Collision and breakup of fractal particle agglomerates in a shear flow

Abstract

A computational study was performed both of a single agglomerate and of the collision of two agglomerates in a shear flow. The agglomerates were extracted from a direct numerical simulation of a turbulent agglomeration process, and had the loosely packed fractal structure typical of agglomerate structures formed in turbulent agglomeration processes. The computation was performed using a discrete-element method for adhesive particles with four-way coupling, accounting both for forces between the fluid and the particles (and vice versa) as well as force transmission directly between particles via particle collisions. In addition to understanding and characterizing the particle dynamics, the study focused on illuminating the fluid flow field induced by the agglomerate in the presence of a background shear and the effect of collisions on this particle-induced flow. Perhaps the most interesting result of the current work was the observation that the flow field induced by a particle agglomerate rotating in a shear flow has the form of two tilted vortex rings with opposite-sign circulation. These rings are surrounded by a sea of stretched vorticity from the background shear flow. The agglomerate rotates in the shear flow, but at a slower rate than the ambient fluid elements. In the computations with two colliding agglomerates, we observed cases resulting in agglomerate merger, bouncing and fragmentation. However, the bouncing cases were all observed to also result in an exchange of particles between the two colliding agglomerates, so that they were influenced both by elastic rebound of the agglomerate structures as well as by tearing away of particulate matter between the agglomerates. Overall, the problems of agglomerate–flow interaction and of the collision of two agglomerates in a shear flow are considerably richer in physical phenomena and more complex than can be described by the common approximation that represents each agglomerate by an ‘equivalent sphere’.

Exponential scaling in early-stage agglomeration of adhesive particles in turbulence

“…First, the current work focuses on the early-stage agglomeration, where the breakage and the rearrangement of agglomerates are not significant. It is unclear to what extend the framework developed here can be extended to situations with large agglomerates [65]. It requires one to construct kernel functions that contain information about breakage and restructuring [66,67].…”

Deagglomeration of cohesive particles by turbulence

“…2010). Several models have recently been developed for deagglomeration due to rotary stress in simple shear flows (Dizaji, Marshall & Grant 2019; Ruan, Chen & Li 2020; Vo et al. 2020).…”

Fractal structures arising from interfacial instabilities in bio-oil atomization

“…In the area of thermofluids, and possibly in general, fractals can be associated with some source of instability leading into organized/chaotic patterns 21 – 23 , namely, Kelvin–Helmholtz (KH) instability in shear layers 24 – 28 and viscous fingers 29 , 30 . Fractal complexities can be related to some important phenomena such as mixing and mass transport at the turbulent/non-turbulent interface of shear layers 31 – 33 and boundary layers 34 as well as turbulent particle agglomeration 35 . One of the most complex yet fascinating classes of fluid dynamics are the two-phase flows in which a competing effect from gravity 36 and inertial forces against resistances arising from the differences in physical properties such as viscosity, surface tension and density produces a broad range of complex interfacial geometries.…”