Hydrodynamic effects of non-uniform feed spacer structures on energy loss and mass transfer in spiral wound module

Abstract

To find a balance between feed flow rate and energy penalty, the hydrodynamic effects of non-uniform feed spacer structures on energy loss and mass transfer in spiral wound reverse osmosis (RO) module were numerically simulated by the CFD method through adopting dumbbell-shaped filament. The user-defined function (UDF) was employed to satisfy the permeability boundary conditions of membrane surfaces. The hydrodynamic effects of different filament diameter ratios (DR, d2/d1) as well as the spacer mesh angle α were studied. The simulation results showed that the spacer configuration greatly affected the energy loss and mass transfer in the spiral wound module. The pressure drop rate increased with increasing space mesh angle and a mesh angle between 60° and 120° was recommended for bearable pressure drop. The specific helicity was introduced to measure the vorticity transported with the flow. The negative region of specific helicity can be used to represent the wake size easily. The feed spacer with varied cross-section has the potential to reduce the specific energy consumption. When DR decreases from 1.0 to 0.45, the wake length becomes longer, and the pressure drop rate decreases significantly by about 58.1%. By increasing the Reynolds number from 50 to 200, the averaged wall shear stress increases by more than 5.6 times, and the averaged permeate water flux increases by 18.2%. Thus, the recommended value of feed flow Re was about 200. This detailed numerical study is of great significance to improve the understanding of concentration polarization and energy loss suppression by the optimized feed spacer configuration.