Flow and mixing dynamics in face-to-face and rear-end collisions of pairs of equal-sized droplets

Abstract

In this work, the behaviors of pairs of equal-sized droplets in rear-end and face-to-face collisions were simulated using the improved lattice Boltzmann method for incompressible two-phase flows. First, the time evolution of the droplet shape was investigated by tracing colored particles, and this was compared between the rear-end and face-to-face collisions. For collinear collisions, the droplet shapes in the rear-end collisions were found to be similar to those in the face-to-face collisions. However, the behaviors of the tracer particles were different: the droplets in the rear-end collisions mixed more easily than those in the face-to-face collisions. For offset collisions, it was found that the rolling motion of the coalesced droplet accelerates the mixing inside it in both face-to-face and rear-end collisions. A new index—the total mixing intensity—was introduced, and the droplet mixing can be quantitatively evaluated by calculating its value. The results indicate that the droplet mixing process of a collinear collision can be characterized by the velocity ratio, which is defined as the ratio of the center-of-mass velocity to the relative impact velocity.