The onset of convection induced by buoyancy during gas diffusion in deep fluids

The onset of convection induced by buoyancy caused by interfacial gas diffu sion in deep fluids is analysed. It was found, as in heat transfer, that tr ansient convection during mass diffusion is dependent on a Biot number. A t ransient diffusive Blot number (Bi,) was defined such that Bi-D = 0 and inf inity correspond to constant mass flux (CMF) and fixed surface concentratio n (FSC) boundaries, which have theoretical critical Rayleigh numbers of 669 and 1100, respectively. Transient Rayleigh numbers were derived for both b oundary conditions. Experiments of soluble and sparingly soluble gases diff using in water were found to agree very well with the theory advanced in th is paper for the onset of convection in accordance with CMF and FSC models. The stable diffusion times were also predicted accurately for both gases. They also represent the theoretical time limits for the gas penetration the ory, which relies on Fick's law that assumes no convection. The horizontal dimension of the plunging plumes was also predicted with reasonable accurac y. Monolayer formed by surfactant was found to produce an FSC boundary for a soluble solute and to render the liquid surface rigid and prolonged the o nset of convection. (C) 1999 Elsevier Science Ltd. All rights reserved.