Impact of feed side filament mesh spacing on wall shear stress and mass transfer coefficient in spacer filled narrow channels

Material build-up on the membrane surface is one of the vital challenges faced by Reverse Osmosis (RO) operations which leads to many operational and maintenance issues. Literature review reveals that Computational Fluid Dynamics (CFD) tools predict flow behaviour and concentration patterns quite accurately when applied to cross-flow membrane operations and can be used to investigate the resulting flow and concentration patterns by changing the membrane secondary structure. In the present work, the mesh length of the feed side spacer is varied in a systematic manner to study the resulting impact on wall shear stress and mass transfer coefficient of a mono-valent solute (such as NaCl). Ansys Fluent enabled flow visualization explaining the variations in wall shear stress and similarities in mass transfer coefficients for the two membrane walls. It was interesting to note that the shear stress on top membrane wall was significantly higher compared to the bottom membrane wall (3 to 8 times higher at Reh=100 for the cases studied), when top and bottom spacer filaments were oriented in axial and transverse direction to the flow respectively. However, the average mass transfer coefficient values for the two walls were estimated to be almost similar for low to moderate filament spacing.