第5章-水蒸气和湿空气课件.ppt

1、ThermodynamicsThermodynamics-Chapter 5 The Water Vapor and-Chapter 5 The Water Vapor and The Air-Water-Vapor Mixture-The Air-Water-Vapor Mixture-The Water VaporThe Water VaporContentsvThe heating process of water at constant pressurevThe states of water vaporvThermodynamic processes of water vaporvS

2、team tables5-1 5-1 The Heating Process of Water at The Heating Process of Water at Constant Pressure Constant Pressure 5-1-1 Basic Conceptions 3.Boiling point（沸点）（沸点）1.Vaporization（汽化）（汽化）2.Liquefaction（液化）（液化）5.Saturation state（饱和状态）饱和状态）saturated liquid saturated vapor 6.Wet vapor（湿饱和蒸汽）湿饱和蒸汽）7.Su

3、perheated vapor（过热蒸汽）过热蒸汽）4.Subcooled liquid（过冷液体）过冷液体）5-1-2 The Heating Process of Water at Constant Pressure（水蒸气的定压产生过程）水蒸气的定压产生过程）The heating process of water at constant pressure in a piston-cylinder（活塞式汽缸中水蒸气的定压产生过程）（活塞式汽缸中水蒸气的定压产生过程）Analysis Assumption：Consider a piston-cylinder device contain

4、ing 1kg liquid water at 0C Under these conditions,keep the outside barometric pressure and the weight of the piston are constant.Heat is now transferred to the water.pHeat 1.Three stages of the heating process of water （水蒸气产生过程的三个阶段）水蒸气产生过程的三个阶段）Subcooled liquidSaturated liquid-vapor mixtureSaturate

5、d vaporSuperheated vaporSaturated liquid水定压预热水定压预热饱和水定压汽化饱和水定压汽化干蒸汽定压过热干蒸汽定压过热pHeatpHeatpHeatpHeatpHeat Heat is transferred to the water,the temperature rises,the liquid water expands slightly,and so its specific volume increases,the piston move up slightly.As more heat is transferred,the temperatur

6、e will keep rising until it reaches saturation temperature corresponding to the saturation pressure.The water be warmed-up at constant pressure （水定压预热）水定压预热）Subcooled liquid Saturated liquid State 2：psat，tsat，v，h，s State 1：psat，t，v，h，sHeat is transferred to the water,the boiling starts,the temperatu

7、re will remain constant during the entire phase-change process,the specific volume increases largely,the liquid lever decreases as a result of more liquid turning to vapor.State 3：psat，tsat，vx，hx，sxContinue transferring heat,the vaporization process will continue until the last drop of liquid is vap

8、orized,The entire cylinder is filled with vapor.State 4：psat，tsat，v”，h”，s”Saturated liquid-vapor mixture Saturated vapor Saturated liquid State 2：psat，tsat，v，h，s The saturated water evaporate at constant pressure（水定压汽化）水定压汽化）The saturated vapor overheat at constant pressure （干蒸汽定压过热）干蒸汽定压过热）Further

9、transfer of heat will result in an increase in both the temperature and the specific volume.The value of temperature is more than that of saturation temperature.Saturated vapor Superheated vapor State 4：psat，tsat，v”，h”，s”State 5：psat，t，v，h，s a b c d epWarm-upvaporationoverheating v v vx v”v v Tsab c

10、 de s sx s”s At certain pressure 2.P-v and T-s diagrams Quality（干度）（干度）The ratio of the mass of vapor to the total mass of the mixturewvvtotalvapormmmmmx Quality has significance for saturated mixture only For saturated liquid x=0,saturated vapor x=1Notes Tsab c de s sx s”sTS 5-1-3 The P-v and T-s D

11、iagramsa1a2a3a4b4b2b3b1d4d3d2d1e4e3e2e1CTcrTABpcr0svppcra1a2a3a4b1e1d1b4b3b2d4d3d2e4e3e2BCATcr0).(4092.4,/9.2085,/003106.0,99.373,064.223KkgkJskgkJhkgmvCtMPapcrcrcrcrcrcritical point（临界点）临界点）saturation liquid line（下临界线）（下临界线）saturation vapor line（上临界线）（上临界线）liquid region（液相区）（液相区）saturated liquid-va

