第四章理想气体的热力过程优秀课件.ppt

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1、第四章理想气体的热力过程第1页，本讲稿共45页 A.It is to determine the change in the properties of working fluid during a process;(在于确定在一个过程中工质状态参数的变化量在于确定在一个过程中工质状态参数的变化量)B.It is to determine the amount of heat,work interaction between the system and its surroundings during a process.(确定在一个过程中系统与外界所交换的功量和热量的多少确定在一个过程中系统

2、与外界所交换的功量和热量的多少)2.本章研究的对象本章研究的对象 The object of this chapterReversible processes of Ideal Gases in closed systems(闭口系统中理想气体的可逆过程闭口系统中理想气体的可逆过程)1.本章的研究任务本章的研究任务The task of this chapter 第2页，本讲稿共45页3.所采取的步骤所采取的步骤 The procedures adoptedPv=RT第3页，本讲稿共45页4.1 Basic Thermodynamic Process(4.1 基本热力过程基本热力过程)1.等容

3、过程等容过程Isochoric-Constant Volume process 第4页，本讲稿共45页 For a constant volume process,the addition or removal of heat will lead to a change in the temperature and pressure of the gas,as shown on the two graphs above.Substitute into(1)过程方程过程方程(Process equation)第5页，本讲稿共45页(2)内能、焓及熵的变化量内能、焓及熵的变化量The change

4、 in Internal Energy,Enthalpy and Entropy The amount of heat added to a closed system during a constant volume process equals to the increase in internal energy.定容过程中加入闭口系统的热量等于系统的内能的增加量定容过程中加入闭口系统的热量等于系统的内能的增加量第6页，本讲稿共45页Entropy Change To find the Entropy change,start with the expression derived fro

5、m the first law,replacing dU using the definition of specific heat at constant volume and using the definition of entropy(3)Work done and Heat Transferred(功量和热量功量和热量)Applying the first law of thermodynamics to the process Replacing W with the reversible worksince the volume is constant dV=0 第7页，本讲稿共

6、45页using the definition of the specific heat at constant volumeto replace dU in the first law.The technical work done 第8页，本讲稿共45页2.等压过程等压过程 Isobaric-Constant Pressureo第9页，本讲稿共45页(1)过程方程过程方程 Process equationFor a constant pressure process,the addition or removal of heat will lead to a change in the t

7、emperature and volume of the gas,as shown on the two graphs above.Substitute into第10页，本讲稿共45页 The amount of heat added to a closed system during a constant pressure process equals to the increase in enthalpy.(2)内能、焓及熵的变化量内能、焓及熵的变化量The change in Internal Energy,Enthalpy and Entropy 第11页，本讲稿共45页 The a

8、mount of heat added to a closed system during a constant pressure process equals to the increase in enthalpy.（3）Work done and Heat Transferred 第12页，本讲稿共45页 During a constant pressure process,heat is added or removed and the temperature and volume changes.The volume at the end of the process can be f

9、ound using the ideal gas law and the work calculated from第13页，本讲稿共45页3.等温压缩和膨胀等温压缩和膨胀Isothermal Compression and Expansion第14页，本讲稿共45页 For a constant temperature process,the addition or removal of heat will lead to a change in the volume and pressure of the gas,as shown on the two graphs above.Substi

10、tute into (1)过程方程过程方程(Process equation)第15页，本讲稿共45页(2)内能、焓及熵的变化量内能、焓及熵的变化量 The change in Internal Energy,Enthalpy and Entropy 第16页，本讲稿共45页(3)功量和热量功量和热量Work Done and Heat Transferred In an isothermal process,the temperature is constant.Applying the first law of thermodynamics to this closed process F

11、or an ideal gas,the internal energy is a function of temperature only,and since the temperature is constant,then dU is zero and第17页，本讲稿共45页 using the ideal gas law and integrating between the start and end of the process During an isothermal process,the work done by the system is equal to the heat a

12、dded to the system,and all the work is technically usable.第18页，本讲稿共45页4.绝热过程绝热过程 Adiabatic Process 第19页，本讲稿共45页(1)Process equation(过程方程过程方程)orQuasi-static,adiabatic process for an ideal gasq=cvdT+pdv and q=cpdT vdp then cvdT=-pdv and cp dT=vdp therefore thenfinally,we arrive at the very useful expre

