraclosedsystemthatconsistsofagascontainedinanadiabaticpiston-cylinderdevice.Thefourreversibleprocessesare:9Process1-2:isothermalexpansion(等温膨胀)Process2-3:adiabaticexpansion(绝热膨胀)Process3-4:isothermalcompression(等温压缩)Process4-1:adiabaticcompression(绝热压缩)6-7TheCarnotCycle(卡诺循环)Con96-7TheCarnotCycle(卡诺循环)Reversibleisothermalexpansion(可逆等温膨胀)：Process1-2:isothermalexpansion(等温膨胀)TH=constantGasexpandsslowly,doingworkonsurroundings,TisgoingtodecreaseQHissuppliedtomaintainTH=constant(定温吸热)106-7TheCarnotCycle(卡诺循环)Rev106-7TheCarnotCycle(卡诺循环)Reversibleadiabaticexpansion(可逆绝热膨胀)：Process2-3:adiabaticexpansion(绝热膨胀)Adiabatic△Q=0Gasexpandsslowly,doingworkonthesurroundings.TemperaturedropsfromTHtoTL116-7TheCarnotCycle(卡诺循环)Rev116-7TheCarnotCycle(卡诺循环)Reversibleisothermalcompression(可逆等温压缩)

Process3-4:isothermalcompression(等温压缩)TL=constantPistonisdoingworkongas,gasiscompressed,TLisgoingtoincreaseQLisrejectedtomaintainTL=const(定温吸热)126-7TheCarnotCycle(卡诺循环)Rev126-7TheCarnotCycle(卡诺循环)Reversibleadiabaticcompression(可逆绝热压缩)Process4-1:adiabaticcompression(绝热压缩)Adiabatic△Q=0Pistonisdoingworkongas,gasiscompressed.TemperatureincreasesfromTLtoTH,CompletetheCarnotcycle136-7TheCarnotCycle(卡诺循环)Rev136-7TheCarnotCycle(卡诺循环)CarnotcycleinP-VdiagramProcess1-2:isothermalexpansion(等温膨胀)(定温吸热)Process2-3:adiabaticexpansion(绝热膨胀)Process3-4:isothermalcompression(等温压缩)(定温放热)Process4-1:adiabaticcompression(绝热压缩)14Areaunder1-2-3istheworkdonebythegasduringtheexpansionpartofthecycleAreaunder3-4-1istheworkdoneonthegasduringthecompressionpartofthecycle.Areaenclosed1-2-3-4-1isthenewworkdoneduringthiscycle.6-7TheCarnotCycle(卡诺循环)Car146-7TheCarnotCycle(卡诺循环)ReversedCarnotcycle(逆向卡诺循环):allthefourprocessesofCarnotcyclearereversed.CalledalsoCarnotrefrigerationcycle（卡诺制冷循环）QLisabsorbedfromTL;QHisrejectedtoTH,withaworkinputWnet,in.156-7TheCarnotCycle(卡诺循环)Rev156-8TheCarnotPrinciples(卡诺定理)CarnotPrinciples(卡诺定理)Efficiencyofanirreversibleheatengineisalwayslessthantheefficiencyofareversibleoneoperatingbetweenthesametworeservoirs（在温度同为TH的热源和同为TL的冷源间工作的一切不可逆循环，其热效率必小于可逆循环）.

Theefficienciesofallreversibleheatenginesoperatingbetweenthesametworeservoirsarethesame.(在相同温度高温热源和相同温度的低温热源之间工作的一切可逆循环，其热效率都相等。)166-8TheCarnotPrinciples(卡诺定166-9thethermodynamictemperaturescale自学176-9thethermodynamictemperat176-10TheCarnotHeatEngine(卡诺热机)ThehypotheticalheatenginethatoperatesonthereversibleCarnotcycleiscalledCarnotheatengine.

