THE INFLUENCE OF SEPARATORS ON HYDRODYNAMICS AND MASS-TRANSFER IN NARROW CELLS - FLOW VISUALIZATION

Commercial large-scale flat sheet membrane plant is, for various reasons, based on narrow cells, created by some kind of separator structure between the membranes. Ideally, the separator should offer minimal flow resistance while inhibiting the formation of mass transfer barriers. Our study examines idealised geometries to gain a better understanding of hydrodynamics around obstructions in narrow cells, and hence their effect on mass transfer. Various flow visualisation methods using a model of a narrow cell with its thickness scaled up ten times, containing attached and suspended rods, have been employed to determine the flow characteristics (100 < Re(h) < 1000). Most geometries exhibit oscillations starting at Re(h) between 180 and 280 i.e. one order of magnitude lower than in the equivalent empty channel. Both types of geometries can cause separation of flow near the wall: most suspended geometries require Reynolds numbers in the unsteady range whereas attached geometries show a recirculation zone on the wall throughout the whole range under consideration. Possible implications for mass transfer will be discussed briefly.