Air Bubbles Displacement in Yield Stress Fluids

The motion of air bubbles in a yield stress fluid is analyzed numerically using a 2D approach and the finite volume technique. The multiphase flow is simulated using the volume of fluid method (VoF), which solves the conservation equations of mass and momentum coupled to a transport equation for the volume fraction of the fluids. The effects of yield stress, bubble size, number and position of bubbles rising in a viscoplastic fluid confined between vertical parallel plates are analyzed and discussed. The results indicate that the yield stress has great impact on the rising velocity. In the case of multiple bubbles flowing vertically, it is observed that the displacement of one bubble influences the rising velocity of the others, causing them to approach each other. As the distance between the bubbles increases the interference is reduced and the bubbles begin to flow as single ones. When two bubbles are horizontally positioned, they can approach or move away from each other, depending on the initial distance between them. Furthermore, the bubbles shape is analyzed as a function of the governing parameters. It is observed that for lower Reynolds number the bubbles present a circular shape, but as inertia increases the bubble becomes ellipsoidal.