12、por region（汽液两相区）（汽液两相区）vapor region（汽相区）（汽相区）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（湿蒸汽）（湿蒸汽）saturated vapor（干饱和蒸汽）（干饱和蒸汽）superheated vapor（过热蒸汽）（过热蒸汽）Two lines Three regionsFive statesOne point是否有是否有400400CC的水？的水？0C0C或或-10C-10C的水蒸气的水蒸气5-2 Propertie

13、s of Water Vapor5-2 Properties of Water Vapor 5-2-1 Reference State and reference valuesThe difference between ideal gases and real gasesContrast Reference State:the state of saturated water at 0.01C.The internal energy and entropy are assigned zero values at that state.Then)/(0,/0,/00100021.0,16.27

14、3,7.6113KkgkJskgkJukgmvKTTPapptptpEnthalpy0/00061.0/21000001.07.61103kgkJkgmPapvuh 5-2-2 Property Tables（水与水蒸气表）水与水蒸气表）Property tables of saturated liquid and saturated vapor(table 5,6)Property table of subcooled liquid and superheated vapor(table7)Properties are listed under temperature（table5）Prop

15、erties are listed under pressure（table6）Above black line:properties of subcooled liquid Under black line:properties of superheated vaporSteam tables:Property table of saturated liquid and saturated vapor-listed under temperature(table 5)Property table of saturated liquid and saturated vapor-listed u

16、nder pressure(table 6)1.Saturated liquid and saturated vapor states 压力压力温度温度 比体积比体积比焓比焓汽化汽化潜热潜热比熵比熵液体液体蒸汽蒸汽液体液体蒸汽蒸汽液体液体蒸汽蒸汽ptv v hhss0.00106.94910.0010001129.18529.212513.292484.10.10568.9735Pressure ps or temperature ts Saturated liquid：Known:ps/ts,Determine:v,h,s,u=h-psv Saturated vapor：Known:ps/t

17、s，Determine:v,h,s,u”=h”-psv”2.Saturated liquid-vapor mixture 压力压力温度温度 比体积比体积比焓比焓汽化汽化潜热潜热比熵比熵液体液体蒸汽蒸汽液体液体蒸汽蒸汽液体液体蒸汽蒸汽ptv v hhss0.00106.94910.0010001129.18529.212513.292484.10.10568.9735wvvmmmxxsxxxxxvphusxsxshxhxhvxvxv )1()1()1(Saturated mixture 1kg saturated liquid-vapor mixture consists of xkg satu

18、rated vapor and(1-x)kg saturated liquid.Pressure ps(or temperature ts)and quality x Property table of subcooled liquid and superheated vapor(table7)3.Subcooled liquid and superheated vapor 8.34222648.916.2688.66392649.732.5669.40802650.3162.956808.23132610.815.3368.55372611.830.7129.29842612.7153.71

19、7600.5723167.510.00100798.43662574.028.8549.18232575.2144.475400.296383.870.00100180.296383.870.00100189.05882537.7135.226200.151042.010.00100030.151042.010.00100038.99382519.0130.59810-0.0002-0.040.0010002-0.0002-0050.0010002-0.0002-0.050.0010020shvshvshv温度温度t0.010MPa0.005MPa0.001MPa压力压力pPressure p

20、 and temperature t1.Determine the state of water from the steam tables （1）t=45 C,v=0.00100993 m3/kg;（2）t=200 C,x=0.9;（3）p=0.5MPa,t=165C;（4）p=0.5MPa,v=0.545 m3/kgExamples解：（解：（1）t=45 C,v=0.00100993 m3/kg;由已知温度，查饱和水与饱和水蒸气表（附表由已知温度，查饱和水与饱和水蒸气表（附表5）。）。知：知：vkgmv/0010993.03确定该状态为饱和水，查饱和水与饱和蒸汽表得：确定该状态为饱和水，

21、查饱和水与饱和蒸汽表得：)./(6386.0,/42.188,01.0KkgkJskgkJhMPaps分析求解过程分析求解过程（2）t=200 C,x=0.9;该状态为湿蒸汽，查饱和水与饱和水蒸汽表，得：该状态为湿蒸汽，查饱和水与饱和水蒸汽表，得：)./(4312.6),./(3307.2,/47.2792,/34.852 KkgkJsKkgkJskgkJhkgkJh )./(0212.6)./(3307.2)9.01()./(4312.69.0)1(/5.2598/34.852)9.01(/47.27929.0)1(KkgkJKkgkJKkgkJsxxsskgkJkgkJkgkJhxxhhx

