Origin and effect of thermocapillary convection in subcooled boiling: observations and conclusions from experiments performed at microgravity.

During the past two decades we have performed pool boiling experiments under microgravity conditions with saturated and subcooled liquids using various fluids, mostly fluorinated hydrocarbons, and various shapes of heaters. The common observation at subcooled boiling is the development of thermocapillary convection around bubbles. This convection forms jet streams above the head of the bubbles that carry the heat from the bubbles into the ambient liquid. Heat transfer measurements demonstrate that this flow does not directly contribute to the overall heat transfer itself, but that it is an important transport mechanism in subcooled boiling, not only at microgravity, but also under terrestrial conditions. The development of a thermocapillary flow is surprising, because it is well known that for this convection a temperature gradient along the interface of a bubble is necessary. However, it is also well known that the interfacial kinetics of evaporation and condensation is very strong. Thus, small temperature differences generate strong mass flow rates across the interface, coupled with high heat transfer rates that immediately equalize even moderate temperature gradients appearing along the bubble interface. This is confirmed by the observation that, during boiling in saturated liquids under microgravity conditions, thermocapillary convection was never observed, thus indicating uniformity of the temperature along the interface. From this we must conclude that the temperature difference of wall superheat, which generates boiling, cannot be the driving force for the observed thermocapillary flow even in subcooled liquids. Therefore, the question about the origin of this flow arises and is discussed in this paper.