Influence of electric field on single gas-bubble growth and detachment in microgravity

The effect of electric and gravitational field on detachment and motion of gas bubbles was studied by injecting nitrogen in a fluoroinert liquid (FC-72) at ambient temperature and pressure through an orifice (about 0.1 mm diameter) drilled in a horizontal tube. In such a way, it was possible to investigate the mechanical effects in bubble dynamics separately from the thermal and mass exchange ones involved in boiling. An electric field of nearly cylindrical geometry was generated around the tube by imposing a D.C. potential drop V (0–18 kV) to a eight-rod cylindrical “squirrel cage” surrounding it. The apparatus was operated in microgravity conditions in the dropshaft of JAMIC in Hokkaido, Japan. Bubble size, detachment frequency and velocity were measured by digital processing of high-speed video images. The results showed that in the absence of electric field bubble detachment did not take place at low gas flow rate; conversely at higher gas flow, the dynamical effects were sufficient to induce bubble detachment even in the absence of the buoyant force. The application of electric field was confirmed to be effective in promoting bubble detachment at values of diameter greater but of the same order of magnitude as in normal gravity, and in providing a force to remove the bubbles away from the orifice.