Droplet breakup and collision dynamics of an internal-mixing twin-fluid (air-assisted) spray were investigated experimentally. Time-resolved spray morphological evolutions were obtained by employing a high-speed spray visualization system, while droplet size and velocity were measured by using a phase-Doppler particle analyzer. The results show that the air-assisted spray is composed of a bulk of tiny droplets entrained by high-speed gas-phase flow. Droplets with a diameter of less than 5 µm account for the majority of samples on the basis of the distribution function. The calculated Stokes numbers of selected tracer droplets (0 µm–5 µm) show that these droplets tend to follow the air flow faithfully and thus can be used to estimate the local air flow velocity. By comparing with the critical Weber number, we found that droplet shear breakup is absent, but some droplets may undergo turbulent breakup. A theoretical analysis accounting for the effects of the turbulent dissipation rate on both droplet breakup and coalescence was performed, and the critical equilibrium length of breakup and coalescence is found to be less than 3.0 mm. In terms of droplet collision dynamics in the spray far-field, coalescence dominates droplet collision outcomes; therefore, the droplet size increases linearly as the spray downstream distance X increases. Particularly, the influence of the droplet size ratio [C. Tang, P. Zhang, and C. K. Law, “Bouncing, coalescence, and separation in head-on collision of unequal-size droplets,” Phys. Fluids 24, 022101 (2012)] was adopted to quantify the droplet coalescence probability. When compared to the n-octane spray, the n-dodecane spray accounts for more droplet bouncing (II) since n-dodecane possesses a relatively larger bouncing (II) region.

Binary collision of unequal-size droplets was investigated numerically by using the front tracking method, with particular emphasis in studying the kinetic energy recovery and the interface hysteresis of bouncing droplets. The numerical results were sufficiently validated against the high-quality experimental data in the literature to verify the quantitative predictivity of the numerical methodology in simulating droplet bouncing. Distinct stages of droplet deformation and viscous dissipation during droplet collision were revealed and explained for their dependence on the Weber number and the size ratio. A linear fitting formula that well correlates the kinetic energy recovery factor of bouncing droplets with various collision parameters was proposed and would be practically useful in modeling inelastic droplet bouncing in Lagrangian spray simulation. As an interesting post-collision characteristic of bouncing droplets, the interface hysteresis was found to favor smaller droplet deformation by decreasing the size ratio or decreasing the Weber number or increasing the Ohnesorge number.