Local flow dynamics in the motion of slug bubbles in a flowing mini square channel

For a slug flow flowing in a square capillary, the surrounding liquid flows through a thin film between the slug bubble and the wall and the corners of the channel and generates an asymmetric two-phase flow field with significant spatial variations. The current literature mainly discusses the bulk features of such a flow field, while local phenomena in these flows are not well known yet. In this study, the local dynamics of the liquid film and corner flows in a flowing square channel with a width of 3 mm were investigated by experiments and numerical simulations. Instantaneous velocity fields in the liquid flow were measured at the channel's central plane using particle tracking velocimetry (PTV). The obtained data were utilized to calculate the axial variations of the film and corner velocities. Three-dimensional numerical simulations with adaptive mesh refinement of the interface were performed for a series of flow conditions. Comparing the film velocity obtained from PTV and simulations demonstrates an overall good agreement between the results, with discrepancies close to the solid walls, where the PTV data is sporadic. Film and corner flows show rapid velocity changes adjacent to the minimum film area. Both experiments and simulations elucidate the backflow phenomena in the analyzed flow fields, where the liquid locally flows in the opposite streamwise direction. The backflow region shrinks as the liquid flow rate and speed of the bubble increase. Film flow shows significant pressure drops as it passes through the minimum film area. On the contrary, the corner flow does not demonstrate any rapid pressure fluctua-tions. The vortex structures' identification revealed dominant vortex loops inside the gas and small-scale vortex filaments along the channel walls, generated from the liquid film rupture. Crown Copyright (c) 2021 Published by Elsevier Ltd. All rights reserved.