Experimental study on bubbly flow transition in vertical annular gap bubble column

Mean and local gas void fractions in vertical upward, two-phase flows contained within the annular gap between two concentric tubes are presented. For a given gas superficial velocity, jg, the mean void fraction, , was found to decrease as the diameter ratio of the two tubes approached unity (i.e. for narrower gaps). Moreover, was significantly lower than that obtained in an open tube at the same jg. Two explanations were proposed: (i) large bubbles, generated in the annular gap, destabilised the flow, and (ii) the driftflux distribution parameter in an open tube was different to that in an annular gap, which results in a different mean . It was shown that injection of large bubbles into a homogeneous flow led to an early transition. Furthermore, the void fraction profiles in the annular gaps depended on both jg and the gap geometry