第六章 水和水蒸气的性质

1、 Chapter 6. The property of Water and Vapor (第第6章章. 水和水蒸气的性质水和水蒸气的性质)6.1 Basic conceptions (基本概念基本概念)6.2 Phase change Process of Pure Substance (纯质的纯质的相变过程相变过程)6.3 Property Tables (水蒸气性质表水蒸气性质表) 6.4 h-s diagrams for Water Vapor (水蒸气的水蒸气的h-s图图)6.5 Thermodynamic Processes of Water Vapor (水蒸气的热力过程水蒸气的热

2、力过程)水和水蒸气是实际气体的代表水和水蒸气是实际气体的代表水蒸气水蒸气在空气中含量极小，当作在空气中含量极小，当作理想气体理想气体一般情况下，为一般情况下，为实际气体实际气体，使用图表，使用图表18世纪，蒸气机的发明，是唯一工质世纪，蒸气机的发明，是唯一工质直到内燃机发明，才有燃气工质直到内燃机发明，才有燃气工质目前仍是火力发电、核电、供暖、化工的工质目前仍是火力发电、核电、供暖、化工的工质 优点优点: 便宜，易得，无毒，便宜，易得，无毒， 膨胀性能好，传热性能好膨胀性能好，传热性能好是其它实际气体的代表是其它实际气体的代表 6-1 纯物质的热力学面及相图纯物质的热力学面及相图物质三种聚集状

3、态：物质三种聚集状态： 固态、固态、 液态、液态、 气态气态水的三态：水的三态：冰、冰、 水、水、 蒸汽蒸汽Pure substance Solid Liquid Gas Ice Water Steam 任何一种物质都可以经历下列所述的相变任何一种物质都可以经历下列所述的相变. Any kind of substance may undergo various types of phase changes as following : 溶解溶解(Fusion/melting) = solid to liquid 凝固凝固(Freezing) = liquid to solid 气化气化(Vap

4、orization)= liquid to gas 凝结凝结(Condensation)= gas to liquid 升华升华(Sublimation) = solid to gas Phase changes (相变相变):纯物质的纯物质的p-T相图相图ppTT液液液液气气气气固固固固水水一般物质一般物质三相点三相点三相点三相点临界点临界点临界点临界点流体流体流体流体升华线升华线升华线升华线融解线融解线融解线融解线汽化线汽化线汽化线汽化线p-T diagram of pure substancespTLiquidVaporSolidTriple pointCritical pointFlu

5、idSublimationMeltingVaporizationThe sublimation line separates the solid and vapor regions;The vaporization line separates the liquid and vapor regions; it ends at the critical point.The melting line separates the solid and liquid regions;The three lines meet at the triple point.Triple point(三相点）：三相

6、点）：all three phases coexist in equilibrium. (三种相态以平衡共存三种相态以平衡共存)Critical point (临界点临界点): The point at which the saturated liquid and saturated vapor states are identical. No distinction can be made between liquid and vapor phases above the critical point. (临界点就是气相与液相一致的状态点临界点就是气相与液相一致的状态点,温度高于临界温度高于

7、临界点之上点之上,气相与液相没有明显的区别气相与液相没有明显的区别) 临界点临界点 Critical pointcoc3c22.129MPa647.30K (374.15 C)m0.00326kgpTv水的临界点水的临界点状态状态饱和液线与饱和气线的交点饱和液线与饱和气线的交点气液两相共存的气液两相共存的pmax,Tmax等温线是鞍点等温线是鞍点临临界界点点 思考题思考题3. 有没有有没有500C的水的水？1. 溜冰冰刀溜冰冰刀2. 北方冬天晾在外边的衣服，北方冬天晾在外边的衣服，是否经过液相是否经过液相4. 有没有有没有-3 的蒸汽的蒸汽？5. 一密闭容器内有水的汽液混合物，对其一密闭容器内

