Forces on bubbles growing and detaching in flow along a vertical wall

Experiments are performed on bubble detachment from an artificial cavity in a plane wall of a vertical rectangular channel. Mean upward velocity is varied. Steam bubbles are generated by local heating of the cavity, nitrogen bubbles of about the same size by injection. The experiments show a difference in take off direction between vapor and nitrogen bubbles. Steam bubbles take off into the liquid, while nitrogen bubbles more or less slide parallel to the wall. The bubble detachment radius decreases for increasing bulk liquid velocity, in a way that merely depends on the detachment radius without convection. Nitrogen bubbles, coming from a capillary with approximately the same radius are larger than water vapor bubbles. A force coefficient fit is performed on force components perpendicular to the wall. By analysing flow-and non-flow experiments separately, some of the forces are quantified. By combining the results of nitrogen bubble and steam bubble experiments, a force due to the temperature difference at the bubble foot is studied.