热力学与传热学_秦萍_习题热工基础 下载本文

理想气体热力过程习题解答

习题4-1:已知:可逆定容过程,T1=470K,m=2.5kg .△U=295.4kJ/kg R=0.4kJ/kg.K , k=1.35 求: W、 Q、△S

解: Cv = R/(k-1)= 0.4/0.35 = 1.143 kJ/kg.K

T2 =△U/m.Cv +T1 =295.4/(2.5×1.143) + 470 = 114.18 + 470 =103.4+470 =573.4K W = 0 ,

Q = m. △u = △U = 295.4 kJ △S =m. Cv。Ln(T2/T1)= 2.5×1.143×ln(573.4/470)=2.8575×0.19885 = 0.572 kJ/K

习题4-7:

已知:空气、双级压缩、to

1 = 17 C , P1 = 0.1 Mpa , P3 = 2.5 Mpa

V = 500 Nm/h ,n = 1.25

求:(1)P2 ; (2)T3 ; (3)N ; (4)Q

1/21/2

解:(1)P2 = √P1P3 = (0.1*2.5)= 0.25= 0.5 Mpa

n-1/n0.2o

(2)T3 = T2 = T1(P2 / P1)= 290(5)= 290 * 1.38 = 400 k = 127 C

0.2

(3)N = mw t = m [ n /(n - 1)]RT1(1 - 5)

= 5 * 0.287 * 290 (1 - 1.38)m = - 158 .m kJ/kg

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m = PV/RT = (0.101325*10*500)/(287*273.15) = 517 kg/h = 0.18 kg/s

(或 m =( V / 22.4)M = (500×29) / 22.4 = 646 kg/h = 0.18 kg/s) N = mw t = 2×0.18 ×(-158)= 56.9 KW

(4)q = CP( T1 - T2 )= 1.004 *(290 – 400)= - 110.44 kJ/kg

Q = m CP( T1 - T2 )= 0.18 * 1.004 *(290 – 400)= - 19.88 KW

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气体流动习题解答

习题4-9 已知:渐缩管,m = 0.6 kg/s ,P1= 0.6 MPa ,t1= 600 oC ,P2= 0.4 MPa 。

求:C2 和A2 (按空气 CP = 7*8.314/2 *28.97 = 1.0045 kJ/kg.k )

解:C2 = {(2k/1-k)RT1[1-(P2/P1)k-1 / k]}1/2

= {(2.8/0.4)287*873[1-(0.4/0.6)0.4/1.4]}0. 5 = 438 m/s

又 C2 = {2Cp(T1-T2)}1/2 ∵ T2 = T1(P2 / P1)k-1 / k = 873*0.8906 = 777.5 k ∴ C2 = {2*1004 (873 – 777.5)}0.5 = 438 m/s

A2 = mv2/C2 ∵ v2 = RT2/ P2 = 287*777.5 / 0.4×10= 0.558 m3 / kg

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