(431f) Enhanced Mixing of Particulate Matter in Multi Orifice Silos

Enhanced Mixing of particulate matter in multi orifice silos

Silos and hoppers have a ubiquitous
existence in several industries which handle material in a powdered form. In
some instances silos/hoppers act as a feeder to a particular process while in
some other instances silos are used for processing material (e.g. nuclear
pebble bed reactors). One of the key factors which affects the efficient design
of a silo is the flow pattern and mixing of the material which drains down the
silo and exits from the orifice. Typically, a silo (flat bottomed) or a hopper
(conical bottomed) consists of a single orifice/exit either located centrally
or at the periphery, which is wide enough to prevent jamming of the flowing
particles.  Such systems have been widely
studied using experiments and numerical simulations to understand the flow and
mixing behaviour of the particles.

         In
this work we provide newer insights about particle dynamics occurring in a flat
bottomed silo having more than one orifice. We employ discrete element method
(DEM) simulations to study the flow and mixing of the spherical particles
draining down the silo. The method employs a Hertzian  contact model and is simulated by the
large-scale atomic/molecular massively parallel simulator (LAMMPS) developed by
Sandia National Laboratories. In parallel, experiments are carried out using
model granular particles, whose motion (near the wall) is captured using high
speed camera imaging. Position of every particle is tracked over several images
to determine their mean and fluctuating properties. The mixing index is
quantified by counting different types of particles exiting from different
orifices.

         For
the silos operated with "n" orifices each being wide enough, we
observed that the overall flow behaviour, measured in terms of the component of
mean velocity profiles, is equivalent to the "n" silos with a single
exit orifice acting in parallel with little or no interaction between different
regions of flow. However, very interesting behaviour is observed when width of every
orifice is equal to that which can induce jamming. This causes spontaneous jamming
at different orifices at different times, leading to co-existence of regions of
flow and no flow which interact with each other dynamically.

         To
explore this behaviour systematically, we created a protocol in the simulations
whereby the orifices are artificially closed and opened in a prescribed
sequence and for a predetermined time period. We observe that such systematic
closing/opening of the orifices caused the particles initially far apart from
each other, to mix whilst coming out of the orifices. Figure 1 shows two
snapshots taken from a typical simulation. The mixing efficiency is obtained by
measuring the concentration of particles coming out of different orifices
relative to their initial position higher up the silo. The observed behaviour
in simulations is being validated by experiments conducted using a model
granular system.  The early results
clearly show that the multi orifice silo operated using the abovementioned
strategy leads to an improvement in particle mixing as compared to silos with a
single orifice.

(a)                                                                                                                                                                                 
(b)

Figure 1. Snapshot
of silo drainage: (a) at initial time, and (b) at a subsequent time when silo
is close to half empty. The silo is filled with identical particles but colored
differently for ease of visualization. Notice the flow of brown colored
particles from three different orifices shown in b.