Reverse segregation in dense granular flow through narrow vertical channel

TL;DR

This study investigates reverse segregation phenomena in dense granular flows through narrow vertical channels, revealing how flow patterns and insert placements influence particle segregation and mixing.

Contribution

It introduces a phenomenological model explaining rolling and bouncing induced segregation and demonstrates how flow modifications can control segregation patterns.

Findings

Large particles form bands away from walls in dense flow.

Symmetrical inserts can enhance mixing or reverse segregation.

Flow pattern modifications significantly influence segregation behavior.

Abstract

Controlling flow-induced segregation in a granular mixture is highly relevant to many industrial settings. To enhance mixing or promote segregation, the continuous gravity flow of a bidisperse granular mixture through a series of narrow vertical channels with exit slots is investigated. The bidisperse mixture is composed of two different sizes of particles, but of the same density. In dense flow, segregation occurs, leading to formation of bands. The bands of large particles appear at a distance away from the walls. This finding is in contrast to that in shear-driven segregation in a dense flow where large particles segregate towards the walls. Using a phenomenological model, it has been shown that rolling and bouncing induced segregation is the dominant mechanism. When cylindrical inserts are placed to modify flow patterns, that significantly influences segregation patterns. The symmetrical placement of a cylindrical insert close to the exit slot vanishes the bands and enhances mixing. However, with two inserts placed symmetrically and close to the exit slot, the degree of segregation in the reverse direction is greatly enhanced compared to that without insert. In the former, small particles accumulate in thin regions adjacent to the walls, and large particles comprise the bulk of the domain and the flowing stream. The heap formation above the insert in a narrow channel, when the insert is close to the exit, enhances mixing in one configuration, whereas it amplifies reverse segregation in the other.