(87e) Cluster-Induced Deagglomeration in Unbounded Fluidization of Cohesive Particles

The dilute gas-solid flows of cohesive particles in unbounded fluidization are studied via CFD-DEM simulations, focusing on two categories of heterogeneities in particle concentration: particle clusters of hydrodynamic origin (instabilities) and agglomerates of cohesive origin. The former refers to the regions of higher solid concentrations than surroundings, while the latter is defined as particles held together by cohesive forces. Both clustering and agglomeration have significant impacts on reaction rates, momentum, heat and mass transfer in multiphase flow applications. By tracking enduring contacts in simulations, we isolate agglomerates from the overall heterogeneities (resulting from both clusters and agglomerates) in the system, which is quantified by local particle number density fluctuations. In simulations with increasing domain volume, the degree of clustering increases (as is the norm for non-cohesive systems) while agglomeration decreases, contrary to the behavior in granular (no fluid) systems, where clustering enhances agglomeration due to the reduced relative velocities of particles in clusters. Based on statistical and theoretical analyses of particle velocities, higher levels of clustering in larger systems are shown to result in increased relative velocities between the gas and solid phases. This increased relative velocity between the phases serves as an added source of granular temperature (collision velocity between particles), thereby enhancing deagglomeration. We coin this newly-identified mechanism as “cluster-induced deagglomeration” and demonstrate its robustness for systems with increasing cohesion. Specifically, as larger cohesion leads to higher energy dissipation during particle collisions that enhances both clustering and agglomeration, the cluster-induced deagglomeration is able to explain the surprising saturation of agglomeration level with increasing cohesion, as opposed to the monotonic behavior in granular systems.