Cluster-induced deagglomeration in dilute gravity-driven gas-solid flows of cohesive grains

TL;DR

This paper reveals a novel mechanism called 'cluster-induced deagglomeration' in dilute gas-solid flows of cohesive grains, where increased clustering actually reduces overall agglomeration due to higher impact velocities at cluster surfaces.

Contribution

It introduces the concept of cluster-induced deagglomeration and provides a theoretical model explaining the saturation of agglomeration levels in gravity-driven cohesive systems.

Findings

Increased clustering correlates with reduced agglomeration.

Higher impact velocities at cluster surfaces cause deagglomeration.

A theoretical model balances agglomerate formation and breakage rates.

Abstract

Clustering is often presumed to lead to enhanced agglomeration between cohesive grains due to the reduced relative velocities of particles within a cluster. Our discrete-particle simulations on gravity-driven, gas-solid flows of cohesive grains exhibit the opposite trend, revealing a new mechanism we coin "cluster-induced deagglomeration." Specifically, we examine relatively dilute gas-solid flows and isolate agglomerates of cohesive origin from overall heterogeneities in the system, i.e., agglomerates of cohesive origin and clusters of hydrodynamic origin. We observe enhanced clustering with an increasing system size (as is the norm for noncohesive systems) as well as reduced agglomeration. The reduced agglomeration is traced to the increased collisional impact velocities of particles at the surface of a cluster; i.e., higher levels of clustering lead to larger relative velocities between the clustered and nonclustered regions, thereby serving as an additional source of granular temperature. This physical picture is further evidenced by a theoretical model based on a balance between the generation and breakage rates of agglomerates. Finally, cluster-induced deagglomeration also provides an explanation for a surprising saturation of agglomeration levels in gravity-driven, gas-solid systems with increasing levels of cohesion, as opposed to the monotonically increasing behavior seen in free-evolving or driven granular systems in the absence of gravity. Namely, higher cohesion leads to more energy dissipation, which is associated with competing effects: enhanced agglomeration and enhanced clustering, the latter of which results in more cluster-induced deagglomeration.