Ligament formation and droplet breakup on disk-type and cup-type rotary atomizers

Behaviors of liquid films scattering from a disk-type or cup-type rotary atomizer are studied using computations based on the three-dimensional Smoothed Particle Hydrodynamics (SPH) method. To reduce computational costs while maintaining a high spatial resolution, the computational domain is limited to a fan-shaped region near the edge of the atomizers using a periodic boundary condition in the circumferential direction. Steady inflow is considered as the inlet condition. In both disk- and cup-type cases, the liquid film leaving the atomizer edge becomes wavy in the circumferential direction and forms elongated ligaments, which break up into droplets. When the atomizer is equipped with grooves on its outer edge, the process of ligament formation and droplet break up is strongly affected by the number and shape of the grooves, particularly for the cup-type atomizer, in which the centrifugal force works more effectively. Two ligament formation patterns are observed: one ligament from each groove and a pairing of ligaments from two neighboring grooves. Droplets of uniform sizes are likely to be generated when the former pattern appears. The results suggest that droplets of uniform sizes can be obtained by choosing the appropriate shape and number of grooves.