Oscillatory Thermocapillary Convection of Half-Zone Liquid Bridge with Consideration of Ambient Air Motion and Heat Transfer

Abstract

Thermocapillary convection is driven by surface tension gradient due to temperature gradient along the free surface of a liquid. Thermocapillarity is of fundamental importance in material processing and in micro scale. A floating-zone method is a material process technique for producing and purifying single crystals of metals and oxides. In a half-zone liquid bridge, which mimics a half of a floating-zone method, the thermocapillary convection of a high Prandtl number fluid is induced by applying the temperature difference ΔT between cylindrical hot and cold rods sustaining a liquid bridge. If ΔT exceeds a critical value ΔT_c, the flow field exhibits a transition from a two-dimensional steady flow to a three-dimensional time-dependent oscillatory one. The onset of the oscillation is known to be sensitive to heat transfer at free surface caused by the ambient air motion. Under the gravity environment, however, the thermocapillary convection is often hidden by the influence of the buoyancy. This problem is solved by experimental system with a small scale and with placing horizontal partition disks near both the top and bottom rods in the ambient air. With partition disk, the buoyant flow in the surrounding air can be suppressed and controlled. In the present study, an effect of the ambient temperature upon the stability of the thermocapillary convection is investigated experimentally and numerically considering the ambient region with or without partition disk. The transition ΔT_c once increases and then decreases with increasing heat gain as pointed out by Kamotani (2001). It turned out that this change in ΔT_c is accompanied by the transition of the azimuthal mode number.