ALBERT

Description: The effect of surface tension on the onset of convection in a horizontal double-diffusive layer was studied both experimentally and by linear stability analysis. The experiments were conducted in a rectangular tank with base dimension of 25 x 13 cm and 5 cm in height. A stable solute (NaCl) stratification was first established in the tank, and then a vertical temperature gradient was imposed. Vertical temperature and concentration profiles were measured using a thermocouple and a conductivity probe and the flow patterns were visualized by a schlieren system. Two types of experiments were carried out which illustrate the effect of surface tension on the onset of convection. In the rigid-rigid experiments, when the critical thermal Rayleigh number, RT, is reached, large double-diffusive plumes were seen simultaneously to rise from the heated bottom and descend from the cooled top. In the rigid-free experiments, owing to surface-tension effects, the first instability onset was of the Marangoni type. Well-organized small plumes were seen to emerge and persist close to the top free surface at a relatively small RMT (where subscript M denotes ‘Marangoni’). At larger RbT 〉 RtT (where subscript t denotes ‘top’) these plumes evolved into larger double-diffusive plumes. The onset of double-diffusive instability at the bottom region occurred at a still higher RbT 〉 RtT (where subscript b denotes ‘bottom’), A series of stability experiments was conducted for a layer with an initial top concentration of 2wt% and different concentration gradients. The stability map shows that in the rigid-free case the early Marangoni instability in the top region reduces significantly the critical RT for the onset of double-diffusive convection. Compared with the rigid-rigid case, the critical RT in the top region is reduced by about 60 % and in the bottom region by about 30%. The results of the linear stability analysis, which takes into account both surface-tension and double-diffusive effects, are in general agreement with the experiments. The analysis is then applied to study the stability characteristics of such a layer as gravity is reduced to microgravity level. Results show that even at 10-4go, where g0is the gravity at sea level, the double-diffusive effect is of equal importance to the Marangoni effect. © 1995, Cambridge University Press. All rights reserved.