Granular Flow - Application
Idler-induced segregation on a conveyor belt
Material is often transported by conveyor belts.
Conveyors can be up to several kilometers in length and are supported
regularly spaced by idlers. Between idlers, the belt has a catenary shape
determined by the belt tension.
The material on the belt is perturbed by the idlers which induce a slow
but complex flow.
A DEM simulation of a conveyor belt moving at 3 m/s with 10 cm diameter
idlers spaced 3 m apart is shown. The geometry is periodic in the flow
direction. The particles are a binary mixture of 25 mm diameter and
smaller 12.5 mm particles filled to a depth of 20 cm. The simulation plane
is a slice through the center of the belt.
The figures below show the material at distances 6, 900 and 4050 m from
the loading point.
As particles pass over an idler the higher ones travel a slightly longer
path. This causes gaps to open in the microstructure, allowing small
particles to percolate downwards. It also generates a modest shear between
the belt and the free surface. The combination of these induces a slow but
relentless segregation.
The segregation has two mechanisms.
-
Relatively quickly the small particles percolate down
through the microstructure of large particles. This leads to a state
where a layer of large particles devoid of fines sits on top of a
saturated layer of fines containing some large particles.
-
Then the fines push the remaining large particles up
to the large particle region above, leading to a fully segregated
state. The segregation rate for this process is also relatively
constant and between 2 to 4 times slower.
The segregation rates increase approximately linearly with
both the size ratio and the mass fraction of large particles.
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