第六章-水和水蒸气的性质

Chapter6.ThepropertyofWaterandVapor(第6章.水和水蒸气的性质)6.1Basicconceptions(基本概念)6.2PhasechangeProcessofPureSubstance(纯质的相变过程)6.3PropertyTables(水蒸气性质表)6.4h-sdiagramsforWaterVapor(水蒸气的h-s图)6.5ThermodynamicProcessesofWaterVapor(水蒸气的热力过程)水和水蒸气是实际气体的代表水蒸气在空气中含量极小，当作理想气体一般情况下，为实际气体，使用图表18世纪，蒸气机的发明，是唯一工质直到内燃机发明，才有燃气工质目前仍是火力发电、核电、供暖、化工的工质优点:便宜，易得，无毒，膨胀性能好，传热性能好是其它实际气体的代表§6-1纯物质的热力学面及相图物质三种聚集状态：固态、液态、气态水的三态：冰、水、蒸汽PuresubstanceSolidLiquidGasIceWaterSteam任何一种物质都可以经历下列所述的相变.Anykindofsubstancemayundergovarioustypesofphasechangesasfollowing:溶解(Fusion/melting)=solidtoliquid凝固(Freezing)=liquidtosolid气化(Vaporization)=liquidtogas凝结(Condensation)=gastoliquid升华(Sublimation)=solidtogasPhasechanges(相变):Thesublimationlineseparatesthesolidandvaporregions;Thevaporizationlineseparatestheliquidandvaporregions;itendsatthecriticalpoint.Themeltinglineseparatesthesolidandliquidregions;Thethreelinesmeetatthetriplepoint.Triplepoint(三相点）：allthreephasescoexistinequilibrium.(三种相态以平衡共存)Criticalpoint(临界点):Thepointatwhichthesaturatedliquidandsaturatedvaporstatesareidentical.Nodistinctioncanbemadebetweenliquidandvaporphasesabovethecriticalpoint.(临界点就是气相与液相一致的状态点,温度高于临界点之上,气相与液相没有明显的区别)思考题3.有没有500ºC的水？1.溜冰冰刀2.北方冬天晾在外边的衣服，是否经过液相4.有没有-3℃的蒸汽？5.一密闭容器内有水的汽液混合物，对其加热，是否一定能变成蒸汽？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VaporandLiquid(汽相和液相)锅炉汽轮机发电机给水泵凝汽器过热器因固相不流动，更关心汽液两相冰蓄冷§6-2汽化与饱和气化(Vaporization):由液态变成气态的物理过程(不涉及化学变化)TheProcessofChangingfromliquidintogas/vaporiscalledvaporization.凝结(Condensation)：由气态变成液态的物理过程Theprocessofchangingfromgas/vaporintoliquidiscalledCondensation6.2.1汽化与凝结Therateofcondensationdependsuponthedensityofvaporinthespaceabovetheliquid.(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)Liquidmoleculehastoovercomethesurfacetensiontobecomevapor,thereforevaporizationconsumesenergy.(液相分子必须克服表面张力才能进入气相空间而气化,因此气化是要耗能的)TherateofVaporizationdependsontheliquidtemperature.(气化的速率取决于液相的温度))()(vcpfgw)(1Tgwv凝结）(oncondensaticvww气化）(zationvaporicvww平衡状态）(mequilibriucvwwTheequilibriumiscalledSaturationState.(饱和状态）ThepropertiesatthisstateiscalledSaturationProperties.(饱和参数）sTsp6.2.2饱和状态Saturationstate饱和状态：汽化与凝结的动态平衡Attheequilibriumstate,thepressurecorrespondingtoonlyonetemperature.Thatis,.饱和温度Ts饱和压力ps一一对应放掉一些水，Ts不变，ps？TspsSaturationtemperatureSaturationpressure)(Tfpps=1.01325barTs=100℃青藏ps=0.6barTs=85.95℃高压锅ps=1.6barTs=113.32℃Thereisadefiniterelationbetween.)(ssTfp6.2.3汽化Vaporization沸腾：表面和液体内部同时发生的汽化Itisanintensivevaporizationphenomenonoccursintheliquid.