Effects of Small Flow Obstacles on the Limiting Current and Pressure Drop in a Square Duct

摘要

The effectiveness of mass transfer enhancement by small obstacles attached to the cathode in a 0.5 by 0.5 cm square cross section 50.8 cm long electrochemical flow cell was investigated. Pressure drops, and limiting currents for the reduction of ferricyanide were measured in the range of 80 less than or equal to Re less than or equal to 3200. Using rectangular obstacles extending 0.0254 to 0.160 cm from the surface, the spacing to height ratio ranged from 9 to 170. Flow patterns around obstacles were visualized using suspensions of inert materials and recorded by means of dark field photography. Depending on the flow rate, addition of the obstacles can increase mass transfer rates by as much as five times over those in the unobstructed system. The degree of enhancement increases with decreasing obstacle spacing until an optimal spacing is reached. At Re approx. = 100, the mass transfer rate is approximately doubled upon addition of the optimal number of obstacles without appreciably affecting the pressure drop. The largest enhancement (5-fold) occurs at the upper laminar flow regime with a 2.7-fold increase in the pressure drop. These comparisons are made at the same flow rate. In the unobstructed system, improved mass transfer rates (limiting currents) are achieved by increasing the flow rate to produce turbulent flow. This approach involves large pressure drops and consequently large pumping power requirements. An increase in the limiting current by the addition of obstacles, on the other hand, requires only a very small fraction (2 to 5 percent) of the pumping power required to obtain the same limiting current achieved by increasing the flow rate in the unobstructed system. Obstacles therefore produce efficient mixing near the electrode surface and correspondingly high mass transfer rates without causing large energy dissipation in the bulk fluid. (ERA citation 09:000601)