22、x ,867.151,5.0CtMPaps ,stt 现现 所以为过热蒸汽状态。所以为过热蒸汽状态。查未饱和水和过热水蒸气表得：查未饱和水和过热水蒸气表得：)./(8647.6,/2.2767,160,5.0KkgkJskgkJhCtMPap )./(9160.6,/6.2789,170,5.0KkgkJskgkJhCtMPap ,165 Ct 焓和熵可从上面两者之间按线性插值求得：焓和熵可从上面两者之间按线性插值求得：)./(8904.6,/7.2778KkgkJskgkJh （3）p=0.5 MPa,t=165C;当当对于对于MPap5.0 3,/37490.0vvkgmv 所以该状态为过

23、热蒸汽状态。所以该状态为过热蒸汽状态。查未饱和水和过热蒸汽表得：查未饱和水和过热蒸汽表得：)./(5297.7,/9.3104,/54164.0,320,5.03KkgkJskgkJhkgmvCtMPap )./(5643.7,/6.3125,/55115.0,330,5.03KkgkJskgkJhkgmvCtMPap 按线性插值求得：按线性插值求得：)./(5422.7,/4.3112,6.323KkgkJskgkJhCt （4）p=0.5 MPa,v=0.545 m3/kg当当 时，饱和蒸汽的比体积时，饱和蒸汽的比体积Conclusion How to judge the state of

24、 substance If know p and t,check property tables of saturated liquid and saturated vapor.When:If know p(or t),as well as property v（or h,or s）,look up property tables of saturated liquid and saturated vapor first,determine the state of vapor.When:)()()(pttpttpttsssSubcooled liquidSaturated state，sti

25、ll given quality xSuperheated vaporvvvvvvv Subcooled liquidSaturated stateSuperheated vaporThen look up property table of subcooled liquid and superheated vapor.Determine the state of vapor.思考思考题题1.临界点时，饱和液体的焓等于干饱和蒸汽的焓。临界点时，饱和液体的焓等于干饱和蒸汽的焓。2.某液体的温度为某液体的温度为T，若其压力大于，若其压力大于T 对应的饱和对应的饱和 压力，则该液体一定处于未饱和液体

26、状态。压力，则该液体一定处于未饱和液体状态。3.干饱和蒸汽被定熵压缩，将变为（干饱和蒸汽被定熵压缩，将变为（）。）。4.知道了温度和压力，就可以确定水蒸气的状态。知道了温度和压力，就可以确定水蒸气的状态。5.水蒸气的定温膨胀过程满足水蒸气的定温膨胀过程满足 Q=W。Tps(T)实际气体实际气体 过热蒸汽过热蒸汽（）（）（x）（x）5-3 Basic processes of water vapor5-3 Basic processes of water vapor （水蒸气的基本热力过程）（水蒸气的基本热力过程）Basic processes of water vapor:Processes

27、of Isochoric,Isobaric,Isothermal,adiabatic.Calculation contents:（1）Properties of the initial and final states:（2）Heat and work.Methods：By means of tables,graphs,and equations,as well as auxiliary calculation of computer.Principles：1st and 2nd laws of thermodynamics Expression of reversible How to fi

28、nd the properties of an initial and final states for an ideal gas?Steps：According to the properties of initial state,usually(p,t)/(p,x)/(t,x),find the rest properties at this initial state by means of the steam tables/graphs.According to one property of final state and the characteristic of the proc

29、ess,find the rest properties of the final state by means of the steam tables/graphs.Calculation q,u,w 2.水蒸气由水蒸气由p1=1MPa,t1=300C可逆绝热膨胀到可逆绝热膨胀到 p2=0.1MPa,求求1kg水蒸气所作的膨胀功和技术功。水蒸气所作的膨胀功和技术功。例例 题题思路：思路：（1）确定初态，及初态参数；）确定初态，及初态参数；（2）确定终态，及终态参数；）确定终态，及终态参数；（3）根据过程特点和能量守恒，）根据过程特点和能量守恒，计算功量。计算功量。Steam tables3.

