Numerical and experimental study of large air bubbles rising by buoyancy in an inclined, rectangular water channel

Large air bubbles rising by buoyancy through inclined rectangular channels containing initially stagnant water are governed by a three-dimensional coupling among translation, deformation, and liquid drainage; both phases are treated as immiscible Newtonian fluids. While previous numerical studies have addressed related inclined-channel configurations, a systematic numerical-experimental validation across aspect ratio and cross section orientation is lacking. This study combines high-speed visualization, wall-shear-rate measurements, and geometric volume-of-fluid simulations to investigate buoyancy-driven air bubbles in five water-channel cross sections at inclination angles ranging from nearly horizontal to vertical. The simulations captured the displacement of the bubble toward the upper wall, its streamlining and transverse broadening at the nose, and the transition from distributed wall films to a thin upper film with bulk return flow beneath the bubble. These changes were consistent with a strongly geometry-dependent, non-monotonic velocity response. An increase in maximum velocity (up to 1.8 times the vertical-channel value) and a corresponding shift in inclination angle (up to approximately 35°) were observed when the channel orientation changed from flat to tall. For four of the five geometries, the mean numerical-experimental velocity differences were below 6%. The tallest low-inclination configuration was the limiting case, as ultrathin films and extended wakes increased sensitivity to near-wall resolution and domain length. The wall shear rate comparison reproduced the main experimental signatures, including the front peak, the reverse film flow region, rear disturbances, and wake fluctuations. The study provides a framework for interpreting and predicting the main Taylor-bubble trends in inclined rectangular channels, with wall shear rate serving as a stringent validation test for interface-resolved simulations.