工程热力学(英文版)第7单元课件.ppt

1、SHANDONG UNIVERSITYChap6 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 ReservoirR

2、eceive heat QH from 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 l

3、aw,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 THRefrigerator wants QLHeat pump wants QHCOP 2nd law,Clausius Statement:Heat d

4、oes 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)SHANDONG UNIVERSITYChap6 Summary-22A process can be reversed without leaving any trace on the surroundings.Carnot CycleCarnot RefrigeratorCarnot Heat PumpReversible ProcessesThe he

5、at engine operates on the reversible Carnot CycleThe best known reversible cycle;four reversible processesCarnot heat engineCarnot Principle 1:Given TL and TH,th,irrev Reversible processIrreversible process 0sf at 0.01=0 kJ/kg.k2,Isentropic process0,adibatic0,Reversible3,T-S,h-s diagrams=4,The 3rd l

6、aw of thermodaynamics:The entropy of a pure crystalline substance at absolute zero temperature is zero 5,T ds relations:6,reversible work outputSHANDONG UNIVERSITYChapter 7 Entropy4SHANDONG UNIVERSITY7-1 Entropy The 2nd law of thermodynamics leads to expressions that involve inequalities.Heat engine

7、:th,irrev th,rev Heat pump:COPHP,irrev COPHP,rev COPR,irrev For irreversible processThe entropy change of a closed system During an irreversible process is greater than the integral of Q/TDuring a reversible process is equal to the integral of Q/TSHANDONG UNIVERSITY7-2 the increase of entropy princi

8、ple(熵增原理)Entropy change of the system:For reversible processes,the entropy change:Represent the entropy transfer with heatQheat transfer between system and surroundings,kJT absolute temperature at the boundary.K For irreversible processes,the entropy change:Always greater than the entropy transferSo

9、me entropy is generated or createdThe entropy generated during an irreversible process is called entropy generation(熵产)Entropy generation(熵产)：Entropy generation is always a positive or zero quantity.Its value depends on the process,thus it is not a property of a systemIn the absence of any entropy t

10、ransfer,the entropy change of a system is equal to the entropy generation.0SHANDONG UNIVERSITY7-2 the increase of entropy principle(熵增原理)In the absence of any entropy transfer,the entropy change of a system is equal to the entropy generation.For an isolated system,the heat transfer is zeroIncrease o

11、f entropy principle(孤立系统的熵增原理，简称熵增原理孤立系统的熵增原理，简称熵增原理):the entropy of an isolated system during a process always increase or,in the limiting case of a reversible process remains constant.（孤立系统的熵可以增大，或保持不变，但不可能减少）（孤立系统的熵可以增大，或保持不变，但不可能减少）0 0orSHANDONG UNIVERSITY System+Surroundings=Isolated system Ent

12、ropy change of a closed system includesEntropy transfer(热熵流)with heat transferEntropy generation（熵产）-irreversibilities Entropy change of a system can be negative during a process；but entropy generation cannot.SHANDONG UNIVERSITY Remarks on Entropy Processes can occur in a certain direction only,not

13、in any direction.A process must proceed in the direction that complies with the increase of entropy principle,that is,Entropy is a nonconserved property.Entropy is conserved during the idealized reversible processes only and increases during all acutal processes.The performance of engineering system

14、s is degraded by the presence of irreversibilities,and entropy generation is a measure of the magnitudes of the irreversibilities present during that process.The greater the extent of irreversibilities,the greater the entropy generation.Entropy generation can be used as a quantitative measure of irr

15、eversibilities associated with a process.It is also used to establish criteria for the performance of engineering devices.SHANDONG UNIVERSITYa)has larger Sgen,so it is more irreversibleSHANDONG UNIVERSITY7-3 Entropy change of pure substances(tables)Reference point:Entropy of saturated liquid water:s

16、f at 0.01=0 kJ/kg.k (Table A-4)SHANDONG UNIVERSITYSuperheated Sat.MixSHANDONG UNIVERSITY7-4 Isentropic processes(等熵过程)Isentropic process is a process during which the entropy remains constant.the entropy of a fixed mass can be changed by 1)heat transfer,(Adiabatic)2)irreversibilities (reversible)A r

17、eversible adiabatic process is an isentropic process.(可逆绝热过程是等熵过程)Many engineering systems or devices such as pumps,turbines,nozzles,and diffusers are essentially adiabatic in their operation.An isentropic process can serve as an appropriate model for actual processesAn isentropic process enable us

18、to define the efficiency for a process to compare the actual performance of these devices as idealized conditions.SHANDONG UNIVERSITYSHANDONG UNIVERSITY7-5 Property diagrams involving entropy(T-S)(H-S)Temperature-Entropy(T-S)diagram The area under the process curve on a T-S diagram represents heat t

