(354d) Tayor-Couette Flows of Suspensions

It is essential to understand the dynamics of fluid-particle mixture flows having applications in a wide range of areas including oil, food and pharmaceutical industries, power plants and biological flows. Fundamental understanding in this field will help in improving existing models and develop new predictive models that can compute quantities such as the energy to drive a flow, or the extent of mixing. Here, we report our experimental findings on the effect of particles on the inertial flow transitions that occur when a neutrally buoyant suspension of fluid-particles mixture is sheared between two concentric cylinders (Taylor-Couette apparatus). The apparatus has a radius ration of η = r_i/r_o = 0.877 and an aspect ratio of Γ = L/δ = 20.5 where the radial gap δ = r_o-r_i, L is the axial height and r_i and r_o are the radii of inner and outer cylinders respectively. A neutrally buoyant suspension consisting of non-Brownian particles in a Newtonian fluid of same density was used. To study the effect of particle loading and particle size, suspension concentration was varied between volume fractions φ = 0 and φ = 0.30 and two particle sizes were chosen such that the ratio of radial gap to the particle size (d_p) is either α = δ/d_p = 30 or 100. It is well known that for a Newtonian fluid without particles, with decreasing inner cylinder Reynolds number (Re) in a quasi-steady manner, the flow between a rotating inner cylinder and a stationary outer cylinder transitions from Wavy vortex flow (WVF) to Taylor vortex flow (TVF) to circular Couette flow (CCF). Here Re = r_iΩρ/μ, where Ω is the inner cylinder rotation rate and ρ and μ are the density and viscosity of the fluid. Our experiments showed that the presence of particles significantly changes these flow transitions, quantitatively altering known transitions, and introducing flow states not seen in the pure fluid flow for similar conditions. In particular, additional non-axisymmetric states such as spiral vortex flow (SVF) and ribbon (RIB) flow were observed between TVF and CCF. At φ = 0.30, the flow transitions only through non-axisymmetric flows: wavy spiral vortices (WSV) to SVF to CCF. At dilute concentrations (φ << 0.01), inertial migration of particles to specific locations was observed for CCF and TVF which suggests the possibility of non-uniform concentration profiles in the annular region at higher concentrations. These non-uniform profiles combined with interactions between particle induced flow disturbances may be the cause of the observed deviations of the suspension flows from pure fluid behavior.