MASS-TRANSPORT IN A WEAKLY STRATIFIED ELECTROCHEMICAL-CELL

A study of natural convection in an electrochemical system with a Rayl eigh number of the order 10(10) is presented. Theoretical and experime ntal results for the unsteady behaviour of the concentration and veloc ity fields during electrolysis of an aqueous solution of a metal salt are given. The cell geometry is a vertical slot and the reaction kinet ics is governed by a Butler-Volmer law. To reduce the effects of strat ification, the flush mounted electrodes are located (symmetrically) in the middle parts of the vertical walls. It is demonstrated, both theo retically and experimentally, that a weak stratification develops afte r a short time, regardless of cell geometry, even in the central part of the cell. This stratification has a strong effect on the velocity f ield, which rapidly attains boundary layer character. Measured profile s of concentration and vertical velocity at and above the cathode are in good agreement with numerical predictions. For a constant cell volt age, numerical computations show that between the initial transient an d the time when stronger stratification reaches the electrode area, th e distribution of electric current is approximately steady.