ThermalefficiencyofCarnotheatengine:Wnet,outQHηth=QH-QLQH=QLQHHeatsources,THHeatsink,TLQHQLWnet,outCarnotHeatenginesηth=1-186-10TheCarnotHeatEngine(卡诺186-10TheCarnotHeatEngine(卡诺热机)Process1-2:isothermalexpansion(等温膨胀)，定温吸热QH,TH,V1V2Process2-3:adiabaticexpansion(绝热膨胀)△Q=0,V2V3,THTLProcess3-4:isothermalcompression(等温压缩)，定温放热QL,

TL,V3V4Process4-1:adiabaticcompression(绝热压缩)△Q=0,V4V1,TLTHQLQHTLTH=1-ηth,rev=1-TLTHAbsolutetemperatures,K推导196-10TheCarnotHeatEngine(卡诺196-10TheCarnotHeatEngine(卡诺热机)TheefficiencyofaCarnotengineisonlyrelatedtoTL,andTH.ηth,rev=

1-TL/TH;

IncreaseTH,ordecreaseTLcanincreaseηth,revForanyreversibleheatengine,ηth,rev=

1-TL/TH;

ηth,rev=

1-TL/TH

isthehighestefficiency,aheatengineoperatingbetweenthetwothermalenergyreservoirsattemperatureTL

andTH

canhave.ηth,rev=

1-TL/THisalwayslessthan1.TL=TH,ηth=0.(2ndlawofthermodynamics)AllirreversibleheatenginesoperatingbetweenTL

andTH

haveefficiency<ηth,revNoactualheatenginecanhaveefficiency>ηth,rev206-10TheCarnotHeatEngine(卡诺206-10TheCarnotHeatEngine(卡诺热机)216-10TheCarnotHeatEngine(卡诺216-10TheCarnotHeatEngine(卡诺热机)226-10TheCarnotHeatEngine(卡诺222323236-11TheCarnotrefrigeratorandheatpumpArefrigeratororaheatpumpthatoperatesonthereversedCarnotcycleiscalledCarnotrefrigerator,oraCarnotheatpump.Thecoefficientofperformance(COP):COPR,rev

=QH/QL-11COPHP,rev

=1-QL/QH1=TH/TL-111-TL/TH1=246-11TheCarnotrefrigeratora246-11TheCarnotrefrigeratorandheatpumpCOPR,rev

andCOPHP,rev

arethehighestCOPthatarefrigeratororaheatpumpoperatingbetweenTLandTHcanhave.AllactualrefrigeratorsorheatpumpsoperatingbetweenTLandTHhavelowerCOPthanCOPR,rev

orCOPHP,rev

NoactualrefrigeratorsorheatpumpsoperatingbetweenTLandTHcanhavehigherCOPthanCOPR,rev

orCOPHP,rev

COPsofrefrigeratororheatpumpdecreaseasTLdecrease256-11TheCarnotrefrigeratora256-11Thecarnotrefrigeratorandheatpump266-11Thecarnotrefrigeratora26272727Chap6Summary-228Aprocesscanbereversedwithoutleavinganytraceonthesurroundings.

CarnotCycleCarnotRefrigeratorCarnotHeatPumpReversibleProcessesTheheatengineoperatesonthereversibleCarnotCycleThebestknownreversiblecycle;fourreversibleprocessesCarnotheatengineCarnotPrinciple1:GivenTLandTH,ηth,irrev<ηth,revSystemSurroundingsInternalRevExternalRevWhyneedRevIrreversible:heattransferIsothermalexpansionIsothermalCompressionAdiabaticcompressionReversedCarnotCycleCarnotrefrigerator/heatpumpCarnotPrinciple2:GivenTLandTH,ηth,allrev=ηth,revCarnotHeatEngineTherefrigerator/heatpumpoperatesonareversibleCarnotCycleAdiabaticexpansionChap6Summary-228Aprocesscan28296-716-128296-7129Chap6Summary-130Whyweneed2ndLaw?Allprocessessatisfy1stLaw;Satisfying1stdoesnotensuretheprocesscanactuallyoccurHeatEnginesRefrigerator,HeatPumpIntroductionto2ndLawRefrigerators/heatpump:ThedevicesdriveheatQtransferfromTLtoTH,ThermalenergyReservoirReceiveheatQHfromahightemperaturesourceTheworkinputtotherefrigerator/heatpumpHeatQLabsorbedfromrefrigeratedspaceTLAprocesshasdirectionEnergyhasqualityandquantityHeatSourceHeatSinkConvertpartQHtoworkWnet,outRejectwasteheatQLtoalowtemperaturesinkHeatengine2ndlaw,Kelvin-PlanckStatement:Itisimpossibleforanydevicethatoperatesonacycletoreceiveheatfromasinglereservoirandproduceanetamountofwork.

Noheatenginecanhaveη=100%HeatQHrejectedtohightemperatureTHRefrigeratorwantsQLHeatpumpwantsQHCOP2ndlaw,ClausiusStatement:Heatdoesnot,ofitsownvolition,transferfromacoldmediumtoawarmerone.(热不能自发地、不付代价地从低温物体传到高温物体)Chap6Summary-11Whyweneed2n30Chap6Summary-231Aprocesscanbereversedwithoutleavinganytraceonthesurroundings.

