Question

A piston/cylinder contains 1 kg water at 150 kPa, 20°C. The piston is loaded so pressure is linear in volume. Heat is added from a 600°C source until the water is at 1 MPa, 500°C. Find the heat transfer and the total change in entropy.

Answer 1

CV H2O out to the source, both 1Q2 and 1W2

State 1: Compressed liquid use saturated liquid at same T:

v1 = 0.001002 m3/kg; u1 = 83.94 kJ/kg; s1 = 0.2966 kJ/kg K

State 2: sup. vap.

v2 = 0.35411 m3/kg

u2 = 3124.3 kJ/kg;

s2 = 7.7621 kJ/kg K

W2 = (1000 + 150) 1 (0.35411 - 0.001002) = 203 kJ

1Q2 = 1(3124.3 - 83.94) + 203 = 3243.4 kJ

m(s2 - s1) = 1(7.7621 - 0.2968) = 7.4655 kJ/K

1Q2 / Tsource = 3.7146 kJ/K (for source Q = -1Q2

1S2 gen = m(s2 - s1) − 1Q2 / TSOURCE = ∆Stotal

= ∆SH2O + ∆Ssource = 7.4655 - 3.7146 = 3.751 kJ/K

Remark: This is an external irreversible process (delta T to the source)