Abstract
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.
Highlights
The collision dynamics of droplets in a gas environment are of great importance in the industrial processes related to spraying, such as spontaneous combustion gel propellants in missile propulsion systems [1], 3D inkjet printing in textile printing [2,3,4], and polymer–solvent sprays in chemical pharmaceutical engineering [5]
Finotello et al [24] experimentally studied the effect of viscosity on the collision outcomes of binary droplets by the Herschel–Bulkley (H–B) model. They found that, in the collision process, the kinetic energy was dissipated due to viscous flow, and the critical Weber number for the reflexive separation increased with the Ohnesorge number
The coupled level-set and volume of fluid (CLSVOF) method is used in our paper, which can accurately capture the complex process of the interface evolution and has good mass conservation [31,32,33]
Summary
The collision dynamics of droplets in a gas environment are of great importance in the industrial processes related to spraying, such as spontaneous combustion gel propellants in missile propulsion systems [1], 3D inkjet printing in textile printing [2,3,4], and polymer–solvent sprays in chemical pharmaceutical engineering [5]. Finotello et al [24] experimentally studied the effect of viscosity on the collision outcomes of binary droplets by the Herschel–Bulkley (H–B) model They found that, in the collision process, the kinetic energy was dissipated due to viscous flow, and the critical Weber number for the reflexive separation increased with the Ohnesorge number. In view of the above research, compared to the Newtonian liquid, the research on binary non-Newtonian droplet collisions is mainly focused on the effect of the viscosity on the deformation process and the boundary model of the droplets. In this paper, based on the coupled level-set and volume of fluid (CLSVOF) method and adaptive mesh refinement (AMR), the effects of viscosity and surface tension on the collision behaviors of the non-Newtonian droplets will be investigated.
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