Question

(20%) Q2. A piston-cylinder assembly initially has 100 kPa pressure and 2 liters of air at 25°C. The air is then compressed to 600 kPa pressure and 150°C temperature. Useful work input for compression is 1.2 kJ. Assuming that the ambient pressure and temperature are 100 kPa and 25°C respectively, calculate a) the exergy of the air in the first and last states, b) the minimum work required for the compression process, c) the second law efficiency of the state change. Note: Rair= 0.287 kJ/kgK, Cp.mean = 1.009 kJ/kgK, Cv,mean = 0.722 kJ/kgK

Answer 1

a ) 1oox 0.002 0.00234 PV RT, kg 0:287 X 298 Alo Pui 2 2 2 2 PV, Tz OT, P2 T 0.473 L V2 = 1oox42 342 600x298 Sa-So = (pang R en P2 Pang enta To Po 0.287 en 600 loo - m &2 11 = 1.009 ln423 288 - 011608 ks leg ke [cow, ang (T2-To) - Tolda-so)} + Po (VG-vo) [0722 1722 (423-298) - 298 (-0,1608 + 100 (0.000473-0-02)] X2 X2 0.00234 o.171 UJ b) Ween, in = X2-X, = 0.171–0 0.71 kJ X, 20 : air is initially at the dead state c) - 0:17) Weer, in wein - 0.1425 1.2 or 14.25%