工程热力学(英文版)第六单元课件.ppt

1、Chap6 Summary-11Why we need 2nd Law?All processes satisfy 1st Law;Satisfying 1st does not ensure the process can actually occurHeat EnginesRefrigerator,Heat PumpIntroduction to 2nd LawRefrigerators/heat pump:The devices drive heat Q transfer from TL to TH,Thermal energy ReservoirReceive heat QH from

2、 a high temperature sourceThe work input to the refrigerator/heat pumpHeat QL absorbed from refrigerated space TLA process has directionEnergy has quality and quantityHeat SourceHeat SinkConvert part QH to work Wnet,outReject waste heat QL to a low temperature sinkHeat engine2nd law,Kelvin-Planck St

3、atement: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=100%Heat QH rejected to high temperature THRefrigerator wants QLHeat pump wants QHCOP 2nd law,Clausius Statement:Heat does not,of its own

4、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热不能自发地、不付代价地从低温物体传到高温物体传到高温物体)Chap6 Summary-22A process can be reversed without leaving any trace on the surroundings.Carnot CycleCarnot RefrigeratorCarnot Heat PumpReversible ProcessesThe heat engine operates on the reversible C

5、arnot CycleThe best known reversible cycle;four reversible processesCarnot heat engineCarnot Principle 1:Given TL and TH,th,irrev th,rev SystemSurroundingsInternal RevExternal RevWhy need RevIrreversible:heat transferIsothermal expansionIsothermal CompressionAdiabatic compressionReversed Carnot Cycl

6、eCarnot refrigerator/heat pumpCarnot Principle 2:Given TL and TH,th,all rev=th,rev Carnot Heat EngineThe refrigerator/heat pump operates on a reversible Carnot CycleAdiabatic expansion6-6 reversible and irreversible processes The second law of thermodynamics states that:no heat engine can have an ef

7、ficiency 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.36-6 reversible and irreversible processes A reversible process(可逆过程)is defined as a process that can be reversed without leaving any

8、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 irrev

9、ersible processes.46-6 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 c

10、hange 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.56-6 reversible and irreversible processes Reversible processes Internally reversible(内部可逆):no irreversibilities occur within the bound

11、ary of the system during the process.Like the quasi-equilibrium process is an example.Externally reversible(外部可逆):no irreversibilities occur outside the system boundary during the process.66-6 reversible and irreversible processes Reversible processes do not occur in nature.They are idealizations of

12、 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 ideali

13、zed models(theoretical limits)to be compared by actual processes.76-7 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 e

14、ither 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）.86-7 The Carnot Cycle(卡诺循环)Consider a closed system that consists of a gas contained in an adiabatic piston-cylinder device.The four reversible processes are:9Process 1-2:isot

15、hermal expansion(等温膨胀)Process 2-3:adiabatic expansion(绝热膨胀)Process 3-4:isothermal compression(等温压缩)Process 4-1:adiabatic compression(绝热压缩)6-7 The Carnot Cycle(卡诺循环)Reversible isothermal expansion(可逆等温膨胀)：Process 1-2:isothermal expansion(等温膨胀)TH=constant Gas expands slowly,doing work on surroundings,

16、T is going to decrease QH is supplied to maintain TH=constant(定温吸热)106-7 The Carnot Cycle(卡诺循环)Reversible adiabatic expansion(可逆绝热膨胀)：Process 2-3:adiabatic expansion(绝热膨胀)Adiabatic Q=0 Gas expands slowly,doing work on the surroundings.Temperature drops from TH to TL116-7 The Carnot Cycle(卡诺循环)Revers

17、ible isothermal compression(可逆等温压缩)Process 3-4:isothermal compression(等温压缩)TL=constant Piston is doing work on gas,gas is compressed,TL is going to increase QL is rejected to maintain TL=const(定温吸热)126-7 The Carnot Cycle(卡诺循环)Reversible adiabatic compression(可逆绝热压缩)Process 4-1:adiabatic compression(

18、绝热压缩)Adiabatic Q=0 Piston is doing work on gas,gas is compressed.Temperature increases from TL to TH,Complete the Carnot cycle136-7 The Carnot Cycle(卡诺循环)Carnot cycle in P-V diagramProcess 1-2:isothermal expansion(等温膨胀)(定温吸热)Process 2-3:adiabatic expansion(绝热膨胀)Process 3-4:isothermal compression(等温压

