工程熱力學(xué)(英文版)第六單元課件

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-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(可逆過程)isdefinedasaprocessthatcanbereversedwithoutleavinganytraceonthesurroundings.

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

56-6reversibleandirreversibl56-6reversibleandirreversibleprocessesReversibleprocessesInternallyreversible(內(nèi)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(卡諾循環(huán))Carnotcycle(卡諾循環(huán))isthebestknownreversiblecycle.Composedoffourreversibleprocesses:TwoisothermalprocessesTwoadiabaticprocessesFirstproposedin1824bySadiCarnotCarnotcyclecanbeexecutedeitherinaclosedorasteadyflowsystem.ThetheoreticalheatengineoperatesonaCarnotcycleisCarnotheatengine（卡諾熱機(jī)）.86-7TheCarnotCycle(卡諾循環(huán))Car86-7TheCarnotCycle(卡諾循環(huán))Consideraclosedsystemthatconsistsofagascontainedinanadiabaticpiston-cylinderdevice.Thefourreversibleprocessesare:9Process1-2:isothermalexpansion(等溫膨脹)Process2-3:adiabaticexpansion(絕熱膨脹)Process3-4:isothermalcompression(等溫壓縮)Process4-1:adiabaticcompression(絕熱壓縮)6-7TheCarnotCycle(卡諾循環(huán))Con96-7TheCarnotCycle(卡諾循環(huán))Reversibleisothermalexpansion(可逆等溫膨脹)：Process1-2:isothermalexpansion(等溫膨脹)TH=constantGasexpandsslowly,doingworkonsurroundings,TisgoingtodecreaseQHissuppliedtomaintainTH=constant(定溫吸熱)106-7TheCarnotCycle(卡諾循環(huán))Rev106-7TheCarnotCycle(卡諾循環(huán))Reversibleadiabaticexpansion(可逆絕熱膨脹)：Process2-3:adiabaticexpansion(絕熱膨脹)Adiabatic△Q=0Gasexpandsslowly,doingworkonthesurroundings.TemperaturedropsfromTHtoTL116-7TheCarnotCycle(卡諾循環(huán))Rev116-7TheCarnotCycle(卡諾循環(huán))Reversibleisothermalcompression(可逆等溫壓縮)

Process3-4:isothermalcompression(等溫壓縮)TL=constantPistonisdoingworkongas,gasiscompressed,TLisgoingtoincreaseQLisrejectedtomaintainTL=const(定溫吸熱)126-7TheCarnotCycle(卡諾循環(huán))Rev126-7TheCarnotCycle(卡諾循環(huán))Reversibleadiabaticcompression(可逆絕熱壓縮)Process4-1:adiabaticcompression(絕熱壓縮)Adiabatic△Q=0Pistonisdoingworkongas,gasiscompressed.TemperatureincreasesfromTLtoTH,CompletetheCarnotcycle136-7TheCarnotCycle(卡諾循環(huán))Rev136-7TheCarnotCycle(卡諾循環(huán))CarnotcycleinP-VdiagramProcess1-2:isothermalexpansion(等溫膨脹)(定溫吸熱)Process2-3:adiabaticexpansion(絕熱膨脹)Process3-4:isothermalcompression(等溫壓縮)(定溫放熱)Process4-1:adiabaticcompression(絕熱壓縮)14Areaunder1-2-3istheworkdonebythegasduringtheexpansionpartofthecycleAreaunder3-4-1istheworkdoneonthegasduringthecompressionpartofthecycle.Areaenclosed1-2-3-4-1isthenewworkdoneduringthiscycle.6-7TheCarnotCycle(卡諾循環(huán))Car146-7TheCarnotCycle(卡諾循環(huán))ReversedCarnotcycle(逆向卡諾循環(huán)):allthefourprocessesofCarnotcyclearereversed.CalledalsoCarnotrefrigerationcycle（卡諾制冷循環(huán)）QLisabsorbedfromTL;QHisrejectedtoTH,withaworkinputWnet,in.156-7TheCarnotCycle(卡諾循環(huán))Rev156-8TheCarnotPrinciples(卡諾定理)CarnotPrinciples(卡諾定理)Efficiencyofanirreversibleheatengineisalwayslessthantheefficiencyofareversibleoneoperatingbetweenthesametworeservoirs（在溫度同為TH的熱源和同為TL的冷源間工作的一切不可逆循環(huán)，其熱效率必小于可逆循環(huán)）.

