In the spray combustion process, the distribution of droplet size and velocity will affect the atomization performance of the fuel and the combustion effect. Compared with binary equal-sized droplet collisions, binary unequal-sized droplet collisions are more in line with the actual situation. In this paper, a numerical investigation of binary unequal-sized droplet collision has been performed under different high Weber numbers (from 210 to 810) and impact parameters (B ≈ 0.3–0.9) by a coupled level-set and volume of fluid method with adaptive mesh refinement technology. Unlike the coalescence and separation phenomena at low and medium Weber numbers, at high Weber numbers, due to the difference in velocity between large and small droplets, the rim expands radially outward in different ways and further breaks up. The splashing behavior of the droplets can also be observed. As the Weber number increases, the breakup moment of the droplets advances and the maximum radial deformation diameter increases first (We = 210–360) and then decreases (We = 360–810). By changing the impact parameters, it can be found that binary off-center collisions are associated with rotational motion. At larger impact parameters, the features of the capillary wave instability can be observed on the surface of the ligament.
Binary droplet collisions are a key mechanism in powder coatings production, as well as in spray combustion, ink-jet printing, and other spray processes. The collision behavior of the droplets using Newtonian and polymer liquids is studied numerically by the coupled level-set and volume of fluid (CLSVOF) method and adaptive mesh refinement (AMR). The deformation process, the internal flow fields, and the energy evolution of the droplets are discussed in detail. For binary polymer droplet collisions, compared with the Newtonian liquid, the maximum deformation is promoted. Due to the increased viscous dissipation, the colliding droplets coalesce more slowly. The stagnant flow region in the velocity field increases and the flow re-direction phenomenon is suppressed, so the polymer droplets coalesce permanently. As the surface tension of the polymer droplets decreases, the kinetic and the dissipated energy increases. The maximum deformation is promoted, and the coalescence speed of the droplets slows down. During the collision process, the dominant pressure inside the polymer droplets varies from positive pressure to negative pressure and then to positive pressure. At low surface tension, due to the non-synchronization in the movement of the interface front, the pressure is not smooth and distributes asymmetrically near the center of the droplets.
Micro-nano droplet collisions are fundamental phenomena in the applications of nanocoating, nano spray, and microfluidics. Detailed investigations of the process of the droplet collisions under higher Weber are still lacking when compared with previous research studies under a low Weber number below 120. Collision dynamics of unequal-sized micro-nano droplets are simulated by a coupled level-set and volume of fluid (CLSVOF) method with adaptive mesh refinement (AMR). The effects of the size ratio (from 0.25 to 0.75) and different initial collision velocities on the head-on collision process of two unequal-sized droplets at We = 210 are studied. Complex droplets will form the filament structure and break up with satellite droplets under higher Weber. The filament structure is easier to disengage from the complex droplet as the size ratio increases. The surface energy converting from kinetic energy increases with the size ratio, which promotes a better spreading effect. When two droplets keep the constant relative velocity, the motion tendency of the droplets after the collision is mainly dominated by the large droplet. On one hand, compared with binary equal-sized droplet collisions, a hole-like structure can be observed more clearly since the initial velocity of a large droplet decreases in the deformation process of binary unequal-sized droplets. On the other hand, the rim spreads outward as the initial velocity of the larger droplet increases, which leads to its thickening.
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