Experimental investigation of batch sedimentation of concentrated bidisperse suspensions

• We used an LIF technique to investigate sedimentation of bidisperse suspensions.
• Interface velocities and sediment structure have been determined.
• Use of some literature approaches can predict falling interfaces velocities.
• Experimentally determined settling types are compared to literature predictions.
• Change in particles size ratio leads to different sediment processes.

In this study, we experimentally investigate sedimentation of concentrated (30% in volume) bidisperse suspensions using a laser-induced fluorescence technique. Particles were of same density and mixed in 1:1 ratio. The small-to-large-particle ratio varied from 0.32 to 0.63. We firstly show that the experimental downward velocities of interfaces are well predicted by the MLB theory and that the Smith effect is less important when the size ratio is increased. We secondly show the sedimentation process to follow a particular type depending on such size ratio. In particular, type 3 is involved at the highest size ratio and for which the mixed zone is connected to the sediment through a rarefaction fan. However, by lowering the size ratio to 0.32, the sedimentation type is found to be then type 1 where no rarefaction fan does exist. These experimental findings are in qualitative agreement with the predictions of Dorrell and Hogg. Finally sediment structure was examined and its constitution was evaluated in the course of time, showing the occurrence of the transition from mixed sediment to mono-sized sediment. The dependence of such a transition and the mixing level on the size ratio was found to agree with theoretical predictions from literature.

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