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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,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 T,L,to T,H,Thermal energy Reservoir,Receive heat Q,H,from a high temperature source,The work input to the refrigerator/heat pump,Heat Q,L,absorbed from refrigerated space T,L,A process has direction,Energy has,quality,and quantity,Heat Source,Heat Sink,Convert part Q,H,to work W,net,out,Reject waste heat Q,L,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,=100%,Heat Q,H,rejected to high temperature T,H,Refrigerator wants Q,L,Heat pump,wants Q,H,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,),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 T,L,and T,H,th,irrev,th,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 T,L,and T,H,th,all rev,=,th,rev,Carnot Heat Engine,The refrigerator / heat pump operates on a reversible Carnot Cycle,Adiabatic expansion,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.,3,6-6 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.,4,6-6 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.,5,6-6 reversible and irreversible processes,Reversible processes,Internally reversible(内部可逆): no irreversibilities occur within the boundary 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.,6,6-6 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.,7,6-7 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.,8,6-7 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:,9,Process 1-2: isothermal 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),T,H,=constant,Gas expands slowly, doing work on surroundings, T is going to decrease,Q,H,is supplied to maintain T,H,=constant,(定温吸热),10,6-7 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 T,H,to T,L,11,6-7 The Carnot Cycle (卡诺循环),Reversible isothermal compression(,可逆等温压缩,),Process 3-4: isothermal compression (等温压缩),T,L,=constant,Piston is doing work on gas, gas is compressed, T,L,is going to increase,Q,L,is rejected to maintain T,L,=const,(定温吸热),12,6-7 The Carnot Cycle (卡诺循环),Reversible adiabatic compression(,可逆绝热压缩,),Process 4-1: adiabatic compression(绝热压缩),Adiabatic,Q=0,Piston is doing work on gas, gas is compressed.,Temperature increases from T,L,to T,H,Complete the Carnot cycle,13,6-7 The Carnot Cycle (卡诺循环),Carnot cycle in P-V diagram,Process 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 (等温压缩) (定温放热),Process 4-1: adiabatic compression(绝热压缩),14,Area under 1-2-3 is the work done by the gas during the expansion part of the cycle,Area 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 Cycle (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.,15,6-8 The Carnot Principles (卡诺定理),Carnot Principles (卡诺定理),Efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs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.,The efficiencies of all reversible heat engines operating between the same two reservoirs are the same. (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。),16,6-9 the thermodynamic temperature scale,自学,17,6-10 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:,W,net,out,Q,H,th,=,Q,H,- Q,L,Q,H,=,Q,L,Q,H,Heat sources, T,H,Heat sink, T,L,Q,H,Q,L,W,net,out,Carnot,Heat engines,th,=1-,18,6-10 The Carnot Heat Engine(卡诺热机),Process 1-2: isothermal expansion (,等温膨胀,),， 定温吸热,Q,H,T,H, V,1,V,2,Process 2-3: adiabatic expansion (,绝热膨胀,),Q=0, V,2,V,3, T,H,T,L,Process 3-4: isothermal compression (,等温压缩,),，定温放热,Q,L,T,L, V,3,V,4,Process 4-1: adiabatic compression (,绝热压缩,),Q=0, V,4,V,1, T,L,T,H,Q,L,Q,H,T,L,T,H,= 1 -,th, rev,=1-,T,L,T,H,Absolute temperatures, K,推导,19,6-10 The Carnot Heat Engine(卡诺热机),The efficiency of a Carnot engine is only related to T,L, and T,H,.,th,rev,=,1-T,L,/T,H,;,I,ncrease T,H, or decrease T,L,can increase,th,rev,For any reversible heat engine,th,rev,=,1-T,L,/T,H,;,th,rev,=,1-T,L,/T,H,is the highest efficiency, a heat engine operating between the two thermal energy reservoirs at temperature,T,L,and,T,H,can have.,th,rev,=,1-T,L,/T,H,is always less than 1.,T,L,=T,H,th,=0. (2nd law of thermodynamics),All irreversible,heat engines operating between,T,L,and,T,H,have efficiency,th,rev,20,6-10 The Carnot Heat Engine(卡诺热机),21,6-10 The Carnot Heat Engine(卡诺热机),22,23,6-11 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):,COP,R,rev,=,Q,H,/Q,L,-1,1,COP,HP,rev,=,1-Q,L,/Q,H,1,=,T,H,/T,L,-1,1,1-T,L,/T,H,1,=,24,6-11 The Carnot refrigerator and heat pump,COP,R,rev,and COP,HP,rev,are the highest COP that a refrigerator or a heat pump operating between T,L,and T,H,can have.,All actual refrigerators or heat pumps operating between T,L,and T,H,have lower COP than COP,R,rev,or COP,HP,rev,No actual refrigerators or heat pumps operating between T,L,and T,H,can have higher COP than COP,R,rev,or COP,HP,rev,COPs of refrigerator or heat pump decrease as T,L,decrease,25,6-11 The carnot refrigerator and heat pump,26,27,Chap6 Summary-2,28,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 T,L,and T,H,th,irrev,th,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 T,L,and T,H,th,all rev,=,th,rev,Carnot Heat Engine,The refrigerator / heat pump operates on a reversible Carnot Cycle,Adiabatic expansion,29,6-71,6-128,
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