课件工程热力学yu chapter.properties of a pure,simple compressible substance

1、CHAPTER 3. PROPERTIES OF A PURE,SIMPLE COMPRESSIBLE SUBSTANCE3.1Pure SubstanceA pure substance is defined as a substance that has a fixed chemical composition throughout.A pure substance can be a mixture of various chemical elements or compounds as long as its homogeneous.A pure substance can be a m

2、ixture of two or more phases as long as the chemical composition of all phases is the same. Water,nitrogen,helium, carbon dioxide, air, a mixture of ice and liquid water Examples of pure substance3.2The State PrincipleThe state principle is a general rule developed as a guide in determine the number

3、 of independent properties needed to specify the state of a system.An independent property can be associated with heat transfer, or one kind of relevant work the energy can be changed.The number of independent properties for a system is one plus the number of relevant work interactions.The state pri

4、nciple is often interpreted in terms of intensive properties for convenience.3.3Simple SystemsA simple system is defined when there is only one way the system energy can be significantly altered by work as the system undergoes a quasi-equilibrium process.A simple system is one which contains only tw

5、o independent properties, one for heat transfer and another for the single work mode.A simple compressible system is defined as a system free of magnetic, electrical, or surface tension effects. The only mode of work that can occur in a simple compressible system is associated with volume change whi

6、ch is given by pdV, or W= pdVTwo independent properties T and v are sufficient to fix the state of a simple compressible system at equilibrium. p=p(T,v)3.4PropertiesProperties of a system are defined as any measurable quantities which possess certain characteristics of the system. Types of property:

7、Dependent property, p = S(x1, x2, xi . xn);Independent property, xi;Intensive property, T,v,p;Extensive property, m,V,E,S,H,U,etc.3.5Phases of a Pure SubstanceSolid- The molecules are at relatively fixed positions in solid phase. Liquid-Groups of molecules move about each other in the liquid phase.G

8、as-Molecules move about at random in gas phase.Phase change-A substance changes its phases and absorbs or releases latent heat as the changing of temperature or pressure.Saturated state-A state at which a phase change begins or ends.Saturated liquid-A liquid that is about to vaporize.Saturated vapor

9、-A vapor that is about to condense.Saturated liquid-vapor mixture-Liquid and vapor phases coexist in equilibrium.Saturated temperature Tsat-At a given pressure, the temperature at which a pure substance changes phase.Saturated pressure psat-At a given temperature, the pressure at which a pure substa

10、nce changes phase. Subcooled liquid (compressed liquid)-A pure substance exists in the liquid phase. The temperature at this state is less than the saturation temperature at the given pressure, TTsatThe quality of the liquid-vapor mixture x-The ratio of the mass of vapor present to the total mass of

11、 the mixture.Latent heat -The amount of energy absorbed or released during a phase-change process. Latent heat of fusion-The amount of energy absorbed or released during melting or solidification process. eg. Water: 333.7kJ/kg at 1 atm pressure.Latent heat of vaporization-The amount of energy absorb

12、ed or released during boiling(evaporation) or condensation process.eg. Water: 2257.1kJ/kg at 1 atm pressure.3.6p-v-T RelationFrom experiment it is known that temperature T and specific volume v can be regarded as independent and pressure p determined as a function of them: p=p(T,v)The graph of the f

13、unction is a surface, p-v-T surface.单元工质的相图与相转变 P-T图饱和状态：不同相平衡共存的状态汽化或凝结 ；熔解或凝固升华或凝华 饱和温度 饱和压力 三相点 临界点，在临界压力以上液、气两个相区不存在明显的、确定的界线。 单元工质的相图与相转变P-v图 T-s图Phase DiagramPlasmaGasVaporLiquidSolidTtripleTcriticalPtriplePcriticalPressureTemperatureCritical PointTriplePointBoilingCondensationSublimationMelti

14、ngFreezingCritical point-At the top of the dome where the saturated liquid line and saturated vapor line meet.The critical temperature Tc-The maximum temperature at which liquid and vapor phases can coexist in equilibrium. For water, Tc=374.14The critical pressure pc-The pressure at the critical poi

15、nt. For water, pc=22.09MPaThe critical specific volume vc-The specific volume at the critical point. For water, vc=0.003155m3/kgValues of the critical point properties for a number of substances are given in Tables A-1 in appendix p.722.At pressure above the critical pressure, there will not be a di

16、stinct phase-change process. Instead, the specific volume of the substance will continually increase, and at all times there will be only one phase present which can be described simply as a fluid. Triple line-Three phases can exist in equilibrium along this line on the p-v-T surface. Triple point-T

17、hree phases can exist in equilibrium on this point on the phase diagram (p-T diagram).For water, Ttp=273.16K, ptp=0.6113kPa三张图三个区（气，液，固）(液相，湿蒸气，过热蒸气）三条线（饱和线）两个点（临界点和三相点）单元工质的相图与相转变3.7Property TablesTables in SI units-Table A-1 through A-32Tables in English units-Table A-1E through A-30EThe steam tab

