A pan contains 5 L of engine oil at 20°C, 100 kPa. Now 2 L of hot 100°C oil is mixed into the pan. Find the entropy generation.
Answer: A
ρ = 885 kg/m3; From energy equation,
T2 = (mA/m2)TA + (mB/m2)TB = (5/7)20 + (2/7)100 = 42.868°C = 316.02 K
S2 – S1 = m2s2 − mAsA – mBsB = mA(s2 – sA) + mB(s2 – sB)
= 0.005 × 885 × 1.9 ln (316.02/293.15) + 0.002 × 885 × 1.9 ln (316.02/373.15)
= 0.6316 – 0.5588 = + 0.0728 kJ/K.