MASS TRANSFER ENHANCEMENT BY SMALL FLOW OBSTACLES IN ELECTROCHEMICAL CELLS

Mass transfer enhancement by small obstacles attached to the cathode in electrolytic flow cells of 5x5 mm cross-section and 500 mm length was investigated. Double beam laser interferometry was used to observe the local mass transfer boundary layer thicknesses preceding and following rod-shaped dielectric obstacles placed normal to the direction of electrolyte flow. Flow patterns have been visualized by use of suspensions of small inert particles and dark field photography. For the evaluation of the effectiveness of mass transport enhancement, pressure drops, and limiting currents for the reduction of ferricyanide have been measured in the range of Reynolds Number 80 to 3200. The degree of enhancement increases with decreasing obstacle spacing until an optimal spacing of approximately 15 times the obstacle size is reached. A three to five-fold increase in the average mass transfer coefficient is achieved by the use of obstacles with a small fraction of the pumping power required to obtain the same limiting current by increasing the flow rate in the unobstructed channel. Small obstacles produce efficient mixing near the electrode surface, and corresponding improvement in uniformity and magnitude of mass transport rates, without increasing the energy dissipation in the bulk fluid.