SINGLE AND DUAL VAPOR BUBBLE EXPERIMENTS IN MICROGRAVITY

A series of experiments for single and dual vapor bubbles of single component liquid (CFC-113) and non-azeotropic binary mixtures (CFC-112/CFC-12 and water/ethanol) have been conducted in a microgravity condition available in a 490-m free-fall drop shaft. The bubbles were nucleated and grown in the microgravity condition, and were observed with the aid of a two-wavelength Mach-Zehnder interferometer for the evaluation of both temperature and concentration profiles. An onset of significant Marangoni effect was observed for subcooled boiling in CFC-113, though appreciate Marangoni effect no longer appeared in the saturation boiling. Both the temperature and concentration distributions around a single bubble were clearly obtained for CFC-112/CFC-12 mixture, and the thickness of thermal and mass boundary layers developed around a bubble was found to be one order of magnitude larger than those predicted by pure diffusion models. The behavior of the temperature and concentration profiles suggested the influence of the Marangoni effect induced by temperature gradient in the early stage of bubble growth and alternative Marangoni effect induced by concentration gradient in the later stage of bubble growth. The boiling behavior of water/ethanol mixture showed the nucleation of secondary bubble from the bottom of a primary bubble, which suggests the existence of liquid layer with an appreciable thickness underneath bubbles. Such liquid layer is considered to be supplied by the Marangoni effect induced by the concentration gradient around three-phase interline, formerly pointed out by the present author.