工程热力学(英文版)第六单元ppt课件.ppt

,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 =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),Chap6 Summary-2,2,A process can be reversed without leaving any trace on the surroundings.,Carnot Cycle,Carnot RefrigeratorCarnot 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, 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 TL and TH, 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 processReversible 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 statesSurroundings returned to initial statesThis 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) processProcesses 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 processesInternally 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 processAct 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 processesTwo adiabatic processesFirst proposed in 1824 by Sadi CarnotCarnot 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)TH=constantGas expands slowly, doing work on surroundings, T is going to decreaseQH is supplied to maintain TH=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=0Gas expands slowly, doing work on the surroundings.Temperature drops from TH to TL,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)TL=constantPiston is doing work on gas, gas is compressed, TL is going to increaseQL is rejected to maintain TL=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=0Piston is doing work on gas, gas is compressed.Temperature increases from TL to TH,Complete the Carnot cycle,13,6-7 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 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 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:,Wnet,out,QH,th =,QH - QL,QH,=,QL,QH,Heat sources, TH,Heat sink, TL,QH,QL,Wnet,out,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 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 compression (绝热压缩) Q=0, V4V1, TLTH,QL,QH,TL,TH,= 1 -,th, rev =1-,TL,TH,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 TL, and TH. th,rev= 1-TL/TH; Increase TH, or decrease TL can increase th,revFor any reversible heat engine, th,rev= 1-TL/TH; th,rev= 1-TL/TH is 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,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):,COPR,rev =,QH/QL-1,1,COPHP,rev =,1-QL/QH,1,=,TH/TL-1,1,1-TL/TH,1,=,24,6-11 The Carnot refrigerator and heat pump,COPR,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 refrigerators 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 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 RefrigeratorCarnot 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, 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 TL and TH, 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,