Experiments on thermocapillary-buoyancy convection of medium Prandtl number liquids in annular pools heated from inner cylinder.

• Thermal convection in annular pools with a heated central column is observed. • The effects of heating mode and Prandtl number on the flow patterns are analyzed. • Evolution of 3D flow patterns with the temperature difference is revealed. • Critical temperature difference depends on the heating mode and Prandtl number. • The wavenumber of 3D flow pattern decreases with the increase the liquid pool depth. A series of terrestrial experiments on thermocapillary-buoyancy convection of medium Prandtl number liquids (Pr = 6.7-25.2) were performed in annular pools with different depths and a heated inner cylinder. The temperature fluctuation pattern on the free surface was recorded by the schlieren method. Experimental results indicate that thermocapillary-buoyancy convection could weaken and the critical Marangoni number of the flow transition increases with the Prandtl number. When the flow destabilizes, the rotating petal-like structure and the spoke pattern appear respectively in the annular pools with the depths of 4-5 mm and 6-8 mm. As the Marangoni number increases, the flow becomes oscillating as a result of the thermal perturbation induced by the thermocapillary effects. Besides, with the increasing liquid pool depth, the strengthened buoyancy force has an impact on the basic flow and the thermal fields. Thus, the temperature fluctuation frequency of the oscillating petal-like structure is more than that of the oscillating spoke pattern. Different from the cylindrical geometry heated from the outer cylinder, the radial waves and the petals appear respectively near the outer cylinder and the inner cylinder. Furthermore, the wavenumber decreases as the liquid pool depth increases. [ABSTRACT FROM AUTHOR]