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 eta = d(i)/d(o) = 0.877, where d(i) and d(o) are the diameters of inner and outer cylinders. The ratio of the axial length to the radial gap of the annulus Gamma = L/delta = 20.5, where delta = (d(o) - d(i))/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 d(p) was either alpha = delta/d(p) 30 or 100. For the experiments with alpha = 100, the particle volume fraction was phi = 0.10 and for the experiments with alpha = 30, phi was varied over 0 <= phi <= 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 delta d(i)Omega rho/2 mu(s), where Omega is the rotation rate of the inner cylinder and rho and mu(s) 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 alpha = 30 and 0.05 <= phi <= 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 phi = 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 <= phi <= 0.30, with the initial transition away from CCF, for example, occurring at Re approximate to 120 for the pure fluid and Re approximate to 75 for the phi =0.30 suspension. When the particle size was reduced to yield alpha = 100, at phi = 0.10, only the RIB (and no SVF) was observed between TVF and CCF.