Feed flow patterns of combined Rayleigh-Bénard convection and membrane permeation

Abstract

In micro- and nano-scale filtration processes, concentration polarization and fouling phenomena degrade the overall performance of porous membranes. To reduce these phenomena we aim to induce hydrodynamic instabilities near the membrane surface, which promote local turbulence in the vicinity of the membrane-solution interface. In this work, we utilize thermal instabilities related to buoyancy-driven Rayleigh-Bénard convection by heating a novel electrically conductive membrane. The heated membrane induces a local temperature gradient and thus a convective flow perpendicular to the membrane. We investigate the evolution of the mixed convection flow patterns arising from combining Rayleigh-Bénard convection with pressure-driven membrane permeation. The flow patterns are analyzed experimentally by particle image velocimetry and computed with CFD simulations. We study the contribution of the individual convection modes on the resulting mixed convection using the Richardson (Ri) number. By inducing natural convection on the steady flow pattern from dead-end permeation, a large convective vortex forms with significant shear velocities near the membrane surface. These shear velocities increase with increasing impact of natural convection. The resulting shear flows show good potential in enhancing the mixing of the bulk and to enable convective mass transport of accumulated solutes away from the membrane and thus reduce concentration polarization and membrane fouling.