Numerical simulation of free ascension and coaxial coalescence of air bubbles using the volume of fluid method (VOF)

The dynamics of a single air bubble, the wake structure, the instantaneous liquid velocity field around it and the coaxial coalescence of two successive bubbles have been widely studied in this work by using the VOF method on the software platform of Fluent. It is observed that the bubble rising trajectory changes from one dimension to three dimensions by decreasing the viscosity of the liquid phase. The different behaviors of air bubbles introduce various instantaneous bubbles wake structures which strongly depend on their shape and on the physical properties of the liquid phase. Indeed, as the solution viscosity decreases, the bubbles' shape changes from non-deformed (ellipsoidal) to the deformed shape. In the case of bubbles chain, the wake of the leading bubble significantly affects the shape, trajectory and velocity of the trailing bubble, as well as the velocity field of the liquid phase surrounding it. For high orifice air velocities and due to the wake of the leading bubble, the trailing bubble accelerates and approaches to the leading bubble and finally coalescence phenomenon occurs. During this process, the shape of the leading bubble becomes oblate while the shape of the trailing bubble is stretched in a vertical direction. Thus, the coalescence time and position of two successive bubbles generally increase with increasing the surface tension of the liquid and reducing its viscosity.