13、ssionfrom which it can also be shown thator第20页，本讲稿共45页 When the temperatures at the start and end of the process are known,the pressure is calculated from(2)The change in Internal Energy,Enthalpy and Entropy(内能、焓及熵的变化量）内能、焓及熵的变化量）Entropy ChangeThere is no heat transfer to or from the gas and the pr

14、ocess is reversible so that第21页，本讲稿共45页 When the temperatures at the start and end of the process are known,the work done is calculated from(3)Work Done and Heat Transferred(功量和热量功量和热量)第22页，本讲稿共45页5.多变过程多变过程Polytropic Process oMany processes can be approximated by the law:owhere,P Pressure,v Volume,

15、n an index depending on the process type.第23页，本讲稿共45页on=0,results in P=constant i.e.isobaric process.on=infinity,results in v=constant i.e.isochoric process.on=1,results in P v=constant,which is an isothermal process for a perfect gas.on=k,which is a reversible adiabatic process for a perfect gas.Po

16、lytropic processes are internally reversible.Some examples are vapors and ideal gas in many non-flow processes,such as:第24页，本讲稿共45页oSome polytropic processes are shown in figure below:The initial state of working fluid is shown by point 0 on the P-V diagram.The polytropic state changes are:0 to 1=co

17、nstant pressure heating,0 to 2=constant volume heating,0 to 3=reversible adiabatic compression,0 to 4=isothermal compression,0 to 5=constant pressure cooling,0 to 6=constant volume cooling,0 to 7=reversible adiabatic expansion,0 to 8=isothermal expansion.第25页，本讲稿共45页 When the temperatures at the sta

18、rt and end of the process are known,the pressure is calculated from(2)内能、焓及熵的变化量内能、焓及熵的变化量The change in Internal Energy,Enthalpy and Entropy Entropy ChangeThere is no heat transfer to or from the gas and the process is reversible so that第26页，本讲稿共45页 When the temperatures at the start and end of the

19、process are known,the work done is calculated from(3)Work Done and Heat Transferred(功量和热量功量和热量)第27页，本讲稿共45页第28页，本讲稿共45页29 第29页，本讲稿共45页4.34.3活塞式压气机的工作过程活塞式压气机的工作过程Working Process of gas piston compressor第30页，本讲稿共45页1.Theoretical processes of single-staged piston compressor (理想单级活塞式压气机的工作原理理想单级活塞式压气机的

20、工作原理)第31页，本讲稿共45页 let the initial pressure of the gas is (令气体初态压力为令气体初态压力为 )the final pressure is (终态压力为 )，pressure ratio is (升压比)（1）b-1:charge stroke(吸气冲程)，吸气量增大，吸入气体的状态（吸气量增大，吸入气体的状态（P,T)不变不变;(3)2-a:discharge process(排气过程),气缸内的气体减少，气体状态不变化气缸内的气体减少，气体状态不变化(2)1-2:compression stroke(压缩冲程)，吸气量不变，压力升高吸

21、气量不变，压力升高.第32页，本讲稿共45页2.Analysis on theoretical work consumed by these processes(压气机的理论耗功量分析压气机的理论耗功量分析)o取活塞右行一次的吸气量为控制质量，并忽略动能差取活塞右行一次的吸气量为控制质量，并忽略动能差及位能差进行分析。及位能差进行分析。吸气过程：气体推动活塞移动，做推挤功吸气过程：气体推动活塞移动，做推挤功 ；压缩过程：气体向活塞做膨胀功压缩过程：气体向活塞做膨胀功 排气过程：气体向活塞做推挤功排气过程：气体向活塞做推挤功 第33页，本讲稿共45页压缩过程功压缩过程功:指指1-2，闭口系的膨胀

