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Hopper Flow dynamics

This research was undertaken by Dr. Romain Guises during his PhD studies while at Imperial College under the supervision of Dr. J-P Latham and Prof. Antonio Munjiza, with assistance from AMCG’s Dr. Jiansheng Xiang.

The flux of granular material though an aperture in a silo configuration has been analysed for frictional and frictionless ellipses of varying aspect ratio.

There is considerable interest amongst bulk solids handling industries to better understand the controls on hopper/silo discharge rates and incidence of hang-ups or “jamming”. Mining companies have particular interest in granular flow through given aperture geometries to better understand isolated extraction zones, (IEZ) and isolated movement zones, (IMZ) above draw points in block caving operations. Understanding the flow and origins of naturally fragmented material arriving at a series of draw points is of great importance in draw point design in caving operations. To model the processes involved, study methods have included; large scale physical models, continuum models of granular flow and increasingly, researchers have turned to discrete particle DEM models.

A remarkable richness of mechanical behaviour for the silo discharge flow problem can be seen in the simulations by Guises (2008) using 2D FEMDEM models of elliptical particles with friction. Experiments were run with 900 particles discharging from a rectangular silo with an opening of ~5.7 times the effective diameter of elliptical particles, i.e. sufficiently narrow to observe intermittent jamming for the frictional case. Discharge rates are dependent on particle shape, with faster flows observed for elongated particles when there is zero friction, see Fig. 1. With friction, the periods of time over which significant jamming occurs is greater for increasingly elongated particles.

Fig.1 (a) Total cumulative mass discharged from the silo aperture for frictional particles, m = 0.5, see also Fig.2 and Fig 3. (b) frictionless particles with insets plotting the fluctuations in value of the discharge rate.

FEMDEM methods allow a more fundamental study of flow of granular materials with realistic shape and friction properties.  Aperture shapes, opening widths and spacings can be optimised for flow rates that retain the most uniform flow within the granular mass of the caved ore. There is also potential to parameterise continuum models, compare results with less costly FEMDEM models employing rigid interior properties, or less costly DEM models based on superquadrics or clustered spheres. Transient phenomena leading to silo wall collapse can be studied with this approach.

Fig. 2 Snapshots of the differential stress field during the flux of grains. It is observed that each system goes through periods of relatively jammed flows with long stress chain arches, which consequently reduces the flux of grains. a) α = 1.0, t = 0s, b) α = 1.2, t = 0s, c) α = 2.5, t = 0s, d) α = 1.0, t = 1s, e) α = 1.2, t = 1s, f) α = 2.5, t = 1s, g) α = 1.0, t = 2s, h) α = 1.2, t = 2s, i) α = 2.5, t = 2s, j) α = 1.0, t = 4s, k) α = 1.2, t = 4s, l) α = 2.5, t = 4s

Fig. 3 Snapshots of the velocity field during the flux of grains through the aperture of the silo at different time for 3 types of frictional (μ = 0.5) particles. The silo width is D = 5.656 times the equivalent grain diameter. a) α = 1.0, t = 0s, b) α = 1.2, t = 0s, c) α = 2.5, t = 0s, d) α = 1.0, t = 1s, e) α = 1.2, t = 1s, f) α = 2.5, t = 1s, g) α = 1.0, t = 2s, h) α = 1.2, t = 2s, i) α = 2.5, t = 2s, j) α = 1.0, t = 4s, k) α = 1.2, t = 4s, l) α = 2.5, t = 4s

Reference

Guises (2008) PhD Thesis