8、有水的汽液混合物，对其 加热，是否一定能变成蒸汽？加热，是否一定能变成蒸汽？复习：纯物质的复习：纯物质的p-T相图相图ppTT液液液液气气气气固固固固水水一般物质一般物质三相点三相点三相点三相点临界点临界点临界点临界点流体流体流体流体coc3c22.129MPa647.30K (374.15 C)m0.00326kgpTv饱和线、三相线和临界点饱和线、三相线和临界点pv饱和气线三相线饱和液线饱和固线T临界点Vapor and Liquid (汽相和液相汽相和液相)锅锅炉炉汽轮机汽轮机发电机发电机给水泵给水泵凝凝汽汽器器过热器过热器因固相不流动，因固相不流动，更关心汽液两相更关心汽液两相冰蓄冷冰

9、蓄冷6-2 汽化与饱和汽化与饱和气化气化(Vaporization): 由液态变成气态的物理过程由液态变成气态的物理过程 (不涉及化学变化不涉及化学变化)凝结凝结(Condensation)： 由气态变成液态的物理过程由气态变成液态的物理过程 6.2.1 6.2.1 汽化与凝结汽化与凝结 The rate of condensation depends upon the density of vapor in the space above the liquid.(凝结的速率取决于液面上方气相空间气体分子的密度凝结的速率取决于液面上方气相空间气体分子的密度,即蒸气压力即蒸气压力)Liquid

10、molecule has to overcome the surface tension to become vapor, therefore vaporization consumes energy. ( 液相分子必须克服表面张力才能进入气相空间而气化液相分子必须克服表面张力才能进入气相空间而气化,因此气化是要耗因此气化是要耗能的能的)The rate of Vaporization depends on the liquid temperature.(气化的速率取决于液相的温度气化的速率取决于液相的温度)()(vcpfgw)(1Tgwv凝结）(on condensati cvww 气化）(

11、zation vaporicvww 平衡状态）( mequilibriu cvww The equilibrium is called Saturation State.(饱和状态）饱和状态） The properties at this state is called Saturation Properties. (饱和饱和参数）参数）sTsp6.2.2 饱和状态饱和状态Saturation state饱和状态：汽化与凝结的动态平衡饱和状态：汽化与凝结的动态平衡At the equilibrium state, the pressure corresponding to only one t

12、emperature. That is, .饱和温度饱和温度Ts饱和压力饱和压力ps一一对应一一对应放掉一些水，放掉一些水，Ts不变，不变， ps？TspsSaturation temperatureSaturation pressure)(Tfp ps=1.01325barTs=100 青藏青藏ps=0.6barTs=85.95 高压锅高压锅ps=1.6barTs=113.32 )(ssTfp 6.2.3 汽化汽化 Vaporization沸腾：沸腾：表面和液体内部同时发生的汽化表面和液体内部同时发生的汽化 It is an intensive vaporization phenomenon

13、 occurs in the liquid.( (气体和液体均处在饱和状态下气体和液体均处在饱和状态下) )汽化汽化: 由液态变成气态的物理过程由液态变成气态的物理过程 (不涉及化学变化不涉及化学变化) 蒸发：蒸发：汽液表面上的汽化汽液表面上的汽化 BoilEvaporationEvaporation occurs at any temperature and pressure.（蒸发可在任何温度和压力下发生）（蒸发可在任何温度和压力下发生） Evaporation rate depends on the free surface area, the temperature, the flow

14、 rate above,etc.（蒸发的速度与自由液面表面积，液体温度，液面风速等有关（蒸发的速度与自由液面表面积，液体温度，液面风速等有关Boiling can only occur as temperature reaches the saturation temperature corresponding to the specified pressure or the pressure drops below the saturation pressure corresponding to the specified temperature. （沸腾只能当温度达到给定压力所对应的饱和温

