Bubble formation and detachment from a wall orifice in a liquid cross flow under low and normal gravity

Bubble formation and detachment is an integral part of the two-phase flow science. The objective of the present work is to experimentally and theoretically investigate the effects of liquid cross-flow velocity, gas flow rate, and orifice diameter on bubble formation in a wall-bubble injection configuration. The effects of high and low gas momentum flux regimes on the bubble diameter at detachment are investigated experimentally. Bubble formation and detachment experiments were performed under normal and low gravity using the NASA DC-9 Reduced Gravity Aircraft facility. The experimental results show that bubble formation and detachment depends on gravity, the orifice diameter, the gas flow rate, and the liquid cross-flow velocity. Using scaling and a force balance, two different detachment mechanisms are identified. When the gas momentum is large, the bubble detaches from the injection orifice as the gas momentum overcomes the attaching effects of liquid drag and inertia. The surface tension force is much reduced because a large part of the bubble pinning edge at the orifice is lost as the bubble axis is tilted by the liquid flow. When the gas momentum is small, the force balance in the liquid flow direction is important, and the bubble detaches when the bubble axis inclination exceeds a certain angle.; A theoretical model based on a force balance on the bubble is proposed in this work. Predictions of the model that reasonably agree with the experimental results, suggest that the surface tension that represents a major attaching force, is being compromised by a detaching force responsible for loss of pinning. We propose in this work that this force scales with the orifice radius and acts upward and contributes to bubble detachment. Two detachment criteria were applicable depending on the gas to liquid momentum force ratio. Inclusion of this proposed force in the low gravity models results in good agreement with the experimental trends of the bubble diameter at detachment. Effect of this detaching force on normal gravity model prediction was negligible. Comparison was made with 2-dimensional normal gravity models based on Kumar-Kuloor formulation.