Bubble-chain flows rising in a two-dimensional slit channel*

We experimentally investigate the bubble-chain flow in a slit (two-dimensional) channel, focusing on the bubble dynamics and resulting bubble-induced liquid flow, while varying the bubble generation frequency up to 21Hz. Using two-phase particle image velocimetry, the flow statistics of liquid and gas phases are measured simultaneously and analyzed to explain mechanism underlying the interactions between two phases. As the bubble generation frequency increases, a series of bubble-bubble interaction pattern occur such as two-bubble pairing, clustering, and merging, and we found that horizontal/vertical distance between bubbles and the wake structure of leading bubbles is associated with such changes. In addition, we show that the interaction between bubbles causes a significant change in the liquid flow in the viewpoint of time-averaged (entrainment) and fluctuating (turbulence) components both in vertical and horizontal directions. In particular, we will discuss the characteristic frequency scales that are associated with the region where such bubble-bubble interaction pattern is dominant.