Wind Turbine

chemical
SMX mixer geometry
Static Mixers
by Design
by Shiping Liu, Andrew Hrymak, and Phil Wood,
McMaster University, Hamilton, Ontario, Canada;
and Rafiqul Khan, Fluent Inc.
Static mixers consist of an array of
similar, stationary mixing elements,
placed one behind the other in a pipe
or channel. Liquids are pumped through
the channel, and the elements act to accelerate
the homogenization of material properties,
such as concentration, temperature,
and velocity. In some types of static mixers,
the elements are rotated by some angle
(say, 90°) relative to the previous element.
The SMX mixer is one example of this type
of mixer. The elements are complex networks
of angled guide blades, positioned
at an angle to the pipe axis, and mixing
occurs through the continuous redirecting,
splitting, stretching, and diffusion of the
fluids as they pass through the available
openings.
Since there are no moving parts
involved, static mixing occurs with low shear,
which is very important for some mixing
processes where gentle treatment of the
materials is required. Processes of this type
are found in the food processing, pharmaceutical,
and biotechnology industries.
Static mixers are also widely used in a host
of other industries, however, including oil
and gas, chemical processing, polymer production
and processing, and water and waste
treatment. Some of the major manufacturers
of static mixers are Sulzer Ltd., Koch-Glitsch
Inc., and Chemineer Inc.
Researchers from the Department of
Chemical Engineering at McMaster University
have been investigating the laminar mixing
characteristics of an SMX static mixer
using the discrete phase model (DPM) in
FLUENT. Typically a series of SMX elements
is used to ensure adequate mixing. The mixing
quality increases with the number of
mixing elements, but so does the power
required to pump the fluids through the
channel. For this reason, the number of mixing
elements used in any given mixer is a
function of the required product quality and
operating budget.
Mixing homogeneity is often rated using
the coefficient of variation, or COV, which
can be approximated using the fluid properties,
operating parameters, and geometry
of the mixing element. It can also be
computed easily using CFD. Furthermore,
CFD can be used to test the COV after the
fluid has passed through different element
designs, and to determine the minimum
number of elements required to achieve the
desired product quality. With CFD, these
parameters can be established long before
construction of an experimental apparatus
begins, saving both time and money.
Using FLUENT, COV values, pressure drop,
and power requirements have been computed
for a series of test cases using four
SMX elements in a pipe. Qualitative
results from the DPM calculations have clearly
shown the expected stretching and layering
of the fluid during the mixing process.
Simulations using a two species model to
track the mixing of epoxy resins have also
been performed, and the results, particularly
the species distribution on several axial
planes, are in close agreement with experimental
data provided by Sulzer for the SMX
mixer. ■
Using the DPM, the particle distribution through the mixer,
using a central feeding of 20,000 tracers is shown
Using the species mixing approach, concentration contours
on the center plane are shown
12 Fluent NEWS spring 2002