18、les-Table A-2 through A-6Refrigerant 12-Table A-7 through A-9 Refrigerant 134a-Table A-10 through A-12Ammonia- Table A-13 through A-15Refrigerant 22-Table A-16 through A-18Linear interpolation can be used with acceptable accuracy for the solved examples and end-of-chapter problems.For a two-phase li

19、quid-vapor mixture: Quality As the total volume V=Vliq+VvapThe average specific volumeorExample 3.13.8 A new property -Enthalpy Define a new property-a combination property : enthalpy H U + pVor the specific enthalpy defined as h=u+pvFor a two-phase liquid-vapor mixture:The specific internal energy

20、The specific enthalpy Reference State and Values:The difference of u,h,and s between two states are important. The tabular values of u,h in the appendix are relative to the following reference states and values: For water, the reference state is saturated liquid at 0.01, where u=0, h=u+pv;For ammoni

21、a and the refrigerants, the reference state is saturated liquid at -40, where h=0, u=h-pv. 参考状态和参考点的意义 对于u，h等状态参数表中给定的数值，并不是通过直接测量得到的，而是通过其它更易于实验测量的数据进行计算得到的。当应用能量平衡分析时，我们最关心的是两个状态前后内能、动能以及势能的差异，而不是它们在各自状态时的数值。状态参数的值是用来决定系统状态的相对差异，这样才能反映系统的状态变化。Example 3.2Example 3.3As u = u(T, v) and h = h(T,p) du

22、= (u/T)v dT + (u/v)T dv dh = (h/T)p dT + (h/p)T dpDefine,Constant volume specific heat cv by cv = (u/T)v ; du = cvdT + (u/v)T dv Constant pressure specific heat cp bycp = (h/T)p ; dh = cp dT + (h/p)T dp3.9 Specific Heats Cv and Cp-Intensive propertiesFor liquid, v,u and h can approximately using sat

23、urated liquid data in many engineering calculations as v and u change very little with pressure at a fixed temperature. v(T,p)vf(T), u(T,p)uf(T) and h(T,p)uf(T)+pvf(T) hf(T)+ vf(T) p-psat(T) or h(T,p)hf(T) when the last term is small.3.10 Approximations for liquidsv=const. cv = du/dT As h(T)=u(T)+pv

24、 , (dh/dT)p = du/dT Thus c= cp = cv (Table A-19,p.756)The specific heat C can be treated as constant as the variation of C with temperature is small over limited temperature intervals. u2-u1=c(T) dT=c(T2-T1) h2-h1= u2-u1+v(p2-p1)=c(T2-T1)+v(p2-p1)3.11 pressible Substances3.12 The Compressibility Fac

25、tor - a dimensionless ratio for various gasesThe virial expansions for gases:orand Z=1 when p0 or v , that the molecular interactions of a gas e weaker.For ideal gases: 压缩因子Z是工质体积Vm与按理想气体状态方程计算体积得的体积值的比值。表达了实际工质性质与理想气体性质的偏差Z离1越远，工质偏离理想气体越远。Generalized Compressibility Chart for various gases - The pr

26、inciple of corresponding statesWhere reduced pressure pR=p/pc , pc is the critical p; reduced temperature TR=T/Tc , Tc is the critical T. And pseudo-reduced specific volume Quantitative similarity with various pairs of the valuables TR,pR, and vR are achieved for various gases on the same coordinate

27、 axes such as shown in Fig. A-1,A-2,A-3.对应态定律实验：具有相同对比（折合）温度和对比（折合）压力时，各种气体的压缩因子相等。The Ideal Gas 理想气体定义： 1、 忽略气体分子间相互作用力和分子本身体积影响,仅具有弹性质点的气体 2、注意：当实际气体p0 的极限状态时，气体为理想气体。 3、 可用简单的式子描述 如汽车发动机和航空发动机以空气为主的燃气、气体燃料、空气等研究意义1、在通常工作参数范围内，按照理想气体计算有足够精度2、是研究工质热力性质的基础，其它工质热力性质往往是在理想气体的基础上修正得到。3.13 The Ideal Gas

28、 Equation of State ( p-v-T Relation for Ideal Gases) where (kJ/kmolK) -universal gas constant, the same value for all gases : (As shown in Fig.3.9)R(kJ/kgK) -a constant for the particular gas whose molecular weight is M (kg/kmol). (m3/kmol): 理想气体状态方程VM：摩尔容积m3/kmol； RM ：通用气体常数，8.314J/kmolK；V：nKmol气体容

29、积m3；P：绝对压力Pa ；v：比容m3/kg； T：热力学温度K V：质量为mkg气体所占的容积；3.14 The Ideal Gas ModelAny gas whose equation of state is given by The specific internal energy depends only on temperature.理想气体热力学能和焓只是温度的函数The Ideal Gas ModelFrom the microscopic point of view: The gas consists of a large number of molecules that are in random motion and obey the laws of mechanics;The volume of the molecules occupied is negligibly small compared with the total gas volume.There is no appreciable forces act on the molecules except during collisions. For a gas obeying the ideal gas