Numerical simulations of bubble formation from a submerged orifice and a needle: The effects of an alternating electric field

In many applications, such as bubble column reactors, electric field is employed to provide a greater control on the sizes of bubbles forming at orifices and needles. In this study, we investigate the effects of an alternating electric field on the bubble dynamics. We perform numerical simulations of an alternating electric field coupled with two-phase flow using a Coupled Level-Set and Volume-of-Fluid method. We show that bubbles forming at orifices and needles decrease in size (up to 30%) only for a range of applied frequency and for other frequencies, the size of bubbles can be much bigger compared to the bubbles forming in the corresponding DC electric field case. The oscillating electric forces excite capillary waves on the bubble interface resulting in applied frequency dependent bubble oscillations. The numerically observed resonance for the needle case corresponds to 2 omega tau(c) = 0.75, where 2 omega is the frequency of the oscillation of the electric field force at the interface and tau(c), is the capillary time scale, indicating that the resonance behavior is indeed governed by the interactions between the capillary and electric field force. A decomposition of bubble profile shapes into Legendre modes shows that for orifice as well as the needle case, second mode is most dominant followed by the fourth mode. (C) 2015 Elsevier Masson SAS. All rights reserved