Direct numerical simulation of flow in a membrane channel under oscillating inlet conditions

Abstract

Effects of inflow oscillation in feed channels of spiral wound membranes are investigated. Pulsation has previously been used as a membrane cleaning method in Reverse Osmosis (RO) and other membrane desalination processes. However, there is still a gap in the fundamental fluid dynamics study of pulsation and fouling mitigation. Three-dimensional, transient, high-fidelity Computational Fluid Dynamics investigation using Direct Numerical Simulation has been performed on a geometry of 9 × 3 feed square space elements at Reynolds number of 350 based on inlet velocity. Sinusoidal-wave and step function pulsations are simulated at three frequencies (1, 4, 8 Hz). Five levels of permeate velocity are investigated, from 0.2 % to 0 % (impermeable). Pulsating inflow produces a fully-unstable flow regime. Flow streamlines show no large recirculation regions, unlike for the steady inflow, and flow structures show higher levels of instabilities with a larger range of scales, resulting in smaller stagnation regions near the surfaces. Consequently, the area of low shear decreases and its distribution is limited to the spacer filament crossings instead of the permeate walls. These findings show that inflow oscillation for a membrane channel creates flow conditions that could be beneficial for reducing fouling propensity in RO and other membrane processes with similar feed channel geometry.