Collision outcome and mass transfer of unequal-sized droplet collision

The investigations of the collision outcome and dynamics of mass transfer process of unequal-sized binary droplets collision are preformed computationally and theoretically. All the possible collision outcomes, bouncing, coalescence, reflexive and stretching separation are obtained numerically. The present study focuses on water droplet interactions in atmospheric air over a wide range of Weber numbers and impact parameters with two droplet diameter ratio, 0.5 and 0.25. The numerical formulation is based on the complete set of conservation equations for both the liquid and the surrounding gas phases. An improved volume-of-fluid (VOF) technique, augmented by an adaptive mesh refinement (AMR) algorithm, is used to track the liquid/gas interfaces. A novel thickness-based refinement methodology is also developed to resolve the extremely thin gas film during early approaching of droplets. Numerous high-fidelity bouncing cases are simulated using proposed efficient methodology. The smallest cell size is up to O(10^-4) of the small droplet diameter with minimum grid size of 0.02 μm. In addition, an advanced visualization technique using the Ray-tracing methodology is implemented to gain direct insight into the detailed physics of droplet interaction and mass transfer process. A simple theory based on the geometric relation of the unequal-sized droplet collision system is established to predict mass transfer ratio of stretching separation outcome. Good agreement is achieved with simulation results.