Numerical Simulation of Liquid Droplet Coalescence and Breakup

Abstract This paper studied the evolution of binary droplet collision in liquid and also a mathematical calculation method of coalescence time. Binary droplet collisions occur in many engineering applications; however, the accurate models to predict the collision of droplets in the liquid are still lacking. In this work, the binary collision processes of droplets were simulated through computational fluid dynamic (CFD) method, where the interfaces between the two phases were tracked by the volume of fluid (VOF) approach. The results reveal that Weber number determines the results of the head-on collisions, and the cases with the same Weber number present similar evolution processes. If coalescence happens, the collision time decreases with increase in relative velocity, whereas the shape recovery time is independent with the relative velocity, but depends on droplet diameter. It is derived from this research that the collision time is proportional to the droplet diameter, and the shape recovery time is proportional to the 3/2 power of droplet diameter. The droplet moving directions play an important role in the collision results, and the case of two droplets moving toward each other with equal velocity is the easiest way to coalesce. When two droplets with different sizes collide, besides relative velocity, the coalescence and breakup are determined by the absolute velocities, the size, and size ratio of the two droplets. The increase in viscosity of continuous phase results an increase in collision time, but decrease in coalescence time.