The Trilateral cycle has attracted more and more attentions because of its better temperature profiles matching in the heater compared to conventional heat recovery cycles, such as the Organic Rankine Cycle and the Kalina Cycle. However, the actual fundamentals of the two-phase expansion process have not been clarified, especially, the evolution mechanisms of the evaporating rate and the degree of superheat during the expansion process are indistinct. In the present study, two-phase expansion process of superheated water in a cylinder with moving piston was analyzed. A model was proposed to calculate the evaporating rate and the degree of superheat of liquid water. A semiempirical Nusselt number was adopted to solve the energy conservation equations. It was shown that the degree of superheat of liquid water was increased rapidly to the maximum at the beginning of the expansion process and then was decreased continuously. The degree of superheat was almost equal to 0 at the end of the expansion process. The simulation results also showed a complex evolution profile of the evaporating rate during the expansion process. As a whole, the evaporating rate was increased from 0 to the maximum, and then it was decreased to 0 at the end of the expansion process. This situation is similar to the experimental results in the published literature. The evaporating rate appeared a transient dropping process at the beginning of the expansion. It was considered that this result was associated with the rapid decline of the degree of superheat. Meanwhile, there were two or three significant inflection points on the evolution profile. From the model prediction, it was concluded that the evaporating rate may be related to the degree of superheat of liquid water, the piston velocity and the mass of liquid water in the cylinder