22、功，是可逆过程；，闭口系的膨胀功，是可逆过程；压气机耗功：指压气机耗功：指a-1-2-b,全过程是开口系，包括流动功。全过程是开口系，包括流动功。气体在全过程中所做的总功为：气体在全过程中所做的总功为：3.三种不同压缩过程的比较三种不同压缩过程的比较(Comparison of three kinds of compression processes)Adiabatic process(绝热过程绝热过程)：压缩过程进行的很快，热量来不及释放；压缩过程进行的很快，热量来不及释放；Isothermal process(等温过程等温过程)：压缩过程中，理想的冷：压缩过程中，理想的冷却条件，压缩产生的

23、热量可及时排出；却条件，压缩产生的热量可及时排出；Polytropic process(多变过程多变过程)：采用了一定的冷却措施，但压：采用了一定的冷却措施，但压缩期间缩期间,温度仍继续升高。温度仍继续升高。第34页，本讲稿共45页(1)Work consumption(耗功量）耗功量）If it is isothermal compression,then若压缩过程为可逆定温过程，则：若压缩过程为可逆定温过程，则：If it is isentropic compression,then若压缩过程为可逆绝热压缩，则:第35页，本讲稿共45页If it is polytropic compres

24、sion,then 若压缩过程为可逆多变过程，则:（2）Discharge Temperature(排气温度）排气温度）第36页，本讲稿共45页4.余隙容积的影响余隙容积的影响The influence of residual volume of inter-space.（1）The influence on discharge volume(对排气量的影响）对排气量的影响）Residual Volume(余隙容积）：余隙容积）：V3We note that if the pressure ratio is too high,then it can not work normally.(压缩压

25、力不能太高，压力升高，效率下降，在极限情况下容积效率可达零）第37页，本讲稿共45页The discharge volume will decrease.(余隙存在将使排气量减少）余隙存在将使排气量减少）（2）The influence on compression work（对耗功量的影响）（对耗功量的影响）There is no influence on compression work.(对耗功量无影响对耗功量无影响)Let thenVolume efficiency is defined as 第38页，本讲稿共45页Multistage compression with interc

26、ooler is especially attractive when a gas is to be compressed to very high pressures.1.two stage compression（两级压缩）（两级压缩）intercooler4.4 Multistage Compression with inter-cooling（多级压缩、中间冷却）（多级压缩、中间冷却）第39页，本讲稿共45页Assumed that the compression process 1-2 and 3-4 are with the same polytropic index n (假设假

27、设1-2和和3-4的多变指数相等。的多变指数相等。)Assumed(假设假设)To minimize the total compression work,the optimal pressure ratio can be determined by (为了使压缩过程的耗功量最小为了使压缩过程的耗功量最小,可以采用如下方法确定最佳中可以采用如下方法确定最佳中间压力）间压力）We obtain第40页，本讲稿共45页 That is,to minimize the compression work during two stage compression,the pressure ratio a

28、cross each stage of the compressor must be the same.（各级压缩比都相等时多级压缩最省功。）（各级压缩比都相等时多级压缩最省功。）When this condition is satisfied,the compression work at each stage becomes identical.(当满足上述条件时，每一级压缩的耗功量都相等）当满足上述条件时，每一级压缩的耗功量都相等）第41页，本讲稿共45页2.Z-Stage Compression(n级压缩）级压缩）Optimal compression ration is 3.Isen

29、tropic Efficiency and Isothermal Efficiency (等熵效率和等温效率）第42页，本讲稿共45页oIsentropic efficiency is defined as the ratio of the work input is required to raise the pressure of a gas to a specified value in an isentropic manner to the actual work input.Isothermal Efficiency(等温效率）等温效率）o Isothermal efficiency

30、 is defined as the ratio of the work input is required to raise the pressure of a gas to a specified value in a reversible isothermal manner to the actual work input.第43页，本讲稿共45页Exercise(练习)oDraw the following polytropic process of air on p-v and T-s diagram.(1)Pressure rises,temperature increases a

31、nd with heat rejection(2)Working medium expands and the temperature drops with heat rejection at the same time第44页，本讲稿共45页Exercise(练习)(3)Expansion process with n=1.6,please judge the sign of (4)Compression process with n=1.3,please judge the sign ofDraw the following process on the T-s Diagram.第45页，本讲稿共45页