Suspension Taylor-Couette flow

Abstract

Taylor-Couette (TC) flow refers to the flow in the annulus between two coaxial, independently rotating cylinders. The TC system has been subject to multiple experiments spanning over decades due to the instability phenomena that occur in the flow. When the rotation rates of the cylinders are increased beyond a critical value, instabilities appear in the system that result in the formation of different flow regimes. Single-phase flow in the TC system has been studied extensively and the various flow transitions have been catalogued for different geometrical parameters and rotational conditions. In the current experiments (Radius ratio= 0.917, Aspect ratio= 22), flow visualisation with anisotropic reflective particles has been used to obtain qualitative and quantitative information about the different flow regimes using Space-Time (S-T) plots and their spectral analyses. An aqueous glycerine solution was used as the working fluid for single-phase flows. The critical Reynolds number (calculated based on inner cylinder shear rate) for the transition from laminar Couette flow was found to be slightly higher for the current setup in comparison to other experiments found in literature, but the order of flow transitions and their spectral characterisation for all the flow regimes showed good agreement, serving as a validation for the setup. With the established single-phase flow as a base, the effect of particle loading on the flow transitions was studied. A neutrally buoyant particle-laden suspension was prepared using an aqueous glycerine solution with Poly(methyl methacrylate) (PMMA) particles of 619-micrometer diameter. The volume fraction of the particles was varied from 0.05 till 0.40 and the flow map was constructed for multi-phase flow. The primary effect of particle addition is an earlier onset of the first transition from laminar Couette flow, thus indicating a destabilisation of the flow by particles. In addition to this, several non-axisymmetric flow structures appeared in the suspension experiments which were absent in the single-phase flow experiments. The particles caused the appearance of flow regimes such as spirals (Taylor vortices that move up the cylinder axis in a helical motion) and ribbons (block-like structures that have alternating light and dark squares), which normally occur in the case of counter-rotating cylinders for single-phase flow. The transitions across all volume fractions were characterised based on the S-T plots and/or spectra to obtain a consolidated flow map for particle-laden suspensions. The results presented point towards intriguing flow behaviour that provides a large parameter space for further research in the years to come.