Optical and electrochemical studies of transient free convection mass transfer at horizontal surfaces

Abstract

The onset of free convection at upward facing horizontal surfaces adjacent to a free fluid environment has been studied using the electrochemical system involving the electrodeposition of Cu 2+ ions. A transient potential step technique was employed and photography of the ensuing developing convective structure was synchronised with the recording of current-time transients. This enabled the interdependence of instantaneous mass-transfer rate and convective flow structure to be visualised. A number of distinctive flow structures in the transient and steady states were observed and these have been related to the correlation regimes for the overall mass-transfer rate. Interesting phenomena involving periodic instabilities have been discovered at transitional Rayleigh numbers leading to pulsations in the convective plume and corresponding regular oscillations in the steady state mass-transfer rate. The effect of surface roughening on mass transfer and hydrodynamic performance during extended experiments has been observed and a method of overcoming surface roughening difficulties in the application of the electrochemical technique is suggested. Diffusion transient undershoot times are correlated in terms of plate Rayleigh number by the equations Dt d 2 = 32·5(Gr d,Sc) − 2 3 in the range 10 7 < Re d < 10 11 and Dt d 2 = 1·80(Gr d,Sc) −0·51 in the range 10 4 < Ra d < 10 7. The periodic fluctuation frequencies are similarly correlated by the equation Dt d 2 = 12·8(Gr dSc) − 2 3 .