Answer to Question #190743 in Mechanical Engineering for Isaac Ferkah

Question #190743

Calculate the change in internal energy, heat transfer and change in enthalpy for 0.5 kg of air expanding according to law PV1.2 = C from 1 MPa and 300⁰C to 100 kPa. What will be the work done by air during expansion?   [ Cv = 0.718 kJ/kg K for air] 


1
Expert's answer
2021-05-19T06:08:13-0400
ΔU=cvm(T2T1)\Delta U=c_v m (T_2-T_1)

T2=T1p2p1(p1p2)11.2T2=(300+273)110(10)11.2=390 KT_2=T_1\frac{p_2}{p_1}\left(\frac{p_1}{p_2}\right)^\frac{1}{1.2}\\ T_2=(300+273)\frac{1}{10}\left(10\right)^\frac{1}{1.2}=390\ K

ΔU=(0.718)(0.5)(390573)=65.7 kJ\Delta U=(0.718)(0.5) (390-573)=-65.7\ kJ

Q=8.3129(0.5)(390573)=26.2 kJQ=\frac{8.31}{29}(0.5) (390-573)=-26.2\ kJ

W=(26.2)1.3951.21.3951=12.9 kJW=(-26.2)\frac{1.395-1.2}{1.395-1}=-12.9\ kJ


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