Settling dynamics and thresholds for breakup and separation of bi-disperse particle clouds

Abstract

The gravity-driven settlement process of bi-disperse particle clouds is investigated by using a computational fluid dynamic–discrete element method coupled model. The purpose of this work is to discuss the effect of particle size ratio and cloud size on cloud settling, with an emphasis on cloud morphological changes and dynamic mechanisms, as well as the spatial distribution and movement rules of each dispersed phase. The simulation results demonstrate that the settling of bi-dispersed clouds is accompanied by cloud breakup and the separation of large and small particles. The rise in particle size ratio will make cloud breakup more difficult, and the increase in cloud size facilitates the separation of large and small particles in the cloud. Essentially, cloud breakup and separation are closely related to weak inter-particle forces, and the thresholds for these two events are, respectively, proposed. Decomposable clouds will continuously flatten into toruses before breaking up, with an increasing aspect ratio γ that suggests a weakening of the interactions. When the aspect ratio exceeds a critical value of 2, the cloud breaks up. The non-dimensional distance ld* between large and small particles, describing the interaction between two dispersed phases, correlates with their separation. The critical distance for separation is 29. Furthermore, the sequence in which the two critical conditions are met determines the order in which cloud breakup and particle separation take place.