山东大学：《工程热力学》课程PPT教学课件（双语）第六章 The second law of thermodynamics（2/2，6.6-6.11）

SHANDONGUNIVERSITYChap6 Summary-1Whyweneed2ndLaw?Allprocessessatisfy1stLawSatisfying1stdoesnotensuretheprocesscanactuallyoccurIntroductionto 2nd LawAprocesshasdirectionEnergyhasqualityandquantityHeat SinkHeat SourceHeatengineThermalenergyReservoirWact.ouOReceiveheatQfromahightemperature source生MthQHWConvert part Qtowork Wnet.outnetoutOHeatEnginesQTihRejectwaste heatQtoalowtemperature sinkO2ndlawKelvin-PlanckStatement:Itisimpossibleforanydevicethatoperatesonacycletoreceiveheatfromasinglereservoirandproduceanetamountofwork.Noheatenginecanhaven=100%Refrigerators/heatpump:ThedevicesdriveheatQtransferfromT,toTHW.RefrigeratorThework inputtotherefrigerator/heatpumpnet,inwants QLQHeatQabsorbedfromrefrigeratedspaceTHeatpumpQHHeatQrejectedtohightemperature THwants QHRefrigerator,HeatPumpDesired outputuDesired outputQ,AirCOPCOPHCOPW.WreLinRequired inputRequired inputConditioner2nd law,Clausius Statement:Heatdoesnot,of its ownvolition,transferfromacoldmediumtoawarmerone.（热不能自发地、不付代价地从低温物体传到高温物体）

SHANDONG UNIVERSITY Chap6 Summary-1 1 Why we need 2nd Law? All processes satisfy 1st Law; Satisfying 1st does not ensure the process can actually occur Heat Engines Refrigerator, Heat Pump Introduction to 2nd Law Refrigerators/heat pump: The devices drive heat Q transfer from TL to TH, Thermal energy Reservoir Receive heat QH from a high temperature source The work input to the refrigerator/heat pump Heat QL absorbed from refrigerated space TL A process has direction Energy has quality and quantity Heat Source Heat Sink Convert part QH to work Wnet,out Reject waste heat QL to a low temperature sink Heat engine 2nd law, Kelvin-Planck Statement: It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. No heat engine can have η=100% Heat QH rejected to high temperature TH Refrigerator wants QL Heat pump wants QH COP 2nd law, Clausius Statement: Heat does not, of its own volition, transfer from a cold medium to a warmer one. (热不能自发地、不付代价地从低温物体传到高温 物体) Air Conditioner

SHANDONG UNIVERSITYChap6 Summary-2SystemAprocesscanbereversedwithoutleavinganytraceonthesurroundingsSurroundingsReversible ProcessesInternal RevExternal RevWhy need RevIrreversible:heattransferThe bestknown reversible cycle; four reversible processesIsothermal expansionAdiabatic expansionIsothermalCompressionAdiabaticcompressionCarnotCycleCarnotheat engineReversedCarnotCycleCarnotrefrigerator/heatpumpCarnot Principle 1:Given Tand Th,Nth.irrev<.Nth,revCarnotPrinciple 2:Given T, and Th,Nth.all rev=Nth,revThe heat engine operates on the reversible Carnot CycleirreversibleheatengineTihrevCarnot HeatEngineTLQLreversible heat engineTth,revThTih.reTHOn二impossibleheatengineThrevThe refrigerator / heat pump operates on a reversible Carnot CycleCarnotRefrigeratorCOPR.ECOPRIEVirreversiblerefrigeratoTH/T,-1Carnot Heat PumpCOPCOPRrevreversiblerefrigeratorCOPHPeCOPR.revimpossiblerefrigerator1-T,/TH2

SHANDONG UNIVERSITY Chap6 Summary-2 2 A process can be reversed without leaving any trace on the surroundings. Carnot Cycle Carnot Refrigerator Carnot Heat Pump Reversible Processes The heat engine operates on the reversible Carnot Cycle The best known reversible cycle; four reversible processes Carnot heat engine Carnot Principle 1: Given TL and TH, ηth,irrev < ηth,rev System Surroundings Internal Rev External Rev Why need Rev Irreversible: heat transfer Isothermal expansion Isothermal Compression Adiabatic compression Reversed Carnot Cycle Carnot refrigerator /heat pump Carnot Principle 2: Given TL and TH, ηth,all rev = ηth,rev Carnot Heat Engine The refrigerator / heat pump operates on a reversible Carnot Cycle Adiabatic expansion

SHANDONGUNIVERSITY6-6 reversible and irreversible processesThe second law of thermodynamics statesthat: no heat engine can have anefficiencyof 100percent??? What is the highest efficiency that aheatengine canhave???Toanswerthisquestion,weneedtodefineanidealizedprocess-Reversibleprocess

SHANDONG UNIVERSITY 6-6 reversible and irreversible processes • The second law of thermodynamics states that: no heat engine can have an efficiency of 100 percent. • ??? What is the highest efficiency that a heat engine can have??? – To answer this question, we need to define an idealized process – Reversible process. 3

SHANDONGUNIVERSITY6-6 reversible and irreversible processesAreversible process（可逆过程）is defined as aprocess that can be reversed without leavinganytrace onthe surroundingsSystemreturnedtoinitial states- Surroundings returned to initial states:This is possible only if the net heat and net workexchange between the system and the surroundings iszero for the combined (original and reverse) processProcesses that are not reversible called irreversibleprocesses