19、缩)(定温放热)Process 4-1:adiabatic compression(绝热压缩)14Area under 1-2-3 is the work done by the gas during the expansion part of the cycleArea under 3-4-1 is the work done on the gas during the compression part of the cycle.Area enclosed 1-2-3-4-1 is the new work done during this cycle.6-7 The Carnot Cycl

20、e(卡诺循环)Reversed Carnot cycle(逆向卡诺循环):all the four processes of Carnot cycle are reversed.Called also Carnot refrigeration cycle（卡诺制冷循环）QL is absorbed from TL;QH is rejected to TH,with a work input Wnet,in.156-8 The Carnot Principles(卡诺定理)Carnot Principles(卡诺定理卡诺定理)Efficiency of an irreversible heat

21、engine is always less than the efficiency of a reversible one operating between the same two reservoirs（在温度同为（在温度同为TH的热源和同为的热源和同为TL的冷源间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）工作的一切不可逆循环，其热效率必小于可逆循环）.The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.(在相同温度在相同温度

22、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。效率都相等。)166-9 the thermodynamic temperature scale 自学176-10 The Carnot Heat Engine(卡诺热机)The hypothetical heat engine that operates on the reversible Carnot cycle is called Carnot heat engine.Thermal efficiency of Carnot heat engine:Wnet,out

23、QHth=QH-QLQH=QLQHHeat sources,THHeat sink,TLQHQLWnet,outCarnot Heat enginesth=1-186-10 The Carnot Heat Engine(卡诺热机)Process 1-2:isothermal expansion(等温膨胀)，定温吸热 QH,TH,V1V2Process 2-3:adiabatic expansion(绝热膨胀)Q=0,V2V3,THTLProcess 3-4:isothermal compression(等温压缩)，定温放热 QL,TL,V3V4Process 4-1:adiabatic com

24、pression(绝热压缩)Q=0,V4V1,TLTHQLQHTLTH=1-th,rev=1-TLTHAbsolute temperatures,K推导推导196-10 The Carnot Heat Engine(卡诺热机)The efficiency of a Carnot engine is only related to TL,and TH.th,rev=1-TL/TH;Increase TH,or decrease TL can increase th,rev For any reversible heat engine,th,rev=1-TL/TH;th,rev=1-TL/TH i

25、s the highest efficiency,a heat engine operating between the two thermal energy reservoirs at temperature TL and TH can have.th,rev=1-TL/TH is always less than 1.TL=TH,th=0.(2nd law of thermodynamics)All irreversible heat engines operating between TL and TH have efficiency th,rev206-10 The Carnot He

26、at Engine(卡诺热机)216-10 The Carnot Heat Engine(卡诺热机)22236-11 The Carnot refrigerator and heat pump A refrigerator or a heat pump that operates on the reversed Carnot cycle is called Carnot refrigerator,or a Carnot heat pump.The coefficient of performance(COP):COPR,rev=QH/QL-11COPHP,rev=1-QL/QH1 =TH/TL

27、-111-TL/TH1 =246-11 The Carnot refrigerator and heat pumpCOPR,rev and COPHP,rev are the highest COP that a refrigerator or a heat pump operating between TL and TH can have.All actual refrigerators or heat pumps operating between TL and TH have lower COP than COPR,rev or COPHP,rev No actual refrigera

28、tors or heat pumps operating between TL and TH can have higher COP than COPR,rev or COPHP,rev COPs of refrigerator or heat pump decrease as TL decrease256-11 The carnot refrigerator and heat pump2627Chap6 Summary-228A process can be reversed without leaving any trace on the surroundings.Carnot Cycle

29、Carnot RefrigeratorCarnot Heat PumpReversible ProcessesThe heat engine operates on the reversible Carnot CycleThe best known reversible cycle;four reversible processesCarnot heat engineCarnot Principle 1:Given TL and TH,th,irrev th,rev SystemSurroundingsInternal RevExternal RevWhy need RevIrreversible:heat transferIsothermal expansionIsothermal CompressionAdiabatic compressionReversed Carnot CycleCarnot refrigerator/heat pumpCarnot Principle 2:Given TL and TH,th,all rev=th,rev Carnot Heat EngineThe refrigerator/heat pump operates on a reversible Carnot CycleAdiabatic expansion296-71 6-128