FORCES ON BUBBLES GROWING AND DETACHING IN FLOW ALONG A VERTICAL WALL

Experiments are performed on bubble detachment from an artificial cavi ty in a plane wall of a vertical rectangular channel. Mean upward velo city is varied. Steam bubbles are generated by local heating of the ca vity, nitrogen bubbles of about the same size by injection. The experi ments show a difference in take off direction between vapor and nitrog en bubbles. Steam bubbles take off into the liquid, while nitrogen bub bles more or less slide parallel to the wall. The bubble detachment ra dius decreases for increasing bulk liquid velocity, in a way that mere ly depends on the detachment radius without convection. Nitrogen bubbl es, coming from a capillary with approximately the same radius are lar ger than water vapor bubbles. A force coefficient fit is performed on force components perpendicular to the wail. By analyzing flow- and non -flow experiments separately, some of the forces are quantified. By co mbining the results of nitrogen bubble and steam bubble experiments, a force due to the temperature difference at the bubble foot is studied . Such a force could explain the observed differences between steam an d nitrogen bubbles. It is found that either a vorticity lift force of the type found by Pluton [The dynamics of bubbles, drops and particles in motion in liquids, Ph.D, thesis, University of Cambridge (1983)] i s negligible, or this temperature difference force may be important. A commonly used criterion to predict detachment radii is found not to b e satisfactory.