Mechanism of reattachment length reduction in a bubbly flow in a pipe with sudden expansion

The reduction of the reattachment length has been targeted in controlling the flow over a backward-facing step because of its relation to the mixing enhancement. In this study, we investigate the bubble-assisted reattachment length reduction in the upward bubbly flow through expansion square pipes with area expansion ratios (ER) 4.0 to 9.0. When air bubbles are injected into the water flow, the liquid-phase turbulence is augmented significantly, which is known to one of the mechanisms of reattachment length reduction in single-phase backstep flow. For the present two-phase flows, however, it is dependent on more than just the global increase of turbulence due to the bubble-induced agitation; its local distribution is also important. Using the two-phase particle image velocimetry, we measure the effects of ER and volume void fraction (0.12-2.80% with the bubble size of 1.6-3.6 mm) on the liquid-phase turbulence with two Reynolds numbers of 420 (laminar) and 6000 (turbulent), based the single-phase flow statistics. We will discuss how the bubble dynamics, origin of bubble-induced turbulence, would affect differently the reattachment length of the flow depending on laminar and turbulent background flows.