CFD modelling of electro-osmotic permeate flux enhancement in spacer-filled membrane channels

Abstract

Electro-osmosis has the potential to reduce concentration polarisation (CP) because it induces the movement of fluid in the vicinity of membrane, thus improving mixing within the boundary layer and enhancing mass transfer. Computational Fluid Dynamics (CFD) is used to simulate steady and unsteady electro-osmotic flow (EOF) in 2D spacer-filled channels, using the Helmholtz-Schmoluchowski slip velocity approximation. The results show that mass transfer enhancement due to EOF is larger in spacer-filled channel than in empty channels. For the steady EOF, the simulation results show that uniform slip velocity reduces the development of stagnant and high concentration regions near spacer filaments when the slip velocity direction is away from the spacer. For unsteady EOF in spacer-filled channels, the simulation results show that an oscillating slip velocity has the potential to induce vortex shedding. This occurs when a resonant slip velocity frequency is used for Reynolds numbers near the transition from steady to unsteady flow. EOF induced vortex shedding due to the resonant slip velocity results in significant increase in maximum wall shear stress along the membrane, therefore potentially delaying the onset of fouling. The data also shows that at the same permeate flux, EOF at the resonant slip velocity frequency results in a significantly lower Power number (a proxy for pumping energy) than the case without EOF.