Lateral coalescence of bubbles in the presence of a DC electric field

Abstract

In this work, the influence of electrohydrodynamic forces on lateral bubble coalescence during nucleate pool boiling is investigated. An experimental pool boiling test facility was used with n-pentane as the working fluid. Boiling took place atop a polished copper surface on which two artificial nucleation sites were fabricated. The nucleation sites were 180μm in diameter and 500μm deep with a centre-to-centre spacing of 660μm. Two diametrically opposed windows allowed for illumination and high speed videography of the bubble growth process from the two nucleation sites. For the saturated boiling tests considered here, bubbles only formed at the two artificial nucleation sites allowing their coalescence behaviour to be scrutinized. A screen electrode above the boiling surface and a high voltage DC power supply facilitated the establishment of the electric field which was varied between 0 and 34.5kVcm−1. Observation of the high speed videos has revealed that bubble coalescence is influenced in such a way that it is delayed in the presence of the electric field to such an extent that, at the highest electric field strength tested, it is avoided all together. To help explain the observed results, a simple numerical model is solved showing that bubbles in close proximity to one another create an electric field distribution with high intensity between them. The overall result is net polarization forces that push the bubbles apart, and the closer they are together the larger this repulsive force becomes.