Analysis of the thermodynamic disequilibrium loss of the two-phase expansion process and its influence on the performance of a reciprocating expander

The screw expander and the reciprocating expander were considered as the suitable technologies to be employed as two-phase expanders of the Trilateral Flash Cycle (TFC) systems. The two-phase expansion process in the screw expander could be approximately viewed as the thermodynamic equilibrium process due to the strong disturbing and mixing between the two-phase working fluids in the chamber volume. However, for the reciprocating expander, the situation was different. The flashed vapor in the reciprocating expander cylinder was separated from liquid in the flash chamber, which would result in a significant thermodynamic disequilibrium loss. To assess this loss, the thermodynamic equilibrium model (TEM) and the flash vaporization model (FVM) were simultaneously proposed in this paper. Water was used as the two-phase working fluid. The two-phase expansion processes, the isentropic efficiencies and the output expansion works of the two models were obtained. Predicted results were analyzed and compared under different rotation speeds and injection temperatures. The results showed that when rotation speed of the expander was increased from 50 r/min to 1000 r/min, the thermodynamic disequilibrium loss was approximately increased from 5.3% to 6.3%, and when injection temperature was decreased from 523.15 K to 473.15 K, the thermodynamic disequilibrium loss was approximately increased from 4.5% to 6.2%.