15、度或压力降到给定（沸腾只能当温度达到给定压力所对应的饱和温度或压力降到给定温度所对应的饱和压力时温度所对应的饱和压力时,才能发生才能发生)6-3 水蒸气的定压发生过程水蒸气的定压发生过程t tsv vv = vv = vv v v未饱和水未饱和水饱和水饱和水 饱和湿蒸汽饱和湿蒸汽 饱和干蒸汽饱和干蒸汽 过热蒸汽过热蒸汽h hh = hh = hh h hs ss = ss = ss s s水预热水预热汽化汽化过热过热6.3.1 P=constant (压力压力 p 是定值是定值) Preheat Stage (预热阶段预热阶段)Compressed liquid to Saturated li

16、quid.(未饱和液体未饱和液体 饱和液体饱和液体)Compressed liquid/subcooled liquid is not about to vaporization.As heat added, t slightly . When It becomes Saturated liquid. The liquid is about to vaporize. It is the state at which it is still a liquid, but any heat addition will cause some of the liquid to vaporize.)(pf

17、ttsvvstt sttt过冷Vaporization Stage (气化阶段气化阶段)Saturated liquid to Saturated vapor (饱和液体饱和液体 饱和蒸气饱和蒸气)Saturated vapor is a vapor about to condense.A substance at state between Saturatedliquid and Saturated Vapor is calledSaturated liquid-vapor mixture/wetvapor. Latent heat of Vaporization (气化潜热气化潜热):Th

18、e amount of energy absorbed during vaporizationvvstt Superheat Stage (过热阶段过热阶段)Superheated vapor is the vapor that is not about to condense. stt vtsttt 过热注意：水蒸气定压发生过程说明注意：水蒸气定压发生过程说明(1)()QUWUpdVUp VUpVH (2)fg0SSS (3) 理想气体理想气体( )hf T实际气体汽化时，实际气体汽化时，TTs不变，但不变，但h增加增加hh汽化潜热汽化潜热(4) 未饱和水未饱和水过冷度过冷度sttt过冷过冷

19、水过冷水过热蒸汽过热蒸汽过热度过热度sttt过热只有熵加热时永远增加只有熵加热时永远增加State of Liquid and vapor未饱和液，过冷液未饱和液，过冷液饱和液饱和液饱和湿蒸气饱和湿蒸气饱和蒸气饱和蒸气过热蒸气过热蒸气Saturated liquidSaturated vaporSaturated liquid-vapor mixtureSuperheated vaporCompressed liquidSubcooled liquid汽化潜热汽化潜热Latent heat of Vaporization压缩液压缩液 6.3.2 定压气化过程的定压气化过程的P-v P-v 图和

20、图和T-s T-s 图图 P-v and T-s diagramscpcTcTABCACBcTTcTT cppcpp3a3b3d3e3c3a3b3d3e3c2c2a2b2d2e2c2a2b2d2e1c1a1b1d1e1c1a1b1d1eTsp-v图，图，T-s图上的水蒸气定压加热过程图上的水蒸气定压加热过程pvcp一点，二线，三区，五态一点，二线，三区，五态Saturated liquid line, SLL is formed by connecting a series of boiling points. saturated vapor line, SVL：Connecting a se

21、ries of points at dry saturated vapor builds a line, known as saturated vapor line, SVL. Vaporization continues by further heat supply to the system until no liquid is left. This state is known as dry saturated vapor, e.g. point d. If the system is slightly cooled at this state, then droplets of liq

22、uid will begin to form. The state of substance between saturated liquid and dry vapor is called wet vapor. Further heating of a dry saturated vapor at constant pressure causes a rise of vapor temperature and it becomes superheated vapor. The state of substance is completely defined by its pressure a

23、nd temperature if it is in liquid or superheated vapor phase.vOne point (一点一点):critical point (临界点临界点)Two lines (两线两线):SLL and SVL (饱和液体线和饱和蒸气线饱和液体线和饱和蒸气线)Three regions(三区三区): Subcooled liquid region (未饱和液体区未饱和液体区) Saturated liquid-vapor mixture region(湿蒸气区湿蒸气区) Superheated vapor region (过热蒸气区过热蒸气区)

