1 kg of air at 300 K is mixed with 1 kg air at 400 K in a constant pressure process at 100 kPa and Q = 0. Find the entropy generation in the process.
Answer: A
U2 – U1 + W = U2 – U1 + P(V2 – V1) = H2 – H1 = 0
H2 – H1 = mA(h2 – h1)A + mB(h2 – h1)B = mACp(T2 – TA1) + mBCp(T2 – TB1) = 0
T2 =(mATA1 + mBTB1)/(mA + mB) = (TA1/2) + (TB1/2) = 350 K
1S2 gen = mACp ln(T2/TA1) + mBCp ln(T2/TB1)
= 1 × 1.004 ln [350/300] + 1 × 1.004 ln[350/400] = = 0.15477 – 0.13407 = 0.0207 kJ/K.