Mass transfer study around a turbulence promoter by electrochemical method

Abstract

To investigate the mass transfer enhanced by the turbulence promotor, local mass transfer rates around a turbulence promoter, in a channel having the geometry of zigzag type and cavity type, characterized by the location of promoter, were measured applying limiting current technique. To measure the local mass transfer rates, currents on the sandwich type nickel cathode, where reduction of ferricyanide takes place, were measured in a solution of equal molar amounts 1 M, of ferricyanide and ferrocyanide and 1 M sodium hydroxide as a supporting electrolyte, which eliminates the migration effect. From experiment, zigzag type promoter was proved to be more effective than cavity typepromoter in breaking the concentration boundary-layer and thus increasing the mass transfer rate. It seemed to be due to the fact that concentration boundary-layer can grow along the wall without promoters for cavity type promoter. Since the eddy sizes of recirculating flow before and behind the turbulence promoter were varied as Reynolds number and have the influence upon the Sherwood number distribution, the relationship between eddy size and Sherwood number distribution, was analyzed. In the comparison of experimental Sherwood number with numerical results, the latter was partially dependent upon the correlation order of concentration profile near the wall, the effect of which was analyzed to show that the first or third order agrees better than second order with experimental results, since second order correlated concentration profile does not satisfy the no slip boundary condition at the wall. Deviation between numerical and experimental results increase as Reynolds number, due to the fact that the accuracy of numerical scheme is reduced as Peclet number increases. Mean Sherwood number in the first stage for the zigzag type promoter was correlated as a function of Reynolds number and aspect ratio (the ratio of distance between successive promoters to channel height) to give the re...