Unsteady Flow and Mass Transfer Induced by Rayleigh-Bénard-Marangoni Convection.

Abstract

Evaporative cooling at the water surface is usually modelled by imposing a constant heat flux at the surface. This boundary condition allows for variations in the water surface temperature T, which then induce variations in surface tension. The resulting Marangoni forces tend to move surface water from low surface tension (high T) regions to high surface tension (low T) regions. To study the combined effect of buoyancy and Marangoni forces on interfacial mass transfer, (fully resolved) direct numerical simulations have been performed. The simulations were carried out for a fixed macro Rayleigh number of RaL = 21200 and a variety of Marangoni (Ma) numbers to assess the relative importance of buoyancy and Marangoni forces on the air-water mass transfer. It is known that both forces tend to reinforce one another even though the underlying physical mechanisms are different. This is highlighted in the present results showing that Marangoni forces, acting at the water surface, induce very efficient mixing of dissolved gases (as well as heat) in a well-defined layer adjacent to the surface. Buoyant convection, on the other hand, tends to cause deep penetration of plumes of cold, saturated water into the bulk.