On Inter-bubble distances and bubble clustering in bubbly Flows: An experimental study

•Positional relationships of bubbles were studied experimentally in the dense swarms.•Dimensionless inter-bubble distances were extracted from X-ray measurement data.•The distance of nearest neighbor bubbles is close to that of FCC structure.•Vertical clustering is observed for Eo ≥ 3.82.•Clustering orientation was found to be independent of the gas holdup in this study. Swarm effects in bubble columns result from the interaction of the bubbles at higher gas holdup. The flow around individual bubbles in a swarm influences the movement of the following bubbles and this has a feedback on the flow regime on a larger scale. That is, hydrodynamics that can no longer be described with models for single rising bubbles. The experimental investigation of the multiscale hydrodynamics in three-dimensional bubble columns was so far limited by the available measurement techniques. In this study, ultrafast X-ray tomography (UFXCT) was applied on bubbly flows in a lab scale bubble column to determine inter-bubble distances and clustering characteristics in the vicinity of individual bubbles for different gas holdups and bubble diameters. Applying the pair correlation function reveals a pronounced vertical clustering for large bubbles with Eo ≥ 3.82, whereas no clustering has been observed for bubbles of lower Eo. It was found that clustering orientation mainly depends on deformability of the bubbles but not on gas holdup.