CFD Modelling of a Twin Screw Expander Using a Single Domain Rotor Grid

Organic Rankine cycle (ORC) is widely recognized as a viable technology to convert low temperature heat into electricity. Compared to other emerging technologies, ORC systems have a number of advantages such as low maintenance, favourable operating pressures and autonomous operation. Among the ORC components, selection of the appropriate expander and working fluid affects the overall cycle’s efficiency, size and cost. Expanders are classified into dynamic and positive displacement machines. Twin screw expanders belong to the latter group and compared to other expanders have the advantage of high isentropic efficiency (up to 70%), high pressure ratios (typically 2-10), simple structure, potential operation at relatively high rotational speeds and high power outputs as well as capability to handle multiphase fluids. Despite the large number of publications on twin screw expanders, the experimental data are not readily available in open literature. Additionally, effects of oil and other liquids in multiphase screw machines are not fully understood. Therefore, better understanding of the distribution of vapour, liquid as well as oil in multiphase expanders and their influence on leakage flows will lead to better designs and improvements in both efficiency and reliability of such machines. Previous publications from the authors of this demonstrated that deforming numerical meshes generated by in-house grid generator for screw machines SCORG when used with commercial Computational Fluid Dynamics (CFD) solver can provide very good results in simulation of single and multiphase screw expanders. It was however noticed that sliding interfaces in most of commercial CFD codes introduce numerical errors which lead to inaccuracy in representing flows locally and affect accuracy of leakage flow calculation. In this work, an analysis of the performance prediction of a twin screw air expander with the novel single domain mesh for both rotors generated in SCORG is compared with the results obtained with the mesh consisting of two domains and measured results obtained on the test rig. Furthermore the performance of the same expander with oil injections is obtained using Euler-Euler multiphase model. The single domain numerical mesh for the screw expander rotors is obtained from SCORG. Stationary grids of the ports are generated using ANSYS commercial grid generator. The performance calculations are performed in ANSYS CFX® solver. The working fluid selected in this study is air. Two cases are explored, firstly the performance of an oil free twin screw expander and then the same methodology is applied for the prediction of the performance of an oil injected expander. The results showed significant improvement in performance prediction of the single phase air screw expander with the single domain numerical mesh especially in representing interlobe leakage flows. The non-homogeneous Euler-Euler multiphase model was used to evaluate effects of oil injection on flow and power. However, more importantly, the distribution of oil in such an expander was visualised. Following this work the future work is dedicated to evaluation of ORC expander with real fluid properties and oil injection.