The role of the electric field in the departure of vapor bubbles in microgravity

Abstract

We present the results of a study aimed at investigating the effects of electric fields on quasi-static bubble departure dynamics during pool boiling of perfluorohexane (FC-72) in microgravity conditions. Analysis was performed through an alternative formulation of the bubble momentum balance in which the contribution of non-uniform electric stress distributions at the bubble interface can be quantified through high-speed video measurements without having to numerically solve the laws of electrostatics. Data used in this study were obtained in the scope of the Multiscale Boiling Project, which included advanced single bubble growth experiments performed aboard the International Space Station. Our results confirm that bubble departure counterintuitively begins before the force resulting from electric stresses starts to pull the bubble up from the wall. When this occurs, it is shown that the shrinking process of the contact line accelerates, in agreement with known theoretical results. It is concluded that the electric force is essentially determined by the electric stress distribution at the bubble cap above the contact area. Furthermore, we show that the electric stress at the bubble interface is also responsible for the increase in bubble internal overpressure, which explains the early departure of the bubble while increasing the intensity of the electric field. The results of this study provide an important step in achieving a more comprehensive understanding of the bubble behavior at the heated surface in the presence of an electric field, which is essential to optimally design electrodes and two-phase heat transfer devices for future space applications.