Experimental study and numerical simulation of thermocapillary convection around gas bubbles

At the interface of a liquid-gas or liquid-liquid system thermocapillary or Marangoni convection develops in the presence of a temperature or concentration gradient. This convection was not paid much attention up to now, because under terrestrial conditions it is superimposed by the strong buoyancy convection. In a microgravity environment, however, it is the remaining mode of natural convection, and was observed during subcooled boiling in microgravity. Therefore, the question arises about its contribution to the heat transfer without phase change. Thus the thermocapillary convection was intensively studied at single gas bubbles in various liquids both experimentally and numerically. The overall heat transfer around single bubbles of different volume was measured inside a temperature gradient chamber, and simultaneously the liquid flow was observed with PIV and LDV. Supplementary a 2D CV-FEM method was developed to simulate the coupled heat transfer and fluid flow. Even in the case where the thermocapillary flow is counteracted by buoyancy flow, a distinct increase on the heat transfer in the liquid volume was found. Under a significantly reduced gravity level the numerical results point out that the thermocapillary convection can partially replace the buoyancy as heat transfer mechanism.