Hydrodynamics of Isolated Air Bubbles Rising Freely in a Thin-Gap Cell at High Reynolds Numbers: Combined effects of Gap Size and Inclination

This study investigates the hydrodynamics of isolated air bubbles rising at high Reynolds numbers in thin-gap cells filled with quiescent water, with an original focus on the combined effects of gap size and inclination. Experiments were conducted at two gap thicknesses (0.95 and 1.78 mm) and inclination angles ranging from 0 ° to 80 ° , over Reynolds numbers in the range 187 ≤ Re ≤ 5012 and Archimedes numbers 362 ≤ Ar ≤ 20240. In the vertical configuration, three main bubble regimes were observed, in agreement with those reported in the literature. The published scaling law for bubble rise velocity allowed reproducing the experimental trends, provided that the Archimedes numbers were corrected to account for the influence of the cell’s width. When the cell was inclined, bubble oscillations were progressively suppressed, bubble's velocity decreased, with stronger effects under tighter confinement. The impact of the cell’s inclination could not be fully described only by projecting buoyancy along the direction of motion, because it modified how buoyancy was converted into inertial motion through deformation of the confined bubble. Consequently, a confinement-weighted deformation parameter was introduced in the scaling law. The experimental rise velocities obtained for all inclinations and gaps could be collapsed onto a single master curve. This scaling provides a basis for modeling inclined bubbly flows in narrow-gap gas-liquid systems.