Oscillatory thermocapillary convection of half-zone liquid bridge with consideration of ambient air motion and heat transfer

摘要

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, 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 ΔTc, 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 by the research in recent years. But under the gravity environment, phenomenon that only thermocapillary convection cause ware not understood, because the influence of buoyant flow is strong. Thermocapillary convection emerges dominantly under microgravity and at a micro scale, where the buoyant effect can be almost neglected. This problem is solved by experimental system is micro scale, and placing horizontal partition disk near both the top rod and bottom rod in the area of ambient air. With partition disk, 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 ambient region with or without partition disk. The transition ΔTc once increases and then decreases with increasing heat gain. It turned out that this change in ΔTc is accompanied by the transition of the azimuthal mode number.