R?4.157?kJ?kg.K??M?8.314 p1?pb?pg1?0.1?0.2?0.3?MPa?Rg?p2?pb?pg2?0.1?0.19?0.29?MPa?
p1qV10.3?106?1.5qm???0.3459?kgmin? 3RgT14.157?10??40?273?(1) 按定值比热
77Rg??4.157?14.55?kg?kg.K??22cV?cp?Rg?14.55?4.157?10.39?kJ?kg.K??cp??U?qmcV?t?0.3459?10.39??66?40??93.44?kJmin????qmcp?t?0.3459?14.55??66?40??130.9?kJmin??T2p2?60?2730.29??????S?qm?cln?Rln?0.3459?14.55ln?4.157lnkJ?K.min???0.450?4g?pT?p1?40?2730.3??1?(2)按平均比热容的直线关系式
66?Cp40?C66?C40?Cc?143.3?0.0005945??66?40??14.39?kJ?kg.K???10.12?0.0005945??66?40??10.18?kJ?kg.K??t2t1cV ?U?qmcV?t?0.3459?10.18??66?40??91.55?kJkg?
?t2T2p2????S?qm?cpt1ln?RglnT1p1???66?2730.29???0.3459??14.39ln?4.157ln??0.4459?kJ?K.min??3130.3??12. 利用内燃机排气加热水的余热加热器中,进入加热器的排气(按空气处理)温度为
300?C,出口温度为80?C。不计流经加热器的排气压力变化,试求排气经过加热器的比热
力学能变化,比焓变化和比熵的变化。
(1)按定值比热容计算;
(2)按平均比热容(表)计算。 答案
(1)
?u??157.9kJkg ?h??221.0kJkg
?s??0.4866kJ?kg.K?(2)
?u??162.1kJkg ?h??225.3kJkg
?s??0.4966kJ?kg.K???600kPa,600C100kPa,300C,略去动能、位13. 进入气轮机的空气状态为,绝热膨胀到
能变化,并设大气温度为
T0?300K,试求:
(1)每千克空气通过气轮机输出的轴功;
(2)过程的熵产及有效能损失,并表示在T?s图上; (3)过程可逆膨胀到100kPa输出的轴功。 解 (1)
ws???h?h1?h2?cp?t1?t2??1.004??600?300??301.4kJkg (2)
?sg??s?cplnT2p573100?Rgln2?1.004ln?0.287ln?0.0913?kJ?kg.K?? T1p1873600i?T0?sg?300?0.0913?27.39?kJkg?
熵产?sg及有效能损失i如图3-36中阴影面积所示。
(3)
k?10.4????kkRgT1??p2??1.4?0.287?8731.4100????351.4?kJkg? ?wsh?wt?1?????1???????600??k?1??p1??1.4?1??????14.由氧气、氮气和二氧化碳组成的混合气体,各组元的摩尔数为
nO2?0.08mol,nN2?0.65mol,nCO2?0.3mol
?试求混合气体的体积分数、质量分数和在p?400kPa,t?27C时的比体积。 解
n??ni??0.08?0.65?0.3??1.03?mol?0.08?0.078n1.03nN20.65xN2???0.631n1.03xCO2?1?xO2?xN2?1?0