Example
The internal energy of a closed system can be increased by adding heat or doing work. In the following example, 1 kg of saturated liquid water at 100°C is contained inside a piston-cylinder assembly. Instead of adding heat, a paddle wheel is doing work on the system and the water is undergoing a transition into 100% saturated vapor. During the process the piston moves “freely” inside the cylinder. Question: Does this make a difference in our calculation? Assume no heat transfer, determine the work done and the amount of entropy production.
m(ug-uf)=-W, ug: saturated vapor, uf: saturated liquid
W=- (ug-uf)=-(2506.5-418.9)=-2087.6(kJ/kg) from Table A-4
(Ds)gen=sg-sf=7.356-1.307=6.049 (kJ/kg K), from Table A-4
Answer to the question: yes, if the piston is fixed, the pressure inside the cylinder will increase as liquid evaporates into vapor. Accordingly, the saturation state will change.