Numerical simulation of Dean vortices: fluid trajectories

Dean vortices enhance the permeate fluxes obtained with ultrafiltration systems. Although this phenomenon has been studied for micro and ultrafiltration processes, the effect of the feed concentration is not yet thoroughly understood. Several studies have reported that increasing the suspension concentration in the feed enhances the permeate flux differences observed between the systems including Dean vortices and those used without secondary flows. Experimental results using laser visualisation to characterise the Dean flow have been carried out: using polystyrene particles, according to these experiments, it could be thought that the particles were more concentrated near the centres of the vortices. Consequently, the concentration near the wall would be lower. However, another interpretation can be proposed. The experiments are based on light intensity measurements which cannot distinguish the number of particles and their velocity. The same intensity measured, in the same time laps, can be obtained either from slow particles concentrated in the observed solution or from fast particles more diluted. In order to clear out this point, we present here a comparison between laser visualisation results and numerical predictions. It appears that at different Reynolds numbers the fluid and the particles suspended follow curved pathlines that successively pass in the core of the flow and near the walls. The resulting swirling motion might take part in the mass transfer observed in the presence of Dean vortices. The present comparison to a straight hollow fibre, gives new insights about the flux improvements observed in membrane processes. (C) 2002 Elsevier Science B.V. All rights reserved.