CONFINEMENT EFFECT ON THE RISING DYNAMICS OF A SKIRTED BUBBLE

In this work, the confinement effect on the buoyancy-driven, axisymmetric motion of a skirted bubble in a liquid-filled, circular cylinder is numerically studied. The gas and liquid phases are assumed to be isothermal, incompressible and immiscible. The volume of fluid (VOF) method is adopted to simulate the deforming interface between gas and liquid. A confinement ratio range of (1:1 ≤ Cr ≤ 10) is considered. The results reveal that the motion of a skirted bubble under Cr ≥ 8 resembles that in an infinite medium in terms of both shape and Reynolds number in terminal state. With decreasing Cr, the wall plays a more significant role in determining the motion of the skirted bubble. For the range of Cr < 8, the drag on the bubble increases as Cr decreases, giving rise to the reduction of bubble rising velocity. As for the terminal shape, the skirted bubble is elongated in the axial direction and may evolve to an ellipsoidal cap or a bullet as a result of increasing wall proximity. The sensitivities of the thickness and length of trailing bubble skirts to the confinement ratio are examined. The skirt length reduces with the decrease of Cr, while the skirt thickness increases with decreasing Cr. The details of fluid field are analyzed both in the global reference frame and in a local reference frame moving with the bubble centroid. The wake effect of the skirted bubble is weaken by the increasing wall effect, suppressing the formations of vortex ring and skirt. Bubble break-up is captured under approximate conditions and can be enhanced by decreasing Cr, confirming the deduction in the literature. The present predictions on terminal velocities agree well with results by the correlation in the literature.