CarnotCycleCarnotRefrigeratorCarnotHeatPumpReversibleProcessesTheheatengineoperatesonthereversibleCarnotCycleThebestknownreversiblecycle;fourreversibleprocessesCarnotheatengineCarnotPrinciple1:GivenTLandTH,ηth,irrev<ηth,revSystemSurroundingsInternalRevExternalRevWhyneedRevIrreversible:heattransferIsothermalexpansionIsothermalCompressionAdiabaticcompressionReversedCarnotCycleCarnotrefrigerator/heatpumpCarnotPrinciple2:GivenTLandTH,ηth,allrev=ηth,revCarnotHeatEngineTherefrigerator/heatpumpoperatesonareversibleCarnotCycleAdiabaticexpansionChap6Summary-22Aprocesscan316-6reversibleandirreversibleprocessesThesecondlawofthermodynamicsstatesthat:noheatenginecanhaveanefficiencyof100percent.???Whatisthehighestefficiencythataheatenginecanhave???Toanswerthisquestion,weneedtodefineanidealizedprocessReversibleprocess.326-6reversibleandirreversibl326-6reversibleandirreversibleprocessesAreversibleprocess(可逆过程)isdefinedasaprocessthatcanbereversedwithoutleavinganytraceonthesurroundings.

SystemreturnedtoinitialstatesSurroundingsreturnedtoinitialstatesThisispossibleonlyifthenetheatandnetworkexchangebetweenthesystemandthesurroundingsiszeroforthecombined(originalandreverse)processProcessesthatarenotreversiblecalledirreversibleprocesses.336-6reversibleandirreversibl336-6reversibleandirreversibleprocessesItshouldbepointedoutthat:Asystemcanberestoredtoitsinitialstatefollowingaprocess,regardlessofwhethertheprocessisreversibleorirreversible.Butforreversibleprocesses:thisrestorationismadewithoutleavinganynetchangeonthesurroundings.Whileforirreversibleprocesses:thesurroundingsusuallydosomeworkonthesystemandthereforedoesnotreturntotheiroriginalstate.

346-6reversibleandirreversibl346-6reversibleandirreversibleprocessesReversibleprocessesInternallyreversible(内部可逆):noirreversibilitiesoccurwithintheboundaryofthesystemduringtheprocess.Likethequasi-equilibriumprocessisanexample.

Externallyreversible(外部可逆):noirreversibilitiesoccuroutsidethesystemboundaryduringtheprocess.356-6reversibleandirreversibl356-6reversibleandirreversibleprocessesReversibleprocessesdonotoccurinnature.Theyareidealizationsofactualprocesses.Alltheprocessesoccurringinnatureareirreversible.Thepossibilityoffindingaperfectmateisnohigherthanthepossibilityoffindingaperfect(reversible)process.Butweneedreversibleprocesses:Easytoanalyze:aseriousofequilibriumstatesduringaprocessActasidealizedmodels(theoreticallimits)tobecomparedbyactualprocesses.366-6reversibleandirreversibl366-7TheCarnotCycle(卡诺循环)Carnotcycle(卡诺循环)isthebestknownreversiblecycle.Composedoffourreversibleprocesses:TwoisothermalprocessesTwoadiabaticprocessesFirstproposedin1824bySadiCarnotCarnotcyclecanbeexecutedeitherinaclosedorasteadyflowsystem.ThetheoreticalheatengineoperatesonaCarnotcycleisCarnotheatengine（卡诺热机）.376-7TheCarnotCycle(卡诺循环)Car376-7TheCarnotCycle(卡诺循环)Consideraclosedsystemthatconsistsofagascontainedinanadiabaticpiston-cylinderdevice.Thefourreversibleprocessesare:38Process1-2:isothermalexpansion(等温膨胀)Process2-3:adiabaticexpansion(绝热膨胀)Process3-4:isothermalcompression(等温压缩)Process4-1:adiabaticcompression(绝热压缩)6-7TheCarnotCycle(卡诺循环)Con386-7TheCarnotCycle(卡诺循环)Reversibleisothermalexpansion(可逆等温膨胀)：Process1-2:isothermalexpansion(等温膨胀)TH=constantGasexpandsslowly,doingworkonsurroundings,TisgoingtodecreaseQHissuppliedtomaintainTH=constant(定温吸热)396-7TheCarnotCycle(卡诺循环)Rev396-7TheCarnotCycle(卡诺循环)Reversibleadiabaticexpansion(可逆绝热膨胀)：Process2-3:adiabaticexpansion(绝热膨胀)Adiabatic△Q=0Gasexpandsslowly,doingworkonthesurroundings.TemperaturedropsfromTHtoTL406-7TheCarnotCycle(卡诺循环)Rev406-7TheCarnotCycle(卡诺循环)Reversibleisothermalcompression(可逆等温压缩)

