DYNAMIC MODEL FOR THE PERFORMANCE PREDICTION OF A TWIN SCREW EXPANDER IN AN ORC

asme-orc2015 Tracking Number 102

The Organic Rankine Cycle (ORC) is well known and proven technology for waste heat recovery. The current generation of twin screw expanders used for low-grade heat recovery are in fact compressors working in the opposite sense. In this paper a mathematical model for calculating the performance of a twin screw expander is presented. The model is based on geometrical parameters which describe volume and leakage areas for every angular position. With these functions the entire design of a screw expander is determined. The differential equations used in the model are derived from the mass and energy conservation laws and are solved together with the appropriate Equation of State in the instantaneous control volumes. Since R245fa is selected as a working fluid, the Aungier Redlich-Kwong Equation of State has been used. The results of the mathematical model are compared to the 3D Computational Fluid Dynamics (CFD) calculations of the same twin screw expander using the same working fluid. To calculate the mass flow rates through the leakage paths formed inside the screw expander, flow coefficients are considered as constant and they are derived from 3D CFD calculations. The outcome of the mathematical model is the P-V indicator diagram which is compared to CFD results of the same twin screw expander. It is shown that the developed model accurately predicts the performance of the expander.