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In this paper, we present a set of linear-stability analysis results of the thermocapillary-buoyancy convection in an annular two-layer system of 5cSt silicone oil and HT-70 with a radial temperature gradient. The annular two-layer system is heated at the outer cylindrical wall and cooled at the inner wall. The effects of the depth ratio, the radius ratio and the aspect ratio of the annular pool on the critical conditions for the convection transition are analyzed. The flow destabilizing mechanism is identified. Results indicate that the hydrothermal wave, or the combined hydrothermal wave and three-dimensional oscillatory flow, should be responsible for the flow destabilization when the stable thermocapillary-buoyancy convection transits to the three-dimensional oscillatory convection. With the increase of the radius ratio of the annular pool, the critical Marangoni number, wave number and phase velocity all decrease. With the increase of the depth ratio η , the critical Marangoni number initially increases before a dramatic decrease. The maximum value of the critical Marangoni number locates in the range of η = 0.625–0.833. [ABSTRACT FROM AUTHOR]

• The energy transfer mechanism in ethanol evaporation is revealed. • 3D oscillatory flow in deep pools is observed at low pressure ratios. • Critical pressure ratio of flow destabilization is determined at different depths. • The evolution process of the flow pattern is exhibited by numerical simulation. • Variation of temperature discontinuity across the liquid–vapor interface is given. For the purpose of revealing the energy-transfer process during ethanol evaporation under low pressure environment, experimental measurement and numerical simulation are adopted for ethanol evaporation at different liquid depths and external radial temperature differences in annular pools. The results show that two thermocapillary vortexes appear below the liquid–vapor interface. Most of the latent heat of vaporization is from the liquid phase, but little from the gas phase. With the increase of liquid layer depth, the steady axisymmetric flow will transform into three-dimensional time-dependent convection. The mechanism of flow instability should be attributed to the variation lag of velocity behind the variation of flow resistance. The critical vapor pressure of flow destabilization depends on radial temperature difference and liquid layer depth. Furthermore, the magnitude of temperature discontinuity across the liquid–vapor interface is the largest near the hot inner cylinder. With the increase of the liquid layer depth, the temperature discontinuity decreases. [ABSTRACT FROM AUTHOR]

• 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]