Oscillatory Convection Due to Combined Buoyancy and Thermocapillarity

An experimental study is performed on oscillatory convection due to combined thermocapillarity and buoyancy in open cylindrical containers. The fluid in the containers is heated by a heating rod placed along the centerline of the containers. Various container diameters, ranging from 0.39 to 3.1 cm, are used to change the effect of buoyancy on steady and oscillatory convection. The container aspect ratio (height/ radius) ranges from 0.75 to 2 and the heater diameter is 10% of the container diameter in all cases. The test fluids are 1- and 2-cS silicone oils (Prandtl number = 13 and 27 at 25°C). The velocity field is studied by flow visualization, whereas the temperature field is measured by a thermocouple probe and thermography. The onset of oscillations and the oscillation frequency are investigated under various conditions. It is found that the basic flow structure is mainly unicellular and thermocapillarity is important for the oscillations in all of the tests. When thermocapillarity is dominant, the Marangoni number is not sufficient to describe the onset conditions. When buoyancy is not negligible, it tends to squeeze the primary cell by thermal stratification and the dynamic Bond number is the parameter characterizing the transition.

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