CFD simulations of net-type turbulence promoters in a narrow channel

The most common spacers or turbulence promoters for membrane processes are net-like materials which enhance mass transfer as well as provide passage for feed solutions. The enhanced membrane performance of spacer-filled channels is determined by the fluid flow patterns induced by the spacer filaments. Insight into the effect of spacer characteristics can be obtained by computational fluid dynamics. In this research, the commercial finite volume package FLUENT was used to visualise the flow pattern in a rectangular membrane channel. Three transverse filament arrangements were simulated. The results show that both high shear stress regions and eddies are present in the channel due to the spacer cylinders. The mass transfer enhancement on the wall/membrane surface is directly related to the high shear stress value, velocity fluctuation, and eddy formation. The peak shear stress and velocity fluctuation are repeated after each spacer cylinder, while the eddies are generally found before and after each cylinder. The CFD simulation also suggests that reducing the transverse filament distance will reduce the distance between shear stress peaks and consequently introduce larger shear stress regions near the wall region and increase the number of eddies, which will benefit membrane mass transfer. However, the penalty for this is that energy losses will also be significantly increased. The selection of optimum spacer geometry design involves a trade-off between these competing effects.