Inertial flow transitions of a suspension in Taylor–Couette geometry

Experiments on the inertial flow transitions of a particle–fluid suspension in the concentric cylinder (Taylor–Couette) flow with rotating inner cylinder and stationary outer cylinder are reported. The radius ratio of the apparatus was [STIX]x1D702=dᵢ/dₒ=0.877 , where dᵢ and dₒ are the diameters of inner and outer cylinders. The ratio of the axial length to the radial gap of the annulus [STIX]x1D6E4=L/[STIX]x1D6FF=20.5 , where [STIX]x1D6FF=(dₒ-dᵢ)/2 . The suspensions are formed of non-Brownian particles of equal density to the suspending fluid, of two sizes such that the ratio of annular gap to the mean particle diameter dₚ was either [STIX]x1D6FC=[STIX]x1D6FF/dₚ=30 or $100$ . For the experiments with [STIX]x1D6FC=100 , the particle volume fraction was [STIX]x1D719=0.10 and for the experiments with [STIX]x1D6FC=30 , [STIX]x1D719 was varied over 0 [STIX]x1D719 0.30 . The focus of the work is on determining the influence of particle loading and size on inertial flow transitions. The primary effects of the particles were a reduction of the maximum Reynolds number for the circular Couette flow (CCF) and several non-axisymmetric flow states not seen for a pure fluid with only inner cylinder rotation; here the Reynolds number is Re=[STIX]x1D6FFdᵢ[STIX]x1D6FA[STIX]x1D70C/2[STIX]x1D707ₛ , where [STIX]x1D6FA is the rotation rate of the inner cylinder and [STIX]x1D70C and [STIX]x1D707ₛ are the density and effective viscosity of the suspension. For purposes of maintaining uniform particle distribution, the rotation rate of the inner cylinder (or $Re$ ) was decreased slowly from a state other than CCF to probe the transitions. When $Re$ was decreased, pure fluid transitions from wavy Taylor vortex flow (WTV) to Taylor vortex flow (TVF) to CCF occurred. The suspension transitions differed. For [STIX]x1D6FC=30 and 0.05 [STIX]x1D719 0.15 , with reduction of $Re$ , additional non-axisymmetric flow states, namely spiral vortex flow (SVF) and ribbons (RIB), were observed between TVF and CCF. At [STIX]x1D719=0.30 , the flow transitions observed were only non-axisymmetric: from wavy spiral vortices (WSV) to SVF to CCF. The values of $Re$ corresponding to each flow transition were observed to reduce with increase in particle loading for 0 [STIX]x1D719 0.30 , with the initial transition away from CCF, for example, occurring at Re≈ 120 for the pure fluid and Re≈ 75 for the [STIX]x1D719=0.30 suspension. When the particle si