Heat transfer near an isolated hemispherical gas bubble: The combined influence of thermocapillarity and buoyancy

Abstract

Thermal Marangoni convection about a 1mm radius hemispherical air bubble attached to a heated wall immersed in a silicone oil layer of constant depth of 5mm was numerically investigated. Tests were performed for a range of Marangoni numbers (145⩽Ma⩽915) with varying levels of gravitational acceleration between zero gravity and earth gravity in order to quantify the rates of heat transfer. For the zero gravity condition, a thermocapillary-driven vortex develops around the bubble extending the entire height of the channel. This flow structure causes a jet-like flow of heated liquid to emanate from the bubble tip which impinged on the cold wall. With the addition of gravity, a counter rotating secondary buoyancy-driven vortex forms which reduces the size of the thermocapillary vortex and disrupts the jet flow from the bubble tip. The wall heat flux profiles indicate that under zero gravity, the peak heat fluxes increase monotonically with Ma with an area of influence that increases asymptotically with Ma. Likewise, under gravity conditions the peak heat flux also increases with Ma and for low Ma is higher than that of 0-g. However, the area of influence is considerably smaller and not sensitive to Ma or the level of gravity. Experimental validation of selected terrestrial gravity numerical results was obtained using particle image velocimetry (PIV) for low to moderate Marangoni numbers. For all experiments, steady-state Marangoni convection was observed. The experimental flow patterns showed good agreement with the numerical solutions.