Effect of bubble-induced turbulence in an upward bubbly flow in a pipe with sudden expansion

The control of the reattachment length is very important in the flow around a backward-facing step, and in this study, we investigate such phenomena in the bubbly flow in the expansion pipe. When gas bubbles are injected into the liquid flow, they significantly modulate the liquid-phase flow characteristics, among which we focus is the turbulence enhancement, which is known to reduce the reattachment length in backstep flow. We experimentally study the bubble dynamics and the liquid-phase flow characteristics in an upward bubbly square pipe with a sudden expansion (area expansion ratios of ER = 4.0-9.0). The effect of ER on the change in the turbulence of the liquid-phase is compared at two Reynolds numbers of 420 (laminar) and 6000 (turbulent), based on the inlet bulk velocities of the single-phase (without bubbles) flow. The mean volume void fractions and averaged bubble size considered ranges from 0.31-1.57% and 2.72-3.74 mm, respectively, and we use the high-speed two-phase particle image velocimetry to measure the gas and liquid phases simultaneously. We will discuss how the bubbles induce the turbulence structure in the expansion pipe flow and explain the mechanism of the controlled flow.