19、ransfer during a reversible process.The area under the process curve on a P-V diagram represents the boundary work for a reversible process.The area has no meaning for irreversible processes.SHANDONG UNIVERSITY Special cases reversible isothermal process Isentropic process:ds=0;Q=0 Vertical like on

20、a T-s diagram12s1s2SHANDONG UNIVERSITY Enthalpy-Entropy(h-s)diagram Commonly used in the analysis of steady-flow devices such as turbines,compressors,and nozzles.Enthalpy,h:a primary property in 1st law analysis of steady-flow devices Entropy,s:the property accounts for irreversibilities during adia

21、batic processes E.g:steady flow of steam through an adiabatic turbine:Vertical distance h:is a measure of the work output of the turbine;Horizontal distance s:is a measure of the irreversibilities associated with the processSHANDONG UNIVERSITYSHANDONG UNIVERSITY7-6 what is entropy?Third law of therm

22、odynamics:The entropy of a pure crystalline substance at absolute zero temperature is zero since there is no uncertainty about the state of the molecules at that instant.There is no entropy transfer associated with energy transfer as work.Work is free of entropy.The quantity of energy is always pres

23、erved during an actual process(the 1st law);But the quality is bound to decrease(the 2nd law).This dicrease in quality is always accompanied by an increase in entropy.Processes can occur only in the direction of increased overall entropy.SHANDONG UNIVERSITY7-7 The T ds relations Energy balance of a

24、reversible process in a closed stationary system.These two results(7-23 and 7-24)are valid for both reversible and irreversible processes.(Entropy is independent of process.)They are applicable whether the change of state occurs in a closed or an open system.SHANDONG UNIVERSITY7-7 The T ds relations

25、 Calculation of S:Integrations of the equationsTabulated dataSHANDONG UNIVERSITY7-10 reversible steady-flow work Energy balance for a steady flow device undergoing an reversible process For incompressible fluid,specific volume is constantSHANDONG UNIVERSITY Neglect changes in KE and PE,we get the re

26、versible work output associated with reversible process in steady flow deviceFrom this equation,we can see that:the larger the specific volume,the larger the reversible work produced or consumed by the steady-flow device.For compression process:keep the specific volume as small as possibleFor expans

27、ion process,keep the specific volume as larger as possible.SHANDONG UNIVERSITY In steam power plants,the pressure rise in the pump or compressor is equal to the pressure drop in the turbine if we disregard the pressure losses in various other components.the pump handles liquid,which has a very small

28、 specific volume the turbine handles vapor,whose specific volume is many times larger.Therefore,the work output of the turbine is much larger than the work input to the pump.This is one of the reasons for the wide-spread use of steam power plants in electric power generation.SHANDONG UNIVERSITYSHAND

29、ONG UNIVERSITY78 ENTROPY CHANGE OF LIQUIDS AND SOLIDS 自学79 THE ENTROPY CHANGE OF IDEAL GASES711 MINIMIZING THE COMPRESSOR WORK712 ISENTROPIC EFFICIENCIES oF STEADY-FLOW DEVICES7-13 Entropy balanceSHANDONG UNIVERSITYChap7 Summary34Clausius inequality:the cyclic integral of is always zeroIncrease of E

30、ntropy Principle(熵增原理熵增原理)Entropy(熵)1,Entropy change of pure substances:Entropy change of a closed system:Increase of entropy principle(孤立系统熵增原理，简称熵增原理孤立系统熵增原理，简称熵增原理)：the entropy of an isolated system during a process always increase or,in the limiting case of a reversible process remains constant.

31、(孤立系统的熵可以增大，或保持不变，但不可能减少孤立系统的熵可以增大，或保持不变，但不可能减少)EntropySome remarksIso TReversible processIrreversible process 0sf at 0.01=0 kJ/kg.k2,Isentropic process0,adibatic0,Reversible3,T-S,h-s diagrams=4,The 3rd law of thermodaynamics:The entropy of a pure crystalline substance at absolute zero temperature i

32、s zero 5,T ds relations:6,reversible work outputSHANDONG UNIVERSITYClausius inequalityEntropy Change 0Entropy change of closed systemIncrease of Entropy Principle(isolated sys)T ds relationsreversible work output A reversible adiabatic process is an isentropic process热力学第二定律的数学表达式热力学第二定律的数学表达式SHANDONG UNIVERSITY367-59C7-60C7-267-317-1C7-2C7-3C7-4C7-5C7-6C7-7C7-8C7-9C7-11C7-12C 7-23C7-30C7-61C7-64C7-65C7-87C7-88C7-89C