Influence of Soret effect on thermal convection of a binary mixture in a shallow cylindrical pool with a free surface

Abstract

In this paper, a series of experiments are conducted to understand the influence of Soret effect on thermal convection of binary mixture in a cylindrical pool with a free surface. The cylindrical pool is filled with the n-decane/n-hexane mixture with an n-decane initial mass fraction of 50%. The cylindrical pool and the disk on the free surface are kept at constant temperatures of Th and Tc (Th Tc), respectively. Temperature fluctuation pattern on the free surface is obtained by the schlieren method. Various temperature oscillatory patterns on the free surface are observed when the thermal convection of the n-decane/n-hexane mixture destabilizes at different aspect ratios. Results show that the critical thermal capillary Reynolds number of the incipience of the three-dimensional oscillatory flow in the n-decane/n-hexane mixture is smaller than that in the n-hexane fluid, and the variation tendency with the aspect ratio in the n-decane/n-hexane mixture is the same as that in the n-hexane fluid. The solute-capillary force caused by Soret effect plays an important role of the thermal convection in the n-decane/n-hexane mixture. Because the solute-capillary force has the same direction as the thermocapillary force, the thermal convection in the n-decane/n-hexane mixture becomes more instable and the critical thermocapillary Reynolds number is smaller than that in the n-hexane fluid. In the n-decane/n-hexane mixture, when the aspect ratio increases from 0.0217 to 0.0392, the critical thermal capillary Reynolds number decreases from 7.2104 to 5.0104. With the increase of the aspect ratio, the effect of the buoyancy is enhanced, and the critical thermocapillary Reynolds number decreases. When the aspect ratio increases from 0.0392 to 0.0434, the cold plume which facilitates destabilizing the thermal convection cannot be obviously enhanced. There is little effect of the cold plume on the fluid near the bottom. Therefore, the critical thermal capillary Reynolds number increases from 5.0104 to 6.4104 in this range. In the deep pool, the critical thermal capillary Reynolds number is almost a constant value. When the aspect ratio is smaller than 0.0848, the three-dimensional oscillatory flow occurs and the hydrothermal waves are observed. After the three-dimensional oscillatory flow appears, two groups of the hydrothermal waves with opposite propagating directions coexist in the pool. With the increase of the thermal capillary Reynolds number, the honeycomb-like patterns appear on the free surface, which are similar to the Bnard cells. In addition, the non-dimensional fundamental oscillation frequency increases with the thermal capillary Reynolds number. When the aspect ratio is bigger than 0.0848, spoke pattern, rosebud-like pattern and thin-longitudinal stripes will appear sequentially with the increase of thermocapillary Reynolds number. Furthermore, the number of the rosebud-like patterns decreases, while the area on the free surface in the pool occupied by the rosebud-like pattern increases with the increase of the thermal capillary Reynolds number.