A theoretical analysis of a spacer filled flat plate membrane distillation modules using CFD: Part I: velocity and shear stress analysis

This work presents the theoretical analysis of the velocity and shear stress in a flat plate direct contact membrane distillation system. ANSYS CFX was used to create 3-D CFD model to study the effect of different hydrodynamic angles on the velocity profiles and shear stress. The membrane was assumed fully permeable and the heat and mass transfer mechanisms through the membrane were modelled by implementing FORTRAN user-subroutines at the boundary intersection between the feed and permeate side. Three hydrodynamic angles were tested: αf = 45°,αf = 90° and αf = 0°. Results have shown that for the system examined, the alignment of the flow does not depend on the increasing Reynolds number. Two types of flow were identified: zig- zag pattern in the bulk flow and flow parallel to the filaments. The types of flow observed depended on the hydrodynamic angle. Finally, it was further shown that the most favourable configuration was when the system was operating with a hydrodynamic angle αf = 45° as it produced better mixing and enhanced shear stress resulting in higher fluxes and the least desirable configuration was when the hydrodynamic angle αf = 0°.