Experiments and CFD modelling of bubble formation at a lateral hole in a top-submerged nozzle

Abstract

Top-submerged nozzles with lateral holes find applications as passive mixing devices in radiochemical/hazardous industries. We report the mechanism of bubble formation and detachment at a lateral hole in such a nozzle for the first time. Three distinct phases – a growth phase, an elongation phase and finally a rise/detachment phase – could be observed during bubble formation. Effect of gas flow rate, diameter of lateral hole (1, 2 and 3 mm) and physical properties of liquid-liquid system on bubble detachment time and bubble equivalent diameter are reported. Three different liquid-liquid systems (DM water, 2 N and 4 N nitric acid) have been considered. Bubble diameter first reduced, reached a minimum and then started to increase in an oscillatory manner with increase in air flow rate. Before the onset of oscillatory behaviour an increase in hole diameter was seen to reduce bubble diameter. Such a behaviour is typical to bubble formation at lateral hole and is not seen for top-submerged nozzles with central opening. A detailed analysis of the results was carried out and it was observed that inertial forces played a major role. The work also reports a fundamental criterion which must be achieved for onset of oscillations in bubble diameter with flow rate. We also report a 2D CFD model that can simulate bubble formation dynamics at a lateral hole. The model was validated qualitatively as well as quantitatively against the experimental data.