Mass transfer under forced laminar and turbulent convection at horizontal plane plate electrodes

Abstract

Mass transfer at horizontal plane plate electrodes under forced laminar and turbulent convection is treated for the two cases: I. Uniform potential (limiting c.d.) and II. Uniform c.d. (normal electrolysis). Expressions are derived for the thickness δ ind of the diffusion boundary layer, the limiting c.d. and the concentration at the electrode surface. The results can be expressed as Nu I,lam = 0·331 × Sc 1 3 × Re 1 2 , Nu II,lam = 0·417 × Sc 1 3 × Re 1 2 , Nu I,turb = 0·143 × Sc 1 3 × Re 3 5 , Nu II,turb = 0·170 × Sc 1 3 × Re 3 5 . Nu and Re are the local Nusselt's and Reynolds' numbers respectively and Sc the Schmidt number. Factors expressing the influence of an inlet length in front of the electrode are also derived. Experimental results on the electrodeposition of copper on a horizontal cathode in a cell of rectangular cross-section are reported. In the laminar range of flow the limiting c.d. is found to increase linearly with Re 1 2 and with a slightly greater slope than that calculated for a plane plate without side walls. At the transition to turbulent flow there occurs a sudden increase in the limiting c.d. The latter is an approximately linear function of Re 1 2 also in the turbulent range. The slope of the straight line is then considerably greater than what would be expected theoretically. This is probably due to the expected disturbing influence of the opposite wall and the side walls of the cell which is of greater importance in the turbulent than in the laminar range. Some practical implications of the results are discussed.