Análise detalhada da dinâmica de bolhas isoladas em canais confinados

Abstract

The main goal of the present study is to evaluate the influence of the walls on the dynamics of isolated bubbles rising in rectangular channels filled with water. To achieve the objective, two experimental techniques were applied, namely the shadowgraph and particle image velocimetry (PIV), and the bubbles were split into three size bins (small, medium and large). In addition, the work was divided in two parts. The first one corresponds to the confinement in one direction, in which it was observed that for an 8.5-mm distance between walls the bubble velocity drastically dropped. Thus, geometries with wall-to-wall distances smaller than 8.5 mm were considered as confined ones. In these cases, a decrease in the bubble deformation was noted, as well as zigzag trajectories and a large vortex shedding. Furthermore, correlations for drag coefficient and velocity were proposed as functions of the confinement degree. The second part of the study corresponds to the confinement of the channels in two directions and bubble paths, shapes, velocities and velocity fluctuations were analyzed. It was observed that the velocity decrease with the confinement depends not only on the hydraulic diameter of the channel but also on the confinement direction and bubble size. The proximity between the walls is responsible for the drag increase, which owes to the energy loss caused by collisions and higher shear stresses on the bubble surface. Furthermore, the confinement degree increase is responsible for high oscillatory frequencies as the amplitude and wavelength are reduced. Also, no correlations were found in the literature to evaluate bubble dynamics in confined rectangular channels such as those presented in this study and, therefore, correlations were proposed for velocity and drag coefficient dependent on the degree of confinement.