24、Five states(五态五态): Subcooled liquid(未饱和液体未饱和液体) Saturated liquid (饱和液体饱和液体) Saturated liquid-vapor mixture (湿蒸气湿蒸气) Saturated vapor (干饱和蒸气干饱和蒸气) Superheated vapor (过热蒸气过热蒸气) 6.4.1 Types of Property Tables (水蒸汽表的分类水蒸汽表的分类) 1. Saturated water Tables (饱和水和饱和蒸汽表饱和水和饱和蒸汽表) Saturated water-T Tables(以温度以温度

25、t为独立变量排列为独立变量排列) Saturated water-p Tables(以压力以压力p为独立变量排列为独立变量排列) 2. Subcooled water and Superheated vapor Table (未饱未饱和水和过热水蒸汽表和水和过热水蒸汽表) P and t are dependent variables (以以p和和t为自变量为自变量) 2、Reference State and Reference Values (基准点的规定基准点的规定) For water, the saturated liquid at 0.01 is taken as referenc

26、e state. (取水的三相点，即取水的三相点，即0.01 oC为基准点，规定在为基准点，规定在此温度下液态水的热力学能和熵为零。此温度下液态水的热力学能和熵为零。) The internal energy and entropy are assigned zero at this state.kgkJu/ 00)/( 00KkgkJs3. Properties of Saturated liquid-vapor mixture at Pressure p (压力为压力为p的湿饱和蒸汽的湿饱和蒸汽) The state of wet vapor can not be defined by j

27、ust pressure and temperature until one other property is given. The condition or quality of wet vapor is often defined by its dryness or wetness fraction. 定义干度定义干度x才可以确定湿蒸汽的状态。才可以确定湿蒸汽的状态。 We define dryness as dryness fractiondryness fraction, x = the mass of dry vapor in 1 kg of the mixture, , x =

28、the mass of dry vapor in 1 kg of the mixture, wetness fractionwetness fraction, 1 - x = the mass of liquid in 1 kg of the mixture. , 1 - x = the mass of liquid in 1 kg of the mixture. mmxvvxvxvx )1 (hxhxhx )1 (sxsxsx )1 ( vxvxvx )1 (hxhxhx )1 (sxsxsx )1 (注意注意:饱和表的起始点饱和表的起始点ptsv(m3/kg)vsskJ/(kg.K)0.0

29、061120.01 0.00100022 206.1750.09.15621.099.63 0.00104341.6946 1.3027 7.36085.0151.85 0.00109280.374811.8604 6.821550.0263.92 0.00128580.03941 2.9209 5.9712221.29374.15 0.003260.00326 4.4294.429(bar)()bdbd4. Exercise. Consulting property tables of water vapor to determine the state of each point and

30、their h，s，x.(利用水蒸气表判断下列各点的状态，并确利用水蒸气表判断下列各点的状态，并确定其定其h，s，x的值。的值。) o （1）由饱和水和饱和蒸汽表查得p=2MPa时， 显然 ，可知该状态为过热蒸汽过热蒸汽。 查未饱和水过热蒸汽表，得p=2MPa，t=300 时 o （2） 查饱和表得查饱和表得p=9MPa时时 可见可见 ， 该状态为该状态为湿蒸汽湿蒸汽，其干度为，其干度为 又查饱和表得又查饱和表得=2489.0kJ/kg按湿蒸汽的参数计算式得按湿蒸汽的参数计算式得 ( 3 ) 显然，该状态为湿蒸汽状态。由已知参数查饱和水和饱和显然，该状态为湿蒸汽状态。由已知参数查饱和水和饱和蒸

