G. Saccone

Electric field effects on the dynamics of bubble detachment from an inclined surface

An experimental apparatus to study bubble detachment from an inclined surface under the action of electric forces is described. It consists of a container filled with FC72 at room temperature and pressure where a train of gas bubbles is injected from an orifice. An electrostatic field can be imposed around the bubble, while the cell can be tilted from 0 to 90°. It is possible to study interface growth with the aid of high-speed cinematography. Since the interface is asymmetrical, a mirror system allowed to acquire, in the same frame, two images at 90° of the bubble. Different inclinations, injection rates and voltages were tested in order to couple the effects of shear gravity and electric field. Curvature and contact angles have been derived with appropriate interpolation methods of the profile. Force balances on the bubble were checked, finding an electric force, which, at first pulls the bubbles from the orifice, then pushes it against the surface. The motion of the center of gravity confirms this behaviour. A power balance has been developed to determine the energy contributions, revealing that surface growth incorporates both the effects of inlet power and electric field.

Effects of Force Fields on Interface Dynamics, in view of Two-Phase Heat Transfer Enhancement and Phase Management for Space Applications

On earth, gravity barely influences the dynamics of interfaces. For what concerns bubbles, buoyancy governs the dynamics of boiling mechanism and thus affects boiling heat transfer capacity. While, for droplets, the coupled effects of wettability and gravity affects interface exchanges. In space, in the lack of gravity, rules are changed and new phenomena come into play. The present work is aimed to study the effects of electric field on the shape and behaviour of bubbles and droplets in order to understand how to handle microgravity applications; in particular, the replacement of gravity with electric field and their coupled effects are evaluated. The experiments spread over different setups, gravity conditions, working fluids, interface conditions. Droplets and bubbles have been analysed with and without electric field, with and without (adiabatic) heat and mass transfer across the interface. Furthermore, the results of the 4 ESA Parabolic Flight Campaigns (PFC 58, 60, 64 & 66), for adiabatic bubbles, adiabatic droplets and evaporating droplets, will be summarized, discussed, and compared with the ground tests.