Theefficienciesofallreversibleheatenginesoperatingbetweenthesametworeservoirsarethesame.(在相同溫度高溫?zé)嵩春拖嗤瑴囟鹊牡蜏責(zé)嵩粗g工作的一切可逆循環(huán)，其熱效率都相等。)166-8TheCarnotPrinciples(卡諾定166-9thethermodynamictemperaturescale自學(xué)176-9thethermodynamictemperat176-10TheCarnotHeatEngine(卡諾熱機(jī))ThehypotheticalheatenginethatoperatesonthereversibleCarnotcycleiscalledCarnotheatengine.

ThermalefficiencyofCarnotheatengine:Wnet,outQHηth=QH-QLQH=QLQHHeatsources,THHeatsink,TLQHQLWnet,outCarnotHeatenginesηth=1-186-10TheCarnotHeatEngine(卡諾186-10TheCarnotHeatEngine(卡諾熱機(jī))Process1-2:isothermalexpansion(等溫膨脹)，定溫吸熱QH,TH,V1V2Process2-3:adiabaticexpansion(絕熱膨脹)△Q=0,V2V3,THTLProcess3-4:isothermalcompression(等溫壓縮)，定溫放熱QL,

TL,V3V4Process4-1:adiabaticcompression(絕熱壓縮)△Q=0,V4V1,TLTHQLQHTLTH=1-ηth,rev=1-TLTHAbsolutetemperatures,K推導(dǎo)196-10TheCarnotHeatEngine(卡諾196-10TheCarnotHeatEngine(卡諾熱機(jī))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(卡諾206-10TheCarnotHeatEngine(卡諾熱機(jī))216-10TheCarnotHeatEngine(卡諾216-10TheCarnotHeatEngine(卡諾熱機(jī))226-10TheCarnotHeatEngine(卡諾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.(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-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(可逆過程)isdefinedasaprocessthatcanbereversedwithoutleavinganytraceonthesurroundings.

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

346-6reversibleandirreversibl346-6reversibleandirreversibleprocessesReversibleprocessesInternallyreversible(內(nèi)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(卡諾循環(huán))Carnotcycle(卡諾循環(huán))isthebestknownreversiblecycle.Composedoffourreversibleprocesses:TwoisothermalprocessesTwoadiabaticprocessesFirstproposedin1824bySadiCarnotCarnotcyclecanbeexecutedeitherinaclosedorasteadyflowsystem.ThetheoreticalheatengineoperatesonaCarnotcycleisCarnotheatengine（卡諾熱機(jī)）.376-7TheCarnotCycle(卡諾循環(huán))Car376-7TheCarnotCycle(卡諾循環(huán))Consideraclosedsystemthatconsistsofagascontainedinanadiabaticpiston-cylinderdevice.Thefourreversibleprocessesare:38Process1-2:isothermalexpansion(等溫膨脹)Process2-3:adiabaticexpansion(絕熱膨脹)Process3-4:isothermalcompression(等溫壓縮)Process4-1:adiabaticcompression(絕熱壓縮)6-7TheCarnotCycle(卡諾循環(huán))Con386-7TheCarnotCycle(卡諾循環(huán))Reversibleisothermalexpansion(可逆等溫膨脹)：Process1-2:isothermalexpansion(等溫膨脹)TH=constantGasexpandsslowly,doingworkonsurroundings,TisgoingtodecreaseQHissuppliedtomaintainTH=constant(定溫吸熱)396-7TheCarnotCycle(卡諾循環(huán))Rev396-7TheCarnotCycle(卡諾循環(huán))Reversibleadiabaticexpansion(可逆絕熱膨脹)：Process2-3:adiabaticexpansion(絕熱膨脹)Adiabatic△Q=0Gasexpandsslowly,doingworkonthesurroundings.TemperaturedropsfromTHtoTL406-7TheCarnotCycle(卡諾循環(huán))Rev406-7TheCarnotCycle(卡諾循環(huán))Reversibleisothermalcompression(可逆等溫壓縮)

