A numerical study of ellipsoidal particle interactions and droplet streams with shape oscillations

Two-phase flows are studied in both Eulerian-Eulerian and Eulerian-Lagrangian approaches, focusing on linear arrays of particles and droplets, both ellipsoidal and spherical, both in contact and non-contacting. In the first part of this study, the drag coefficients for a stagnant array of particle pairs with different ellipsoidal aspect ratios and different separation distances in an air flow are determined in Eulerian Computational fluid dynamics (CFD) simulations at steady state. From the steady state simulations (first type of simulations), new correlations for the drag coefficients of the leading and trailing droplets in a pair are obtained. Such correlations could potentially reveal the ratio of the drag coefficients of the ellipsoidal droplets in the stream to that of an isolated ellipsoidal droplet. In the second part of this study, transient discrete phase model (DPM) simulations of the grouping process in monodisperse streams of isopropanol droplet pairs in initially stagnant air are presented, utilizing Eulerian-Lagrangian modeling. For these simulations, a drag coefficient model is supplied initially. The usage of the drag coefficients obtained in the steady state Eulerian simulations (first type of simulations) for the subsequent transient DPM simulations (second type of simulations), is justified by the viscosity ratio between the droplet liquid and the ambient air, which is 131. For high viscosity ratio and low Weber number, the drag coefficient of a viscous sphere is similar to that of a solid sphere. Using these drag correlations from the first part of this study in transient DPM simulations of spherical droplets and oscillating droplet shapes in the second part, a slight decrease in collision time was observed for the oscillating droplets relative to the spherical droplets. Larger initial droplet velocity and smaller initial distance between droplets in a pair were found to increase the collision time difference between ellipsoidal and spherical droplets. These results coincide with the DNS results reported recently. Two mechanisms of grouping enhancement due to droplet shape oscillations are suggested. The first is the negative drag coefficient of the trailing droplet at specific geometries, which can be regarded as a weak attraction between the droplets. The second is the distance between the droplet centers at the collision time, which can either promote or delay coalescence.