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Electric Field Effects on Bubbles of Nitrogen in FC-72 Originating from a Flat Plate

The understanding of the dynamics of bubbles in “adiabatic” (i.e. absence of mass transfer) conditions is useful to open the way to the study of the more complex thermal events. To this aim, an experimental apparatus was set up to study the behavior of nitrogen bubbles growing and detaching from a small orifice (0.1 mm diameter) drilled in a flat plate immersed in a fluoroinert fluid (FC72, by 3M) under the influence of an externally applied electric field (EF). The EF was generated by imposing a D.C. potential drop V (0-20 kV) to a grid of 5 parallel rods, laid parallel to the plate. A high-speed camera (up to 2000 fps) was used to monitor the bubble evolution. Bubble equivalent diameter and trajectory were determined by digital processing of the camera images. Three different configurations were tested. In the first one (configuration A) the orifice was directly underneath one of the rods, where the theoretical analyses showed that the electrical force should press the bubble against the surface, hindering detachment. In the second configuration (B), the orifice was placed mid-way between two rods, where the electrical force should have a detaching component. The third configuration (C) was analogous to A, with the grid tilted 10° in the attempt to generate a net electrical force parallel to the surface. Finally, the apparatus (in configuration A) was turned up side down, giving origin to bubbles directed against the gravitational force. The results are consistent with the hypothesis that the bubbles are mainly driven by the dielectrophoretic force, and showed that the gradient of the square of electric field intensity (grad E 2 ) is a good parameter for a first characterization of the EF actions on bubbles. At high void fraction, however, the EF was significantly altered in proximity of the plate by the massive presence of bubbles, and its alteration should be considered for a more accurate analysis.