Numerical methodology and CFD simulations of a rotary vane energy recovery device for seawater reverse osmosis desalination systems

Abstract

• Novel analytical grid generation methodology for rotary vane machines. • Removal of backflows and torque peaks through flow port optimisation. • Sensitivity analysis to reduce cavitation and improve volumetric efficiency. • Volumetric efficiency with optimal speed and blade design equal to 91.6%. Energy recovery devices in Seawater Reverse Osmosis Systems (SWRO) reduce energy consumption and may facilitate the large-scale deployment of desalination systems. In this paper, a Rotary Vane Energy Recovery Device (RVERD) is analysed and optimised by aiming at weakening cavitation and improving the volumetric performance of the machine. An innovative analytical methodology based on user defined nodal displacement is proposed to address the need to discretise the rotating and deforming computational domain of double-acting vane machines. The generated grids are interfaced with the ANSYS FLUENT solver for multi-phase computational fluid dynamics simulations. The flow topology is analysed to reveal the flow and cavitation features especially in the blade tip regions. A port optimisation is then carried out followed by a sensitivity analysis on the design parameters to improve RVERD performance. The results show that delaying the discharge angle at the high-pressure outlet port by 3° and an optimal port to stator length ratio of 70% helped to prevent backflows and eliminate torque peaks. The sensitivity analysis has identified the rotational speed and the blade tip clearance as the two most influential factors affecting cavitation and, in turn, the volumetric efficiency of the machine. With respect to the baseline design configuration, at the optimal rotational speed of 1000 RPM and with a tip clearance gap of 50 μm, the volume-averaged vapour volume fraction in the core decreased from 20.6 × 10 −3 to 0.6 × 10 −3 while the volumetric efficiency increased from 85.7% to 91.6%. The axial clearance gap of 70 μm contributed to 2.9% of the volumetric losses.