Process3-4:isothermalcompression(等温压缩)TL=constantPistonisdoingworkongas,gasiscompressed,TLisgoingtoincreaseQLisrejectedtomaintainTL=const(定温吸热)416-7TheCarnotCycle(卡诺循环)Rev416-7TheCarnotCycle(卡诺循环)Reversibleadiabaticcompression(可逆绝热压缩)Process4-1:adiabaticcompression(绝热压缩)Adiabatic△Q=0Pistonisdoingworkongas,gasiscompressed.TemperatureincreasesfromTLtoTH,CompletetheCarnotcycle426-7TheCarnotCycle(卡诺循环)Rev426-7TheCarnotCycle(卡诺循环)CarnotcycleinP-VdiagramProcess1-2:isothermalexpansion(等温膨胀)(定温吸热)Process2-3:adiabaticexpansion(绝热膨胀)Process3-4:isothermalcompression(等温压缩)(定温放热)Process4-1:adiabaticcompression(绝热压缩)43Areaunder1-2-3istheworkdonebythegasduringtheexpansionpartofthecycleAreaunder3-4-1istheworkdoneonthegasduringthecompressionpartofthecycle.Areaenclosed1-2-3-4-1isthenewworkdoneduringthiscycle.6-7TheCarnotCycle(卡诺循环)Car436-7TheCarnotCycle(卡诺循环)ReversedCarnotcycle(逆向卡诺循环):allthefourprocessesofCarnotcyclearereversed.CalledalsoCarnotrefrigerationcycle（卡诺制冷循环）QLisabsorbedfromTL;QHisrejectedtoTH,withaworkinputWnet,in.446-7TheCarnotCycle(卡诺循环)Rev446-8TheCarnotPrinciples(卡诺定理)CarnotPrinciples(卡诺定理)Efficiencyofanirreversibleheatengineisalwayslessthantheefficiencyofareversibleoneoperatingbetweenthesametworeservoirs（在温度同为TH的热源和同为TL的冷源间工作的一切不可逆循环，其热效率必小于可逆循环）.

Theefficienciesofallreversibleheatenginesoperatingbetweenthesametworeservoirsarethesame.(在相同温度高温热源和相同温度的低温热源之间工作的一切可逆循环，其热效率都相等。)456-8TheCarnotPrinciples(卡诺定456-9thethermodynamictemperaturescale自学466-9thethermodynamictemperat466-10TheCarnotHeatEngine(卡诺热机)ThehypotheticalheatenginethatoperatesonthereversibleCarnotcycleiscalledCarnotheatengine.

ThermalefficiencyofCarnotheatengine:Wnet,outQHηth=QH-QLQH=QLQHHeatsources,THHeatsink,TLQHQLWnet,outCarnotHeatenginesηth=1-476-10TheCarnotHeatEngine(卡诺476-10TheCarnotHeatEngine(卡诺热机)Process1-2:isothermalexpansion(等温膨胀)，定温吸热QH,TH,V1V2Process2-3:adiabaticexpansion(绝热膨胀)△Q=0,V2V3,THTLProcess3-4:isothermalcompression(等温压缩)，定温放热QL,

TL,V3V4Process4-1:adiabaticcompression(绝热压缩)△Q=0,V4V1,TLTHQLQHTLTH=1-ηth,rev=1-TLTHAbsolutetemperatures,K推导486-10TheCarnotHeatEngine(卡诺486-10TheCarnotHeatEngine(卡诺热机)TheefficiencyofaCarnotengineisonlyrelatedtoTL,andTH.ηth,rev=

1-TL/TH;

IncreaseTH,ordecreaseTLcanincreaseηth,revForanyreversibleheatengine,ηth,rev=

1-TL/TH;

ηth,rev=

1-TL/TH

isthehighestefficiency,aheatengineoperatingbetweenthetwothermalenergyreservoirsattemperatureTL

andTH

canhave.ηth,rev=

1-TL/THisalwayslessthan1.TL=TH,