A Multi-Scale Study of Droplet Collision Dynamics: From Gas Film Lubrication to Liquid Phase Mixing

Binary droplet collision plays an elementary role in dense spray combustion. The collision outcome influences the spray characteristics, for example, the distributions of droplet size and velocity, local mixing rate, etc., and thus the overall combustion performance. In this paper, the dynamics and mixing of colliding droplets are preformed computationally and theoretically. All possible collision outcomes (including bouncing, coalescence, reflexive and stretching separation) are obtained over a wide range of the Weber numbers and impact parameters. Generalized correlations of the collision regime boundaries are obtained for a wide variety of liquid droplets and ambient fluids. The theoretical formulation is based on a complete set of conservation equations for both the liquid and 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 interface. Gradientand value-based refinement criteria are employed to substantially improve computational accuracy and efficiency. In additional, a thickness-based refinement is developed and implemented to resolve extremely thin gas films during the droplet contact. The minimum cell size is on the order of 0.02 μm, which is O(10 -4 ) of a typical droplet diameter. An advanced visualization technique using the Raytracing methodology is implemented to gain direct insight into the detailed physics. Various underlying processes and mechanisms of droplet dynamics, as well as gas film lubrication, energy and mass transfer are investigated systematically. Theories are also established to predict the regime boundary of stretching separation and the mass transfer ratio for colliding droplets. * Corresponding author: [email protected] ILASS Americas, 24 Annual Conference on Liquid Atomization and Spray Systems, San Antonio, TX, May 2012 1