twin screw, expander, R245fa, Redlich-Kwong, ORC
With the increase in energy prices and environmental constraints, recovering the energy from low grade waste heat presents an important challenge. The Organic Ranke Cycles (ORCs) are widely used, but there is still need to improve their efficiency (especially for small scale energy production). This paper presents the Computational Fluid Dynamics (CFD) analysis of a twin screw expander which is used for power generation in an ORC system with refrigerant R245fa. The deforming mesh motion is handled by an in-house code which generates a block-structured grid with the help of the solutions of Laplace problems. The properties of refrigerant R245fa are derived from the Augnier Redlich-Kwong cubic equation of state which is incorporated in the computational model. From the results of a CFD analysis, the pressure-volume diagram, mass flow rates and power output for different pressure ratios and different designs were obtained. In order to evaluate the effects of individual clearance gaps on the performance of the expander, time dependent mass flow diagrams for each of them are provided and studied. It can be concluded that the influence of the leakage flows increases with decrease in rotational speed or with the increase in pressure ratio. To avoid losses during the filling, an optimized design of the inlet port is necessary.