Effect of flow geometry on the evolution of concentrated suspensions flowing through a fracture

Flow of rigid-particle suspensions is prevalent in nature and is important for a range of engineering applications across many disciplines. However, understanding of the three-dimensional flow behavior of suspensions remains limited and studies primarily focus on suspensions consisting of idealized fluids (Newtonian) and particles (neutrally buoyant spheres). Here, we explore the effects of flow geometry on the behavior of a concentrated suspension (ϕ=0.5) composed of dense, polydisperse particles suspended in a shear-thinning fluid. Fully developed flow in a tube transitioned through a tapered manifold to a high-aspect-ratio rectangular duct (fracture), which allowed direct visualization of the flow and quantification of the velocity and solid concentration fields. Obstructions added to the fracture led to increasingly three-dimensional flow fields and allowed direct observation of the role of in-plane shear and extension and contraction of the suspension. We observed centimeter-scale reduced-ϕ regions adjacent to the lateral no-flow boundaries and only millimeter-scale reduced-ϕ regions along no-flow boundaries caused by the obstructions placed in the middle of the flow field. This resulted in regions of increased velocity near the lateral no-flow boundaries, but negligible perturbations to the velocity away from the millimeter-scale shear zone along the internal no-flow boundaries. Additionally, recorded ∇P within the fracture exhibited a transient response that persisted throughout the experiment and was independent of the flow rate or obstruction configuration. Simulations using a suspension balance model provided additional insights into the source of the low-ϕ regions and the mechanisms controlling the transient ∇P. The simulation results support the hypothesis that the non-uniform ϕ-distribution developed in the inlet tubing expanded laterally as the suspension flowed through the manifold, creating the low-ϕ (high velocity) regions next to the no-flow boundaries. Additional simulations showed that particle rearrangement across the fracture aperture was directly related to the transient ∇P observed in our experiments.