Rise velocities of single bubbles in a narrow channel between parallel flat plates

Effects of the bubble lateral motion and the liquid viscosity, mu(L), on the rise velocity, V-B, and the drag coefficient of a single bubble in a narrow channel were investigated. The gap thickness of the channel was 3 mm. The bubble diameter, d(B), was from 7 to 20 mm. Clean water and glycerol-water solutions were used for the liquid phase. The mu(L) ranged from 0.9 to 65.7 mPa s. Air was used for the gas phase. The conclusions obtained are as follows: (1) the bubble motion transits from zigzagging to rectilinear as d(B) increases and the transition abruptly takes place at a certain critical d(B), which decreases with increasing mu(L), (2) the abrupt transition causes a stepwise increase in V-B, (3) the curvature of the bubble nose in the rectilinear regime strongly affects V-B, resulting in unavoidable scatter in V-B data in low viscosity systems due to large shape oscillation, and (4) Filella's V-B correlation for air-water systems is deducible from the force balance and applicable to high viscosity systems, provided that the model coefficient is tuned for each mu(L), implying that the drag coefficient can be expressed in terms of the Morton number and the gap-to-bubble diameter ratio. (C) 2018 Elsevier Ltd. All rights reserved.