Effect of flow rate conditions on bubble formation

An experimental study has been carried out on bubble generation by means of air injection through an orifice submerged in water. An orifice, drilled in an hydrophobic horizontal plate and a radius a = 1 mm, have been used to investigate the effect of the flow rate working conditions on the bubble formation process; a wide range of volumetric gas flow rates (2.0 × 10 ⩽ Q ⩽ 1.8 × 104 mm3/s) has been used, including different chamber volumes before the injection orifice. Two volumetric gas flow rates are apparent: the one into the experimental setup, which could be assumed as a constant, and a higher one into de bubble through the injection orifice; there is a first and significant time step with zero gas flow through the orifice. This drives to two different working conditions, named: constant volumetric gas flow rate, when both flow rates are equal, and non-constant volumetric gas flow rate, when both flow rates are different. First, a short experimental study, for constant volumetric gas flow rate, is presented; it represents a link between both working conditions. The main part of the work are devoted to non-constant volumetric gas flow rate and, it showed that the experimental data can be reduced approximately to a single bubble volume/flow rate relationship, as in the case of constant volumetric gas flow rate, if the properly scaled volumetric gas flow rate is used. Finally a simplified model, to estimate this proper volumetric gas flow rate, is presented and checked experimentally.