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AGGREGATE EVOLUTION IN SHEAR FLOW: NUMERICAL INVESTIGATION OF THE ROLE OF COHESIVE AND HYDRODYNAMIC FORCES
- Author / Creator
- Saxena, Akash
Multiphase flows with a dispersed phase of micron-sized solid particles are common to many industrial applications, ranging from wastewater treatment to metal purification. These particles are in a size range where surface forces are significant, and thus are likely to form aggregates. When these aggregates are exposed to hydrodynamic forces, their size and structure evolve which consequently impacts the process efficiency. This research investigates the size and structure of aggregates as they evolve, under the action of hydrodynamic shear stresses. The goal of this project is to establish the effect of short-range particle-particle interactions and hydrodynamic forces on aggregate size, structure and breakage kinetics. The effect of flow inertia on aggregate behaviour is also investigated.
An Eulerian-Lagrangian approach is used to model aggregates in a dilute system. The initial aggregates are generated using an algorithm, and consist of 50 or 70 discrete spherical primary particles. Aggregate morphology is characterized by their size, their number of primary particles, and their density. The particle-particle interactions are represented using widely accepted models, and are implemented in a Discrete Element Method (DEM) that tracks the motion of every primary particle. The flow dynamics are solved by a Lattice Boltzmann Method (LBM), and the two phases are coupled through an Immersed Boundary Method (IBM). Therefore, the particles are fully resolved in the flow. For each simulation, the free-to-move aggregates are placed at the center of the domain, and their size and structure evolution is studied over time.
Initially, the role of particle-particle interaction forces relative to the viscous drag was established. Aggregates were assigned different normal and tangential components of the inter-particle cohesive forces, and submitted to a shear flow by imposing a shear stress in the liquid phase. Hydrodynamic forces are also estimated using the free draining approximation, where hydrodynamic interactions between particles are not included, and the results are compared with those obtained from a fully resolved flow. It is found that while the normal forces contribute significantly to the overall bond strength of the aggregates, they have no impact on aggregate restructuring. On the other hand, tangential forces are found to play a two-fold role. While tangential forces contribute to the overall bond strength, they also make the aggregates brittle. This leads to enhanced aggregate breakage when the tangential forces are large compared to normal forces and viscous drag. Furthermore, it is discovered that the resistance to deformation at the aggregate scale induces a flow disturbance that reduces drag forces compared to the free-draining approximation, and significantly impacts aggregate breakage and restructuring.
With the role of interaction forces established, the impact of flow inertia on aggregate evolution was then explored. The aggregates were exposed to shear flow with non-negligible flow inertia. Initially, the breakage rate depends on the strength of particle-particle interactions relative to viscous forces. However as the drag force increases, the breakage rate is governed by momentum diffusion, which induces a delay for the imposed stresses to reach the aggregate.
Simulations with scaled particle-particle forces demonstrated that flow inertia has no impact on the aggregates’ stable morphology, but significantly favors breakage; a power-law relationship was found between breakage time and aggregate-scale Reynolds number.
Since flow inertia at finite Reynolds number (that also controls momentum diffusion) was found to play a significant role in aggregate breakage, an attempt to simulate aggregate evolution in accelerating flows mimicking turbulent flows at sub-Kolmogorov length scales is explored. In these simulations, the imposed shear stress is increased linearly with time. It is found that although aggregates restructure due to shear flow, their structure at breakage does not depend on the shear stress in the flow. Furthermore, their breakage is found to be delayed on increasing the flow acceleration. A possible explanation for this phenomenon is inertial effects in the flow at aggregate scale. The delay results in aggregates undergoing more rotations before breaking for higher flow accelerations. These findings suggest that models of breakage rate, as used in population balances for example, should probably consider the effect of flow inertia under accelerated flows, although no such models are readily available in the literature.
- Graduation date
- Fall 2021
- Type of Item
- Doctor of Philosophy
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