SHANDONG UNIVERSITY 6-6 reversible and irreversible processes • A reversible process (可逆过程) is defined as a process that can be reversed without leaving any trace on the surroundings. – System returned to initial states – Surroundings returned to initial states • This is possible only if the net heat and net work exchange between the system and the surroundings is zero for the combined (original and reverse) process • Processes that are not reversible called irreversible processes. 4

SHANDONG UNIVERSITY6-6 reversible and irreversible processes. It should be pointed out that:- A system can be restored to its initial state following aprocess, regardless of whether the process isreversible or irreversible.- But for reversible processes: this restoration is madewithout leaving any net change on the surroundings- While for irreversible processes: the surroundingsusually do some work on the system and thereforedoes not return to their original state

SHANDONG UNIVERSITY 6-6 reversible and irreversible processes • It should be pointed out that: – A system can be restored to its initial state following a process, regardless of whether the process is reversible or irreversible. – But for reversible processes: this restoration is made without leaving any net change on the surroundings. – While for irreversible processes: the surroundings usually do some work on the system and therefore does not return to their original state. 5

SHANDONGUNIVERSITY6-6 reversible and irreversible processesReversible processes do not occur in nature.: They are idealizations of actual processes.All theprocesses occurringinnatureare irreversibleThe possibility of finding a perfect mate is no higher thanthepossibility offindingaperfect(reversible)process.But we need reversible processes:- Easyto analyze:a serious of equilibrium states during a process Act as idealized models (theoretical limits) to be compared byactualprocesses

SHANDONG UNIVERSITY 6-6 reversible and irreversible processes • Reversible processes do not occur in nature. • They are idealizations of actual processes. • All the processes occurring in nature are irreversible. • The possibility of finding a perfect mate is no higher than the possibility of finding a perfect (reversible) process. • But we need reversible processes: – Easy to analyze: a serious of equilibrium states during a process – Act as idealized models (theoretical limits) to be compared by actual processes. 6

SHANDONGUNIVERSITYExpansionCompressionExpansionCompressionPressuredistributionWaterWaterWaterWater(b)Fast(irreversible)process(a)Slow(reversible)processReversible processes deliver themost and consume theleast work

SHANDONG UNIVERSITY 7 Reversible processes deliver the most and consume the least work

SHANDONG UNIVERSITY6-6 reversible and irreversible processesIrreversibilities: The factors that cause a process to beirreversible are called irreversibilities, including=Friction(摩擦)Fnction(e)Fast co20cGASHeunrestrainedexpansion（自由膨胎(b)Fastespansion7OLP50kP(c) Umestnined expansioeW-heattransferacrossafinitetemperaturedifference（温差传热）DuiMehcuimaferfeesmixing of two fluids, electric resistance, inelastic deformation ofsolids,andchemicalreactions

SHANDONG UNIVERSITY 6-6 reversible and irreversible processes • Irreversibilities: The factors that cause a process to be irreversible are called irreversibilities, including – Friction(摩擦) – unrestrained expansion（自由膨胀） – heat transfer across a finite temperature difference （温差传热） – mixing of two fluids, electric resistance, inelastic deformation of solids, and chemical reactions. 8

SHANDONGUNIVERSITYInternallyandExternallyReversibleProcessesInternallyreversibleprocess:Ifnoirreversibilitiesoccurwithintheboundariesofthesystemduringtheprocess.Externallyreversible:IfnoirreversibilitiesoccuroutsidethesystemboundariesTotallyreversibleprocess:ItinvolvesnoirreversibilitieswithinthesystemoritssurroundingsAtotallyreversibleprocessinvolvesnoheattransferthroughafinitetemperaturedifference,nononquasi-equilibriumchanges,andnofriction or otherdissipativeeffects

SHANDONG UNIVERSITY 9 Internally and Externally Reversible Processes • Internally reversible process: If no irreversibilities occur within the boundaries of the system during the process. • Externally reversible: If no irreversibilities occur outside the system boundaries. • Totally reversible process: It involves no irreversibilities within the system or its surroundings. • A totally reversible process involves no heat transfer through a finite temperature difference, no nonquasi-equilibrium changes, and no friction or other dissipative effects

SHANDONGUNIVERSITY6-7 The Carnot Cycle（卡诺循环)Carnot cycle(卡诺循环)is the bestknownreversible cycleComposedoffourreversibleprocesses:Twoisothermalprocesses.Two adiabaticprocessesFirstproposed in1824bySadiCarnotCarnotcyclecanbeexecutedeitherinaclosedorasteadyflow system.Thetheoretical heatengineoperates on aCarnotcycleisCarnotheatengine（卡诺热机）10

SHANDONG UNIVERSITY 6-7 The Carnot Cycle (卡诺循环) • Carnot cycle(卡诺循环) is the best known reversible cycle. – Composed of four reversible processes: • Two isothermal processes • Two adiabatic processes – First proposed in 1824 by Sadi Carnot – Carnot cycle can be executed either in a closed or a steady flow system. – The theoretical heat engine operates on a Carnot cycle is Carnot heat engine（卡诺热机）. 10