Bubble formation at a submerged orifice in reduced gravity

Abstract

We consider gas injection through a circular plate orifice into an ideally wetting liquid which results in the successive detachment of bubbles, each of which is regarded as a separate entity. At normal gravity and at relatively low gas flow rates, the growing bubble is modelled as a spherical segment that touches the orifice perimeter during the whole time of its evolution. If the gas flow rate exceeds a certain threshold value, a second stage of the detachment takes place that follows the first spherical segment stage. In this second stage, a nearly cylindrical stem forms at the orifice that lengthens as the bubble rises above the plate, and this stems feeds an almost spherical gas envelope stituated at the stem upper end. At high gas flow rates, bubble shape resembles that of a mushroom, and its upper envelope continues to grow until the gas supplied through the stem is completely cut off. This second stage always develops when gravity is sufficiently low, irrespective of the gas flow rate. There are two major factors that determine the moment of bubble detachment: the buoyancy force and a force due to the momentum flowing into the bubble with the injected gas. The buoyancy force dominates the process at normal gravity whereas the inflowing momentum force plays the key role under negligible gravity conditions. As gravity fluctuates, the interplay of these forces drastically influences bubble growth and detachment. At sufficiently low gravity, the bubble formation frequency is proportional to gas flow rate whereas the bubble detachment volume is independent of gas flow rate. At normal and moderately reduced gravity conditions, when the gas flow rate grows, bubble formation frequency slightly decreases and bubble detachment volume increases almost linearly. Effects of other parameters, such as the orifice radius, gas and liquid densities and surface tension coefficient are discussed.