Flow visualization through spacer filled channels by computational fluid dynamics I.: Pressure drop and shear rate calculations for flat sheet geometry

Computational fluid dynamics (CFD) simulations were carried out for fluid flow through rectangular channels filled with several commercially available spacers for membrane modules. Simulation results were compared with literature experimental data. Excellent agreement was found between the experimentally determined dependence of the total drag coefficient on the Reynolds number and the CFD simulations in this work. Analysis of the flow structure through spacer filled channels revealed that bulk of the fluid does not change direction at each mesh as suggested previously in the literature, but that the bulk fluid flows parallel to the spacer filaments. The pressure drop through the channel was found to be largely governed by a loss of fluid momentum caused due to an almost abrupt change in the direction of the velocity vectors across a thin transition plane corresponding to the plane of intersection of the spacer filaments. It was observed that spacers with equal filament diameters usually result in a higher pressure drop across the channel and such symmetric spacers also result in a more uniform shear rate at the top and bottom faces of the test cell. Asymmetric spacers (spacers with unequal filament diameters) resulted in lower pressure drop and also induced unequal shear rate on the top and bottom faces of the test cell. Such unequal shear rates at the top and bottom faces would be expected to have an adverse impact on the membrane module performance because of different mass transfer characteristics for adjacent membrane leaves. It was found that a higher overall bulk turbulent flow would not necessarily result in higher shear rates at the top and bottom faces.