工程热力学课后题答案 下载本文

400kJkg,问焓变化了多少?

答案?h?557.9kJkg

??1kgt?30C400C,分别用定值比热容,平均比热容(表)计算其热力学能18.将氮气由定压加热到

和焓的变化。 用定值比热容计算

?U?274.7kJ,???384.6?kJ?

用平均比热容计算

?U?281.7kJ,???391.6kJ

9.

2kg的CO2,由p1?800kPa,t1?900?C膨胀到p2?120kPa,t2?600?C,试利用定值

比热容求其热力学能、焓和熵的变化。 解

R8.314??0.189?kJ?kg.K??M4499cp?Rg??0.189?0.850?kJ?kg.K??2277?kJ?kg.K??cV?Rg??0.189?0.66122?U?mcV?t?2?0.661??600?900???396.6?kJ?Rg????mcp?t?2?0.850??600?900???510?kJ??T2p2?873120????S?m?cln?Rln?2?0.85ln?0.189ln???0.215?kJK?g?pT?p1?1173800??1?

10. 在体积为V?1.5m3的钢性容器内装有氮气。初态表压力为

?pg1?2.0MPa,温度为230C,问

?应加入多少热量才可使氮气的温度上升到750C?其焓值变化是多少?大气压力为0.1MPa。 (1)按定值比热容计算;

(2)按真实比热容的多项式计算; (3)按平均比热容表计算;

(4)按平均比热容的直线关系式计算。 解

R8.314??0.297?kJ?kg.K??M28

p1V?2.0?0.1??106?1.5m???21.09?kg?3RgT10.29?10??230?273?Rg?(1)

77Rg??0.297?1.04?kJ?kg.K??2255cV?Rg??0.297?0.742?kJ?kg.K??

22Q?mcV?t?21.09?0.742??750?230??8.137?103?kJ?cp????mcp?t?21.09?1.04?750?230??1.141?104?kJ?(2)查得

Cp,m?a0?a1T?a2T2a0?27.3146,a1?5.2335?10?3,a2??0.42?10?8n?M21.09??0.7532?kmol?m282222211111Q??nCV,mdT?n??Cp,m?R?dT?n?nCp,mdT?nR?dT?n?a0?a1T?aT2dT?nR?Taa??2?n?a0T?1T2?2T3?TT1?nR?T2?T1?23??1?0.7532?[27.3146??1023?503???5.2335?10?3?2110232?5032????0.42??10?8?10232?5032?0.7532?8.314??1023?503?]?9.005?103?kJ?3??????????nCp,mdT?Q?nR?T12?9.005?103?0.7532?8.314??1023?503??1.226?104?kJ?(3)查得

cpcV23002300?1.045kJ?kg.K?,cp?0.748kJ?kg.K?,cV75007500?1.092kJ?kg.K??0.796kJ?kg.K?3Q?mc?t2p0.t2?ct1p0.t1?21.09??0.796?750?0.748?230??8.962?10?kJ??

???mcpt02.t2?cpt01.t1?21.09??1.092?750?1.045?230??1.22?104?kJ?(4)查得

??cpcVt2t1t2t1?1.032?0.00008955t?1.032?0.00008955??230?750??1.120?kJ?kg.K???cpt2t1t2t1?Rg?1.12?0.297?0.823?kJ?kg.K??Q?mcV???mcp.?t?21.09?0.823??750?230??9.026.?t?21.09?1.12??750?230??1.228?104?kJ?

t2t111. 某氢冷却发电机的氢气入口参数为

pg1?0.2MPa,t1?40?C,出口参数为

pg2?0.19MP,at2?66?C。若每分钟入口处体积流量为1.5m3,试求氢气经过发电机后的热力学

能增量、焓增量和熵增量。设大气压力为(1)按定值比热容计算;

(2)按平均比热容直线关系式计算。 解

pb?0.1MPa。

Rg?

R?4.157?kJ?kg.K??M?8.314?p1?pb?pg1?0.1?0.2?0.3?MPa

?p2?pb?pg2?0.1?0.19?0.29?MPap1qV10.3?106?1.5?kgmin? qm???0.34593RgT14.157?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????kJ?K.min???S?qm?cln?Rln?0.3459?14.55ln?4.157ln???0.4504g?pT?p1?40?2730.3??1?(2)按平均比热容的直线关系式

66?C40?C66?C40?CcpcV

?143.3?0.0005945??66?40??14.39?kJ?kg.K???10.12?0.0005945??66?40??10.18?kJ?kg.K??t2t1?U?qmcV??S?qm??cp??t?0.3459?10.18??66?40??91.55?kJkg?ln

T2p??Rgln2?T1p1??66?2730.29???kJ?K.min???0.3459??14.39ln?4.157ln??0.44593130.3??t2t112. 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300C,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、位能变化,

13. 进入气轮机的空气状态为,绝热膨胀到

并设大气温度为

T0?300K,试求:

?s图上;

(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解 (1)

(3)过程可逆膨胀到100kPa输出的轴功。

ws???h?h1?h2?cp?t1?t2??1.004??600?300??301.4kJkg

(2)

?sg??s?cplnT2p573100?kJ?kg.K?? ?Rgln2?1.004ln?0.287ln?0.0913T1p1873600i?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)