twin-screw expander, trilateral flash cycle, flash evaporation, two phase expansion, metastable liquid
The present paper focuses on the thermodynamic modeling of screw expanders in a trilateral flash cycle. In this process a liquid is pumped from low to high pressure and then heated up close to saturation point. Instead of a vaporous working fluid as usual, a hot liquid is filled in the working chamber of the screw expander. During the filling process the pressure of the liquid drops below the saturation pressure while the temperature remains almost constant due to a fast process. Hence the liquid is in a metastable state. The liquid aspires a stable state and therefore is vaporized during the filling and expansion process due to its increasing chamber volume. Thus a two-phase mixture, vapor and liquid, exists in the working chamber. The simulation presented includes the calculation of fluid states within each chamber to be in thermodynamic equilibrium, which assumes that a sufficient heat transfer exists between the phases to reach a stable state within a time step. Thermodynamic simulations are carried out using water as the working fluid for an exemplary screw expander geometry. The conclusive assessment of the thermodynamic model is demonstrated by the comparison of the simulation results with available experimental measurement results for a corresponding two-phase screw expander.