Experimental study on bubble pairs and induced flow fields using tomographic particle image velocimetry

Abstract

Owing to their unique fluid dynamics, gas‒liquid two-phase flows, such as bubbly flows, are widely used in various engineering applications. This study utilized three-dimensional shadow image reconstruction (3D-SIR) and laser-induced fluorescence tomographic particle image velocimetry (LIF-TPIV) to perform a quantitative analysis of the three-dimensional morphology and motion of bubbles, as well as bubble-induced flow fields. By systematically varying the orifice spacing (\(s/{D}_{n}\), where \({D}_{n}\) is the orifice inner diameter) among the values 3.3, 5, 6.7, and 8.3, we investigate the effects of distance on bubble interactions and flow dynamics. Our findings reveal that at \(s/{D}_{n}\) = 3.3, the bubbles move very close to each other while rising. Distinct vortex rings form around each bubble near the orifice and merge as they rise, which increases flow transport, dissipation, and flow velocity between bubbles, leading to earlier bubble instability. Spectral analysis indicates that bubble spacing is coupled with the dominant flow frequency. As orifice spacing increases, bubble interactions weaken, resulting in independent vortex rings near the orifice that grow to approximately twice the bubble’s diameter before shedding secondary vortices. In these cases, regions of strong transport and dissipation are concentrated in the wake, and the flow velocity between bubbles remains relatively weak. Bubble instability primarily originates from the wake vortices. The aspect ratios of the bubbles align with the dominant flow frequency, indicating a coupling between flow dynamics and bubble morphology, although periodicity in bubble spacing weakens at larger spacings. These findings provide valuable insights into two-phase flow dynamics, especially in multiorifice bubbly flows.