Experimental Study on Thermal Convection in Annular Pools Heated from Inner Cylinder

In order to understand the effect of the radius ratio on thermocapillary-buoyancy convection, a series of experimental observations on thermocapillary-buoyancy convection of 0.65cSt and 1cSt silicone oils have been conducted in annular pools with a smaller heating inner cylinder and the radius ratio of 0.25. The results show that the rotating petal-like structure or spoke pattern first appears on the free surface when thermocapillary-buoyancy convection destabilizes. As the Marangoni number increases, the spoke pattern becomes oscillating at the depth of 6–7 mm and the oscillating frequency is decreased compared with the results in the liquid pool with a radius ratio of 0.5. The wavenumber of the flow pattern decreases when the radius ratio decreases from 0.5 to 0.25. Especially at the liquid depth of 7–8 mm, a mode switching of the flow patterns has been observed and the flow pattern becomes more like cellular structures in cylindrical pools instead of the spoke pattern with the increasing Marangoni number. With the increase of the Prandtl number, this flow transition becomes difficult. Besides, the short spokes appear near the inner cylinder at a large Marangoni number and the deep fluid layer. Based on the experimental results, the general relationship of the threshold complex with the dynamic Bond number and the radius ratio is achieved.