31、汽表得蒸汽表得 按湿蒸汽的参数计算公式得按湿蒸汽的参数计算公式得 （4）由饱和水和饱和蒸汽表查得）由饱和水和饱和蒸汽表查得:当当p=1.0MPa时，时，ts=179.9，显然显然tts，所以该状态为，所以该状态为未饱和水未饱和水。通常通常t=175的状态参数可利用的状态参数可利用t=170与与t=180的对应状态的对应状态参数内插得到，参数内插得到，但此处但此处t=170与与t=180跨越了未饱和表中的黑粗线，跨越了未饱和表中的黑粗线，说明它们分别处于不同相区。说明它们分别处于不同相区。应使内插在未饱和水区内进行，选取离最接近的应使内插在未饱和水区内进行，选取离最接近的t=170与与ts=17

32、9.9的未饱和水参数内插。的未饱和水参数内插。查未饱和水和过热蒸汽表得查未饱和水和过热蒸汽表得p=1.0MPa，t=170时 p=1.0MPa，t=179.9时 于是t=175时 对于未饱和水干度对于未饱和水干度x无意义。无意义。 （5） 时，饱和蒸汽比体积时，饱和蒸汽比体积 ， 该状态该状态为为过热蒸汽过热蒸汽。查过热蒸汽表得。查过热蒸汽表得 t=260时，时， , t=270时，时， 该状态的温度可由比体积值求得该状态的温度可由比体积值求得 o 讨论讨论应该注意，在利用未饱和水与过热蒸汽作应该注意，在利用未饱和水与过热蒸汽作内插时，不允许跨越表中的粗折线，如遇内插时，不允许跨越表中的粗折线

33、，如遇这种情况，应选用更详细的表，或使内插这种情况，应选用更详细的表，或使内插计算在未饱和水（或过热蒸汽）区内进行。计算在未饱和水（或过热蒸汽）区内进行。 6.5 h-s diagram of water vapor (水蒸汽的水蒸汽的h-s图图)1.SLL and SVL (界限曲线，界限曲线，x=0、x=1两条曲线两条曲线)2. Constant Pressure Line and Constant Temperature Line (定压线和定温线定压线和定温线)In Saturated liquid-vapor region, constant pressure line and co

34、nstant temperature line are identical. ( 在湿蒸汽区定压线与定温线重合。在湿蒸汽区定压线与定温线重合。)3. Constant Volume Line（定容线。（定容线。)4. Constant Dryness Line (定干度线。定干度线。) Given Two Properties, all the other state properties can be determined from the Diagram.(给定两个参数，即可查出其余全部参数)。 Priorities (优点优点)：Fast and convenient. 方便、快速。方便

35、、快速。Drawbacks (缺点缺点)：Not so accurate as Property tables 精确性稍差精确性稍差。 s (kJ/kg*K)h (kJ/kg)6.6.1 Characteristics of water vapor (水蒸汽的特点水蒸汽的特点)1. Water vapor can not be treated as ideal gas (水蒸汽不水蒸汽不是理想气体是理想气体) pv=RT is not suitable. （理想气体状态方程不适用。）（理想气体状态方程不适用。）2. Analysis and Calculation on Water vapor

36、 can be solved by using Tables and Diagrams ( 水蒸汽的分析和水蒸汽的分析和计算采用水蒸汽图和表。计算采用水蒸汽图和表。)3. The First and the Second Laws must be obeyed during processes of water vapor (热力学第一定律和第二热力学第一定律和第二定律成立定律成立)6.6 Thermodynamic Processes of Water Vapor (水蒸气的热力过程水蒸气的热力过程)wuqpvuh For reversible processes, (对于可逆过程：对于可逆

37、过程：)21pdvw21Tdsqwzgchq2216.6.2 Procedures (步骤步骤)(1) Based on the given conditions, determine the initial state and its properties .(根据已知条件，确定初始状态，查出其余参数。根据已知条件，确定初始状态，查出其余参数。)(2) Based on the characteristics of the process and one of the properties of the final state, determine the final state and its properties. ( 根据过程特点和一个终态参数，确定终态，再查出其余终态参数。根据过程特点和一个终态参数，确定终态，再查出其余终态参数。)(3) Based on the initial and final state, ca