CFD modelling of unsteady electro-osmotic permeate flux enhancement in membrane systems

Electro-osmotic flow (EOF) perturbations induced near the membrane surface are a promising approach to increase wall shear, which in turn has the potential to slow the onset of fouling for nanofiltration and reverse osmosis processes. As such, it is important to understand the mechanisms that increase wall shear and mass transfer resulting from time-varying EOF perturbations. In this paper, a Computational Fluid Dynamics (CFD) model is used to simulate spatially-uniform unsteady EOF with permeation inside a 2D unobstructed empty membrane channel. Time-averaged permeate flux is used to measure productivity and maximum shear rate is used as a proxy measure for fouling reduction/prevention. The dependencies of solute concentration amplitude and maximum wall stress on slip velocity amplitude are shown to be linear, both in terms of homogeneity and additivity. This implies that time-averaged hydrodynamics and mass transfer do not vary significantly regardless of changes in the frequency and amplitude of the slip velocity, because the effect is cancelled within the time oscillation period. Nevertheless, there are still advantages for this type of perturbation, as larger slip velocity frequency and amplitude increase the maximum wall stress with a negligible change in time-averaged pressure drop, which may have advantages for limiting concentration polarisation and fouling. CFD simulations of unsteady EOF were carried out for unobstructed membrane channels.The effect of an unsteady EOF on flux and wall shear was analysed.Unsteady EOF shows similar or lower time-averaged permeate flux than steady EOF.Unsteady EOF increases maximum wall shear, hence may reduce the onset of fouling.There is a negligible change in time-averaged pressure drop with unsteady EOF.