(气体和液体均处在饱和状态下)汽化:由液态变成气态的物理过程(不涉及化学变化)蒸发：汽液表面上的汽化ItoccursabovethefreesurfaceofliquidBoilEvaporationEvaporationoccursatanytemperatureandpressure.（蒸发可在任何温度和压力下发生．）Evaporationratedependsonthefreesurfacearea,thetemperature,theflowrateabove,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．Boilingcanonlyoccurastemperaturereachesthesaturationtemperaturecorrespondingtothespecifiedpressureorthepressuredropsbelowthesaturationpressurecorrespondingtothespecifiedtemperature.（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,才能发生)§6-3水蒸气的定压发生过程ttst=tst=tst=tsttsvv’v=v’v=v’’v’vv’’vv’’未饱和水饱和水饱和湿蒸汽饱和干蒸汽过热蒸汽hh’h=h’h=h’’h’hh’’hh’’ss’s=s’s=s’’s’ss’’ss’’水预热汽化过热6.3.1P=constant(压力p是定值)PreheatStage(预热阶段)CompressedliquidtoSaturatedliquid.(未饱和液体饱和液体)Compressedliquid/subcooledliquidisnotabouttovaporization.Asheatadded,tslightly.WhenItbecomesSaturatedliquid.Theliquidisabouttovaporize.Itisthestateatwhichitisstillaliquid,butanyheatadditionwillcausesomeoftheliquidtovaporize.)(pfttsvvsttsttt过冷VaporizationStage(气化阶段)SaturatedliquidtoSaturatedvapor(饱和液体饱和蒸气)Saturatedvaporisavaporabouttocondense.AsubstanceatstatebetweenSaturatedliquidandSaturatedVaporiscalledSaturatedliquid-vapormixture/wetvapor.LatentheatofVaporization(气化潜热):TheamountofenergyabsorbedduringvaporizationvvsttSuperheatStage(过热阶段)Superheatedvaporisthevaporthatisnotabouttocondense.sttvtsttt过热注意：水蒸气定压发生过程说明(1)()QUWUpdVUpVUpVH(2)fg0SSS(3)理想气体()hfT实际气体汽化时，T＝Ts不变，但h增加'''hh汽化潜热(4)未饱和水过冷度sttt过冷过冷水过热蒸汽过热度sttt过热只有熵加热时永远增加StateofLiquidandvapor未饱和液，过冷液饱和液饱和湿蒸气饱和蒸气过热蒸气SaturatedliquidSaturatedvaporSaturatedliquid-vapormixtureSuperheatedvaporCompressedliquidSubcooledliquid汽化潜热LatentheatofVaporization压缩液6.3.2定压气化过程的P-v图和T-s图P-vandT-sdiagramscpcTcTABCACBcTTcTTcppcpp3a3b3d3e3c3a3b3d3e3c2c2a2b2d2e2c2a2b2d2e1c1a1b1d1e1c1a1b1d1eTsp-v图，T-s图上的水蒸气定压加热过程pvcp一点，二线，三区，五态Saturatedliquidline,SLLisformedbyconnectingaseriesofboilingpoints.saturatedvaporline,SVL：Connectingaseriesofpointsatdrysaturatedvaporbuildsaline,knownassaturatedvaporline,SVL.Vaporizationcontinuesbyfurtherheatsupplytothesystemuntilnoliquidisleft.Thisstateisknownasdrysaturatedvapor,e.g.pointd.Ifthesystemisslightlycooledatthisstate,thendropletsofliquidwillbegintoform.Thestateofsubstancebetweensaturatedliquidanddryvaporiscalledwetvapor.Furtherheatingofadrysaturatedvaporatconstantpressurecausesariseofvaportemperatureanditbecomessuperheatedvapor.Thestateofsubstanceiscompletelydefinedbyitspressureandtemperatureifitisinliquidorsuperheatedvaporphase.vOnepoint(一点):criticalpoint(临界点)Twolines(两线):SLLandSVL(饱和液体线和饱和蒸气线)Threeregions(三区):Subc