Gas–Liquid Flow Dispersion in Micro-Orifices and Bubble Coalescence With High Flow Rates

The flow of microbubbles in millichannels with typical dimensions in the range of few millimeters offers a reduced pressure loss with simultaneous large specific contact surface. The transformation of pressure into kinetic energy creates secondary flow in micro-orifices, which results in continuous bubble dispersion. In this work, bubble flow through different orifices and channel modules with widths up to 7 mm are experimentally and numerically studied. The effect of the orifice dimensions on bubble sizes is evaluated for hydraulic diameters of 0.25–0.5 mm with different aspect ratios. To provide larger residence times of the generated dispersions in the reactor, several channel structures are analyzed to offer less coalescence. Volume flow rates of 10–250 mL/min are studied with various phase ratios. Bubble diameters are generated in the range of less than 0.1–0.7 mm with narrow size distributions depending on the entire flow rate. Opening angles of the orifices above 6 deg resulted in flow detachments and recirculation zones around the effluent jet. The first break-up point is shifted closer to the orifice outlet with increasing velocity and hydraulic diameter. The entire break-up region stays nearly constant for each orifice indicating stronger velocity oscillations acting on the bubble surface. Linear relation of smaller bubble diameters with larger energy input was identified independent from Reynolds number. Flow detachment and coalescence in bends were avoided by an additional bend within the curve based on systematically varied geometrical dimensions.