Mechanism of natural convection of water-or aqueous solution-fine particle suspension

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Natural convection of water-or aqueous solution-fine particle (micro glass beads) suspension was investigated experimentally. Two kinds of experiments were carried out. In the experiment (1), a rectangular cell with 80mm width and 120mm height was used. Both the vertical walls were cold surfaces maintained at 10ﾟC and a central part of the bottom wall with 80mm width was heated. During the fall of particles, the suspension forms many layrs and the layrs vanish gradually. As the concentration of the solution increases, the sedimentation velocity in the case of saccharose aqueous solution, becomes slower but it becomes larger in the case of NaCl aqueous solution. When the concentration of NaCl aqueous soliton is larger than lwt%, the layrs do not appear.In the case of water-particle suspension, as the temperature difference between the hot and cold walls increases, the layrs vanish in shorter time.In the above experiments mean diameter of particles was 9.76mum and standard deviation was 3.62mum. When particles with a narrow distribution of particle size (2.04mum mean diameter and 0.52mum standard deviation) are dispersed in water, only two layrs can be observed. The interface of the two layrs goes down gradually and after then goes up quickly to the top of the suspension and vanishes.In the experiment (2), glass beads with 4.75mum mean diameter were dispersed in water in a rectangular cell with 40mm width and 160mm height. The temperature difference between the opposing vertical walls was kept at 10ﾟC.Several layrs were also observed. The falling velocities of the interfaces are constant and become slower as the initial concentration of particle increases. The lower interfaces have the larger falling velocities.