Bouncing and coalescence of binary droplets undergoing off-center collisions : a numerical study based on volume-of-fluid method

Abstract

Binary droplet collision in gaseous environment is of relevance to many natural and industrial processes, which has been studied substantially to probe more complicated physics in the context of various spray processes. Compared to the extensively studied head-on droplet collision, the thesis attempts to focus on the three-dimensional (3D) off-center droplet collision, which is more general and practical but less investigated, and strives to numerically reveal both macroscopic and microscopic dynamics for bouncing and coalescence based on a volume-of-fluid (VOF) method. There are four parts in the thesis. 1. Considering the facts that the elevated pressure environment promotes droplet bouncing in the real combustion chambers, and to serve for the modeling of Lagrangian simulations of sprays, the off-center collision of binary bouncing droplets of equal size was studied numerically by a volume-of-fluid (VOF) method with two marker functions. A non-monotonic kinetic energy recovery with varying impact parameters was discovered, and it can be explained by the prolonged entanglement time and the enhanced internal-flow-induced viscous dissipation for bouncing droplets at intermediate impact parameters. 2. The collision between initially spinning droplets, which occurs frequently in the practical sprays but is generally ignored in the previous studies, was numerically studied, with emphasis on the influences of rotation axis of spinning droplet on the droplet collision dynamics. The helicity analysis can be used to describe the "orthogonality" of droplets translational and spinning motions. 3. To help understand the non-monotonic ignition delay time with impact parameters for the collision between two hypergolic ignition droplets, the off-center coalescence between two nonreactive droplets of unequal sizes were numerically studied. A general non-monotonic internal mass entanglement with varying the impact parameter was observed, which verifies that the ignition process upon the collision between two hypergolic ignition droplets is probably dominated by the internal mass entanglement in the preliminary collision stage. 4. The internal mass entanglement, also referred as jet-like internal "mixing", were analyzed to be attributed to a main vortex ring generated during the binary droplet coalescence, which was motivated by the vortex-ring generation during droplet colliding with liquid pool. The correlation between the main vortex ring and the jet-like "mixing" were verified and presented by using a vortex-ring-based Reynolds number.