30、Volume and energy change during evaporation A mass of 200 g of saturated liquid water is completely vaporized at constant pressure of 100 kPa.Determine (a)the volume change and (b)the amount of energy added to the water.例例 题题4.Pressure and volume of a mixture A rigid tank contains 10 kg of water at

31、90.If 8 kg of the water is in the liquid form and rest is in the vapor form.Determine (a)the pressure in the tank and (b)the volume of the tank.例例 题题例例 题题5.Properties of saturated liquid-vapor mixture An 80-L vessel contains 4 kg of refrigerant-134a at a pressure of 160 kPa.Determine (a)the temperat

32、ure of the refrigerant,(b)the quality,(c)the enthalpy of the refrigerant,and (d)the volume occupied by the vapor phase.The Air-Water-Vapor MixtureThe Air-Water-Vapor MixtureContentsvThe saturated air and unsaturated air vThe conceptions of relative humidity and specific humidity 5-4 The Air-Water-Va

33、por Mixture5-4 The Air-Water-Vapor Mixture（湿空气）湿空气）Atmospheric Air（湿空气）：（湿空气）：Air that contains some water vapor(or moisture).Dry Air（干空气）：（干空气）：Air that contains no water vapor.Assumption：Atmospheric air can be treated as an ideal gas mixture.水蒸气分压力很小，多处过热状态，水蒸气分压力很小，多处过热状态，v 较大。较大。The total pressu

34、re is the sum of the partial pressure of dry air Pa and that of water vapor Pv 湿空气的总压力：湿空气的总压力：p=pv+pa The amount of dry air in the air-water-vapor mixture remains constant,but the amount of water vapor changes.湿空气来自湿空气来自环境大气环境大气(pb)5-4-1 The Saturated Air and Unsaturated Air The partial pressure of

35、 the water vapor（水蒸气分压力）（水蒸气分压力）the amount of water vapor in the air 1.The Unsaturated Air Unsaturated air is a mixture of dry air and superheated vapor.If pv ps,the water vapor is in the superheated state.Assume:air temperature T(corresponding to its saturated pressure ps),partial pressure of the w

36、ater vapor pv.Analysis“Unsaturated”means:the value of partial pressure of water vapor is less than that of saturated pressure at air temperature T.max,)(vsvpTppCAvp0pvpsT pv can increase up to ps(T)the amounts of water vapor can increase unsaturated air can absorb waterT0sAPSPVTC如何将过热蒸汽如何将过热蒸汽转变为饱和蒸

37、汽？转变为饱和蒸汽？Method 1:T一定时，增加水蒸气含量，使一定时，增加水蒸气含量，使pv不断增大。当不断增大。当pv=ps(T)，湿空气中水蒸气达到饱和，过热蒸汽湿空气中水蒸气达到饱和，过热蒸汽干饱和蒸汽，干饱和蒸汽，不再具有吸收水分的能力。不再具有吸收水分的能力。“Saturated”means：pv=pv max=ps(T)，the value of the partial pressure of water vapor gets maximum value.At this temperature T,the air can not absorb water.Add water v

38、apor to the air,some vapor condenses out.2.The Saturated Air：Saturated air is a mixture of dry air and saturated water vapor.Assume:air temperature T(corresponding to its saturated pressure ps),partial pressure of the water vapor pv.Analysis How to change the unsaturated air to the saturated airAnal

39、ysisC 法法1：T一定时，增加一定时，增加 pv=pvmax=ps（T）,AC。法法2：保持水蒸气含量（：保持水蒸气含量（pv）不变，）不变，降低湿空气温度降低湿空气温度 T=TS（PV），），AB。BCBDDAvp0pvpsTT0sAPSPVTTd 5-4-2 Dew-Point Temperature（露点露点温度）温度）1.Dew Formation（结露）结露）：保持湿空气中水蒸气的分压力不变，保持湿空气中水蒸气的分压力不变，对饱和湿空气继续冷却降温，则其中对饱和湿空气继续冷却降温，则其中 的部分水蒸气将凝结为水的部分水蒸气将凝结为水.这种现象称为结露。这种现象称为结露。2.Dew

40、-Point Temperature Td is the saturated temperature of water corresponding to the vapor pressure.（湿空气中水蒸气的分压力（湿空气中水蒸气的分压力pv 所对应的饱和温度）所对应的饱和温度）T0sAPSPVTTdNotes If Td 0C,就会出现结霜。就会出现结霜。析湿过程析湿过程)(vdpfT C 5-4-3 Specific and Relative Humidity of Air （相对湿度和含湿量）相对湿度和含湿量）Humidity（湿空气的湿度）湿空气的湿度）The amount of w

