Water model study of the frequency of bubble formation under reduced and elevated pressures

Air was injected vertically upward into a water bath through a bottom nozzle or a bottom orifice. The surface pressure was reduced or elevated from an atmospheric pressure in order to change the hydrostatic pressure around the nozzle and orifice. The gas delivery system was designed so that bubbles were generated in the middle and high gas flow rate regimes under a constant flow condition. The frequency of bubble formation, fB, decreased as the surface pressure, Ps, decreased when the volumetric gas flow rate, Qg, was kept constant. The measured fB values were predicted satisfactorily by an empirical equation proposed previously by the present authors. This equation was derived originally to correlate the frequency of bubble formation both in aqueous and molten metal systems under an atmospheric surface pressure. The effect of surface pressure on the frequency of bubble formation was considered in terms of the density of gas, ρg, and the volumetric gas flow rate Qg in the aforementioned empirical equation. These two quantities, ρg and Qg, were evaluated at the nozzle exit by using the hydrostatic pressure there.