Thermocapillary convection during subcooled boiling in reduced gravity environments

Complete understanding of the origin of thermocapillary convection during subcooled boiling remains elusive. Some recent studies have suggested that the presence of dissolved gas inside the vapor bubble leads to the formation of a localized concentration and temperature gradient along the liquid-vapor interface and thus to the onset of thermocapillary convection. These studies reported that the dissolved gas content determines the onset of thermocapillary convection and that no thermocapillary motion was observed for subcooled boiling with pure liquid. However, other experiments performed in reduced gravity suggest that the strength of the thermocapillary convection around the primary bubble in a gas-saturated fluid is much weaker than that in a degassed fluid. This paper presents a qualitative study of the effects of dissolved gas content, bubble shape and size, and heat transfer coefficient on the strength of thermocapillary convection and offers possible explanations for the existing confusion. Owing to the presence of different complex and interrelated mechanisms, we decoupled the individual mechanisms to study their overall effect on thermocapillary convection. The results indicate that variations in the heat transfer coefficient and in the shape and the size of the bubble play a major role in the development of thermocapillary motion.