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By Mike Slack, Fluent Europe Ltd.
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The cyclone separator is perhaps the most
widely used separation device found in
industry. It contains no moving parts, and
can be easily manufactured from a range of
materials. Predicting the flow inside the
cyclone is a challenging task, however. The
correct choice of turbulence model is a critical
factor in capturing the anisotropic turbulent
features of this flow, which is further complicated
by the high streamline curvature.
Pathlines colored by residence time in the cyclone
The air core that
develops at the center
of the hydrocyclone
The cyclone works by inducing spiral rotation
in the primary phase (liquid or gas) and
using this rotation to induce radial acceleration
on a particulate suspension. In conventional
cylindrical cyclone devices, there are
two outlets, both on the axis of symmetry.
The underflow outlet is situated at the apex
of the cone at the base of the cyclone, and
the overflow outlet is an inner tube (or socalled
vortex finder) that descends from the
top. The density of the suspended particulate
phase is normally greater than that of the primary
phase. Due to the imposed swirl, large
particles migrate rapidly to the outer wall
and then spiral down to the underflow.
Smaller particles migrate more slowly, so are
captured in an upward spiral near the center
of the cyclone, and leave through the top.
Running liquid cyclones (commonly known
as hydrocyclones) operate with the top open
to the atmosphere. Due to the low pressure
on the cyclone axis, a back-flow of air can
occur, forming an air core and increasing the
complexity of the physics.
Most practical CFD modeling of hydrocyclones
has been limited to cases without an
air core and low volume loadings of the particulate
phase. A single-phase flow calculation
is normally done, and the Lagrangian
particle tracking approach (DPM) is used to
predict the separation efficiency. Since
hydrocyclones commonly operate with air
cores and slurry feed concentrations in
excess of 10% by volume, simplified modeling
approaches are not suitable. The air core
shape is strongly coupled to the slurry concentration
and swirl, and it governs the flow
split between the outlets.
FLUENT 6.2 is the first commercially available
CFD software to couple an Eulerian multiphase
algorithm with a full Reynolds stress turbulence model. To demonstrate the
potential of this new technology, a 75mm
diameter hydrocyclone, open to the atmosphere
and with a stable air core has been
studied. The inlet feed is a limestone and
water slurry. Six phases are used in the CFD
model: a primary liquid (water), a gas phase
that develops into an air core, and four granular
phases with particles ranging from 10 to
40 microns. The nineteen Navier-Stokes and
six Reynold stress equations were solved on a
70,000 cell model.
The results show that the finest particles
experience a small radial force and therefore
remain suspended in the water. These particles
divide between the under- and overflow
outlets in the same proportion as the water,
as expected. In contrast, the larger 30 - 40
micron particles experience a greater radial
force, so less of this material remains in suspension.
Shortly after entering the cyclone,
the large particles form an even layer on the
cyclone walls. The layer gradually thickens as
the particles approach the underflow, where
most are removed.
The predicted separation efficiency curve compared
with measured cyclone performance for a 10.47%
by weight slurry
Contours of volume fraction for smallest (10 microns, left) and
largest (40 microns, right) particles show the size-dependent
behavior of the limestone
The predicted separation efficiency follows
the correct trend [1]. Considering the
factors that can impact both the experimental
measurement of slurry classification and
the stability and sensitivity of this strongly
swirling multiphase system, the results are
very impressive for this type of analysis.
Reference:
1 T.C. Monredon, K.T. Hsieh and R.K. Rajamani,
International Journal of Mineral Processing, 35,
p. 65-83, 1990.
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