Process3-4:isothermalcompression(等溫壓縮)TL=constantPistonisdoingworkongas,gasiscompressed,TLisgoingtoincreaseQLisrejectedtomaintainTL=const(定溫吸熱)416-7TheCarnotCycle(卡諾循環(huán))Rev416-7TheCarnotCycle(卡諾循環(huán))Reversibleadiabaticcompression(可逆絕熱壓縮)Process4-1:adiabaticcompression(絕熱壓縮)Adiabatic△Q=0Pistonisdoingworkongas,gasiscompressed.TemperatureincreasesfromTLtoTH,CompletetheCarnotcycle426-7TheCarnotCycle(卡諾循環(huán))Rev426-7TheCarnotCycle(卡諾循環(huán))CarnotcycleinP-VdiagramProcess1-2:isothermalexpansion(等溫膨脹)(定溫吸熱)Process2-3:adiabaticexpansion(絕熱膨脹)Process3-4:isothermalcompression(等溫壓縮)(定溫放熱)Process4-1:adiabaticcompression(絕熱壓縮)43Areaunder1-2-3istheworkdonebythegasduringtheexpansionpartofthecycleAreaunder3-4-1istheworkdoneonthegasduringthecompressionpartofthecycle.Areaenclosed1-2-3-4-1isthenewworkdoneduringthiscycle.6-7TheCarnotCycle(卡諾循環(huán))Car436-7TheCarnotCycle(卡諾循環(huán))ReversedCarnotcycle(逆向卡諾循環(huán)):allthefourprocessesofCarnotcyclearereversed.CalledalsoCarnotrefrigerationcycle（卡諾制冷循環(huán)）QLisabsorbedfromTL;QHisrejectedtoTH,withaworkinputWnet,in.446-7TheCarnotCycle(卡諾循環(huán))Rev446-8TheCarnotPrinciples(卡諾定理)CarnotPrinciples(卡諾定理)Efficiencyofanirreversibleheatengineisalwayslessthantheefficiencyofareversibleoneoperatingbetweenthesametworeservoirs（在溫度同為TH的熱源和同為TL的冷源間工作的一切不可逆循環(huán)，其熱效率必小于可逆循環(huán)）.

Theefficienciesofallreversibleheatenginesoperatingbetweenthesametworeservoirsarethesame.(在相同溫度高溫?zé)嵩春拖嗤瑴囟鹊牡蜏責(zé)嵩粗g工作的一切可逆循環(huán)，其熱效率都相等。)456-8TheCarnotPrinciples(卡諾定456-9thethermodynamictemperaturescale自學(xué)466-9thethermodynamictemperat466-10TheCarnotHeatEngine(卡諾熱機(jī))ThehypotheticalheatenginethatoperatesonthereversibleCarnotcycleiscalledCarnotheatengine.

ThermalefficiencyofCarnotheatengine:Wnet,outQHηth=QH-QLQH=QLQHHeatsources,THHeatsink,TLQHQLWnet,outCarnotHeatenginesηth=1-476-10TheCarnotHeatEngine(卡諾476-10TheCarnotHeatEngine(卡諾熱機(jī))Process1-2:isothermalexpansion(等溫膨脹)，定溫吸熱QH,TH,V1V2Process2-3:adiabaticexpansion(絕熱膨脹)△Q=0,V2V3,THTLProcess3-4:isothermalcompression(等溫壓縮)，定溫放熱QL,

TL,V3V4Process4-1:adiabaticcompression(絕熱壓縮)△Q=0,V4V1,TLTHQLQHTLTH=1-ηth,rev=1-TLTHAbsolutetemperatures,K推導(dǎo)486-10TheCarnotHeatEngine(卡諾486-10TheCarnotHeatEngine(卡諾熱機(jī))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,