Dynamics of single bubbles rising in confined square and rectangular channels

Abstract

The influence of the confinement direction on the dynamics of single bubbles rising through a vertical channel was experimentally investigated by using the shadowgraph technique. Three different sets of bubble sizes, with diameters between 1.32 and 3.06 mm, were released from the bottom of a tank filled with water. In order to vary the tank cross sectional area, the channel was equipped with moving glass sheets. The confinement degree, defined as the ratio of bubble diameter (D) to the channel hydraulic diameter (Dh) was varied between 0.13 and 0.98. The trajectories, shapes, terminal velocities and velocity fluctuations of the bubbles were analyzed. It was noted that even for the less confined geometries the aspect ratios presented a decrease when compared to an unconfined system. The terminal velocity decrease depends not only on the hydraulic diameter but also on the confinement direction and bubble size. The walls proximity is responsible for the drag increase, which owes to the energy loss caused by collisions and higher shear stresses on the bubble surface. The confinement direction was also important to the stabilization of the zig-zag motion, since the bubble preferable oscillatory plane corresponds to the shorter distance between the walls. The confinement degree increase is responsible for high oscillatory frequencies as the amplitude and wavelength are reduced. At the same time, an attenuation in the velocity fluctuations is observed. It was observed that the velocity correction proposed by Clift et al. (1978) for bubbles rising in tubes is not valid for rectangular geometries. Those differences are attributed to recirculations generated in the corners of the channels. Therefore, new velocity and drag coefficient correlations were proposed as functions of the confinement degree