A CFD study of unsteady flow in narrow spacer-filled channels for spiral-wound membrane modules

In spiral-wound membrane modules, spacers are used to enhance wall shear stress and to promote eddy mixing, thereby reducing wall concentration and fouling. Insights into the effect of spacer filaments on flow patterns in narrow channels were obtained using a computational fluid dynamics (CFD) code. The flow patterns were visualized for different filament configurations incorporating variations in mesh length, filament diameter and for channel Reynolds numbers up to 1000. The simulated flow patterns revealed the dependence of the formation of recirculation regions on the filament configuration, mesh length, filament diameter and the Reynolds number. When the channel Reynolds number is increased above 300, the flow becomes super-critical showing time-dependent movements for a filament located in the center of a narrow channel; and when the channel Reynolds number is increased above 500, the flow becomes super-critical for a filament adjacent to the membrane wall. For multiple filament configurations, flow transition can occur at channel Reynolds numbers as low as 80 for the submerged spacer at a very small mesh length (lm/hch = 1) and at a slightly larger Reynolds number at a larger mesh length (lm/hch = 4). The transition occurs above Rech of 300 for the cavity spacer (lm/hch = 4) and above Rech of 400 for the zigzag spacer (lm/hch = 4).

Computational fluid dynamics calculations reveal complex relationships between filament configurations, mesh length, filament diameter, Reynolds number and the formation of recirculation regions. The transition to time-dependent flows in an obstructed channel occurs at much smaller Reynolds numbers than in an empty narrow channel. For a single filament in a narrow channel the wall damps the transition which occurs at slightly higher Reynolds numbers than for a cylinder in a “free flow” channel. Different stages are observed for the transition to unsteadiness from slight movement to downstream convection of recirculation regions. Accurate evaluation of the two-dimensional time-dependent flows demands extremely fine numerical grids and small time steps which increases the computational costs. Coarse grids and large time steps suppress the development of the time-dependent flow movements. The complexity of the two-dimensional results suggest that extension of the calculations to three dimensions will require careful and systematic identification of appropriate modeling procedures. Time-dependent movements of threedimensional recirculation regions are likely to be extremely complex and sensitive to small changes in modeling conditions.