BUBBLE FORMATION AT A SUBMERGED OFFICE IN REDUCED GRAVITY

We consider gas injection through a circular plate orifice into an ide ally wetting liquid which results in the successive detachment of bubb les, each of which is regarded as a separate entity. At normal gravity and at relatively low gas dow rates, the growing bubble is modelled a s a spherical segment that touches the orifice perimeter during the wh ole time of its evolution. If the gas flow rate exceeds a certain thre shold value, a second stage of the detachment takes place that follows the first spherical segment stage. In this second stage, a nearly cyl indrical stem forms at the orifice that lengthens as the bubble rises above the plate, and this stems feeds an almost spherical gas envelope situated at the stem upper end. At high gas how rates, bubble shape r esembles that of a mushroom, and its upper envelope continues to grow until the gas supplied through the stem is completely cut off. This se cond stage always develops when gravity is sufficiently low, irrespect ive of the gas dow rate. There are two major factors that determine th e moment of bubble detachment: the buoyancy force and a force due to t he momentum flowing into the bubble with the injected gas. The buoyanc y 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 infl uences bubble growth and detachment. At sufficiently low gravity, the bubble formation frequency is proportional to gas flow rate whereas th e bubble detachment volume is independent of gas dow rate. At normal a nd moderately reduced gravity conditions, when the gas how rate grows, bubble formation frequency slightly decreases and bubble detachment v olume increases almost linearly. Effects of other parameters, such as the orifice radius, gas and liquid densities and surface tension coeff icient are discussed. Copyright (C) 1996 Elsevier Science Ltd