41、ater in the air（湿空气中水蒸气的含量）湿空气中水蒸气的含量）1.Absolute Humidity（绝对湿度）绝对湿度）（1）The mass of water vapor present in 1m3 of air.（1m3的湿空气中所含水蒸气的质量）的湿空气中所含水蒸气的质量）（2）ExpressionTRpVmvgvvv,（3）For saturated air TRpVmvgsvs,水蒸气的体积水蒸气的体积 2.Relative Humidity（相对湿度）相对湿度）（1）Definition：The ratio of the amount of moisture th

42、e air holds relative to the maximum amount of moisture the air can hold at the same temperature.（湿空气的绝对湿度与同温度下湿空气的最大绝对湿度之比）（湿空气的绝对湿度与同温度下湿空气的最大绝对湿度之比）svsvpp（2）Notes：相对湿度相对湿度越小，湿空气中水蒸气偏离饱和状态越远，越小，湿空气中水蒸气偏离饱和状态越远，空气越干燥，吸湿能力越强。空气越干燥，吸湿能力越强。10 For dry air:=0；For saturated air=1，TRpvgvv,TRpvgss,3.Specifi

43、c Humidity（含湿量含湿量/比湿度）比湿度）（1）Definition:The mass of water vapor present in a unit mass of dry air.（在湿空气中，与单位质量干空气共存的水蒸气含量）（在湿空气中，与单位质量干空气共存的水蒸气含量）（2）Expressionavavmmd（3）The Relationship of PV,d and TRpVmvgvvv,According to ideal-gas equation of stateTRpTRpagaavgvv,avppp)/(287)/(5.461,KkgJRKkgJRagvgss

44、pppd622.0vvavpppppd622.0622.0So：svsvpp 5-4-45-4-4 湿空气的相对分子质量，气体常数及密度湿空气的相对分子质量，气体常数及密度 1.湿空气的相对分子质量：湿空气的相对分子质量：ppMMMvvrvarar954.1097.28,016.1897.28,vrarMM说明说明arrMM,2.湿空气的气体常数：湿空气的气体常数：)/(378.01287KkgJppMRRvrg说明说明Rg随水蒸气分压力随水蒸气分压力 pv的提高而增大。的提高而增大。3.湿空气的密度：湿空气的密度：TpTpVmvva001317.0287说明说明湿空气的密度永远小于干空气的密

45、度。湿空气的密度永远小于干空气的密度。思考思考题题1.相对湿度为相对湿度为=0时，说明空气中完全没有水蒸气。时，说明空气中完全没有水蒸气。2.相对湿度为相对湿度为=1时，说明空气中都是水蒸气。时，说明空气中都是水蒸气。3.空气的相对湿度越大，其含湿量越高。空气的相对湿度越大，其含湿量越高。4.对未饱和湿空气，露点温度即水蒸气分压力所对未饱和湿空气，露点温度即水蒸气分压力所 对应的水的饱和温度。对应的水的饱和温度。sspppd622.05.pV=nRT 公式适用于：公式适用于：（1）水蒸气（）水蒸气（2）湿空气（）湿空气（3）理想气体）理想气体6.湿空气的湿空气的d一定时，温度越高，其吸湿能力：

46、一定时，温度越高，其吸湿能力：（1）越强（）越强（2）越弱（）越弱（3）相等）相等7.湿空气压力一定时，其中水蒸气的分压力取决于湿空气压力一定时，其中水蒸气的分压力取决于 （1）含湿量（）含湿量（2）相对湿度（）相对湿度（3）露点）露点思考思考题题sspppd622.0问答问答题题1.冬季，室内玻璃窗内侧为什么会结霜？冬季，室内玻璃窗内侧为什么会结霜？2.为什么阴天晾衣服不宜干，而晴天易干？为什么阴天晾衣服不宜干，而晴天易干？3.冬季室内供暖，为何要加湿？冬季室内供暖，为何要加湿？4.霉季时，冷水管的表面常有水滴出现，为什么？霉季时，冷水管的表面常有水滴出现，为什么？3.房间的容积为房间的容积为50m3,室内空气温度为室内空气温度为30C，相对湿度为相对湿度为60%，大气压力为，大气压力为0.1013MPa，求：湿空气的露点温度求：湿空气的露点温度td，含湿量含湿量d，空气的，空气的 质量质量ma，水蒸气质量，水蒸气质量mv。例题例题 总结：总结：要确定湿空气的状态，除压力要确定湿空气的状态，除压力 p，温度，温度 t 外，还需知道湿空气中水蒸气的含量，外，还需知道湿空气中水蒸气的含量，即知道即知道v,d,pv,td 中的一个。中的一个。