The effect of gas and liquid properties and droplet size ratio on the central collision between two unequal-size droplets in the reflexive regime

A numerical investigation of various parameters affecting the collision process of two unequal sized droplets in a gaseous environment in the reflexive regime is presented. The investigation is based on the finite volume numerical solution of the Navier–Stokes equations, in their axial-symmetric formulation, expressing the flow field of the two phases, liquid and gas, coupled with the volume of fluid method (VOF) for tracking the liquid–gas interfaces; a recently developed adaptive local grid refinement technique is used in order to track more accurately the liquid–gas interface with reduced computational cost. A colour function is used in order to follow the mixing process of the two droplets after collision. The reliability of the procedure is first established by comparing predictions with available experimental data and important details as regards the time evolution of the deformed shapes, the ligament formation, the maximum deformation of the droplets and the penetration of one droplet into the other are presented. The collision process and its outcome are investigated having the liquid properties (Ohnesorge number based on the liquid properties), the gas phase properties (Reynolds number based on gas properties) and droplet size ratio as the controlling parameters.