2003 Volume 29 Issue 5 Pages 614-621
An experimental study of double-diffusive convection in a two-layer, solutally stratified aqueous solution destabilized by lateral heating and cooling was performed for a rectangular cavity using Na2C 03 or NaCl as a solute. The mechanism and characteristics of mass transfer across the boundary between the two layers were investigated by visualization of temperature and flow fields and measurement of mass transfer rate across the boundary. Solute is transferred from the lower layer to the upper one by the following mechanism. Initially, a mid-layer is formed between the two layers. Part of fluid in the mid-layer is released intermittently into the upper layer near the heating wall, and disperses into the bulk after reaching the ceiling of the upper layer. During the subsequent pseudo-steady period, solute is transferred by molecular diffusion across the interface formed between the two layers. In the final stage just before the interface breaks down, part of fluid in the lower layer directly flows into the upper one along the heating wall. A correlation was presented for the Sherwood number of the pseudo-steady period as a function of the thermal Rayleigh number, Rat, the Schmidt number, Sc, and the initial buoyancy ratio, No. The proposed equation agrees well with the results of mass transfer measurements from other studies using NaCl aqueous solutions. The equation also explains the linear relation between the time when the mixing of the two layers starts and the initial buoyancy ratio, No.
