Question

A mass and atmosphere loaded piston/cylinder contains 2 kg of water at 5 MPa, 100°C. Heat is added from a reservoir at 700°C to the water until it reaches 700°C. Find the work, heat transfer, and total entropy production for the system and surroundings.

Answer 1

C.V. Water out to surroundings at 700°C. This is a control mass.

Work is found from the process (area in P-V diagram)

1W2 = mP(v2 - v1) = 2 × 5000(0.08849 – 0.00104) = 874.6 kJ

The heat transfer from the energy equation is

1Q2 = U2 - U1 + 1W2 = m(u2 - u1) + mP(v2 - v1)= m(h2 - h1)

1Q2 = 2(3900.1 - 422.72) = 6954.76 kJ

Entropy generation from entropy equation

1S2 gen = m(s2 - s1) - 1Q2/Tres

= 2(7.5122 - 1.303) - 6954/973 = 5.27 kJ/K