A NUMERICAL STUDY OF DROPLET GROUPING AND ITS POTENTIAL CONTROL USING ACOUSTIC STANDING WAVES

In monodisperse droplet streams, the inter-droplet distances can change in a manner that brings pairs or triplets of droplets closer to one another, a process known as grouping. In the advanced stages of this process, droplet pairs can coalesce to form larger droplets. The grouping mechanisms in these droplet streams are not yet fully understood. Potentially, such a process can be controlled by an acoustic field. In the present study, computational fluid dynamics (CFD) simulations of isopropanol droplet streams in air are performed in ANSYS Fluent using the Eulerian-Lagrangian approach to analyze this process and to provide insight into grouping mechanisms. User-defined functions (UDFs) are used to tailor the code to the problems addressed here. Three scenarios are investigated. For the case of a single stream of droplet pairs, the mechanism of drag coefficient differences between the leading and trailing droplets enables reproduction of the results of longitudinal grouping experiments. For the case of two parallel streams, the lift force enables reproduction of lateral grouping trends, which are observed in experiments. Finally, for a single droplet stream in an acoustic standing wave, the experimentally observed sequence of single droplets and droplet pairs, induced by the acoustic wave, is reproduced computationally. It is found that the acoustic field significantly affects both grouping behavior and the droplet distribution in the computational domain, thereby either enhancing or delaying grouping tendencies. These results strongly indicate the potential that lies in employing an acoustic field to exercise control over how, where, and if droplet grouping occurs.