Abstract
We address the issue of segregation in bidisperse suspensions of glass beads, by using a liquid fluidized bed in the inertialess regime and an acoustic technique for acquiring the axial composition along the column. Fluidization balances the buoyancy of the particles by a constant uniform upward flow, and therefore enables long-time experiments. From the analysis of the transient segregation fronts, we have collected precise measurements on the sedimentation velocities of small and large beads, Us and Ul, in homogeneous suspensions at the same volume fraction, , for both the bead species, and for different size ratios, 1.13⩽λ⩽1.64, and solid concentrations, . Our measurements provide evidence for a difference in the sedimentation velocities, Us and Ul, over all the ranges of λ and covered. These results make one expect that a long-term fluidization should then result in a stationary segregated state, which was indeed always obtained for large enough particle size ratios, λ⩾1.43. However, at high concentration and for particles of close sizes, λ⩽1.41, we observed a surprising pseudo-periodic intermittency of slow segregation and quick mixing phases. The intermittency time is much longer than the batch sedimentation time and becomes noisy at very high concentration, for which metastable states have been observed. The origin of the mixing destabilization remains an open issue, but we note however that the domain of occurrence, λ⩽1.41, also corresponds, in our experiments, to a continuous size distribution of the particles.
Highlights
On bidisperse sedimentation have reported on the presence of a streaming instability
We note that an inhibition of the segregation process was reported for a size ratio λ = 1.68 and for an overall volume fraction > 0.45 [26], whereas recent experiments [35] have evidenced some segregation for a size ratio, λ, as small as 1.05, and volume fractions, as large as 50%
We note that this segregation inhibition issue is, difficult to address through batch sedimentation experiments, as the segregation velocity at high concentration becomes so small that a segregated layer is usually hardly formed within the experimentation time
Summary
A sketch of the experimental setup is displayed in figure 1. For the sake of accuracy, the acoustic measurements are averaged over 500 signals, which requires 9 s for each scan step and results in a typical scan duration of around 10 min (≈60 measurement locations) Such a procedure enables us to get rid of any radiative noise emitted by the brushless motor of the scanner and does not wipe out any accessible physical phenomenon, as the maximum displacement of any bead during the averaging procedure (evaluated with the largest Stokes velocity, ul, reported in table 1) remains smaller than the transducer diameter. Once the acoustic scans were realized on the stationary fluidized monodisperse suspensions, the flow rate was cut in order to measure the batch sedimentation velocity U ( ), from the resulting top front displacement.
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