ANSYS® CFX®
Particle Transport Model
Technical Brief
Multiphase Flow Modeling
Because better understanding
of multiphase flow will increase
yield, extend longevity and
permit the assessment of new
designs or processes, ANSYS
CFX computational fluid
dynamics (CFD) software
contains industry-leading
technology providing accurate
and robust models for
multiphase flows.
ANSYS CFX is available in
the ANSYS® Workbench™
environment providing a
common interface and file
management system with other
ANSYS tools.
The multiphase flow models
within ANSYS CFX include
capabilities for:
Free surface
Multi-fluid analysis,
including chemical
reactions
Multiple size group
Cavitation
Particle transport
www.ansys.com
Many fluid flows involve the transport of particulates. These can be solid
particles in a gas or liquid, liquid drops in a gas, or gas bubbles in a liquid.
Depending on the size of the particles, this type of flow can be modeled as
a single-phase multi-component flow, as Eulerian multiphase flow or using
the ANSYS CFX particle transport model feature. Examples of fluid flows
involving particulates that are best modeled using the particle transport
model, include water–sand mixtures in which erosion is of interest, water
spray into an air stream, oil droplet injection in a combustion chamber and
coal particulates burning in an air mixture.
The ANSYS CFX particle
transport model has the
ability to model complex
geometry, as shown in
the model of a wet SO2
scrubber. Picture
courtesy URS
Corporation.
In the ANSYS CFX particle transport model, a representative sample of the
actual particles in the flow is tracked through the flow. Differential equations
for the position and velocity of each particle are solved, and the effect of
the particles on the fluid is calculated as source terms for the fluid
momentum equations, given that each sample particle represents many
actual particles in the flow. The equations take into account the various
forces that can act on the particle, such as drag and gravity.
Additional physical models for the particles can be included by calculating
further differential equations for particle temperature, or for masses of
different constituents of the particles. The particle mass can change due to
processes such as evaporation (for example water droplets, oil droplets) or
chemical reactions (for example combusting coal particles) with the fluid.
Other effects that can be modeled include droplet breakup and wall
erosion. Additional effects on the particle can be modeled by the user,
either through the built-in expression language, or by user subroutines.
Technical Brief
In comparison with the multi-component or multiphase methods of
modeling particle-laden flows, the ANSYS CFX particle transport model is
especially useful when the particles have a range of sizes, often in the
range of 10 microns to a millimeter. Further, as each representative particle
is tracked, its diameter, temperature, constituent masses and other particle
properties are known precisely. This makes the ANSYS CFX model a most
convenient way of calculating heat and mass transfer to and from particles,
in addition to reactions including particles.
Particle transport modeling with ANSYS
CFX software was used by the
University of Canterbury, New Zealand,
to predict the trajectory of pollen
around a kiwi fruit flower. Increasing the
number of pollen grains fertilizing the
flower can increase fruit size.
Transient particla tracking includes
interoperability across transient rotor
stator interface.
Understanding flow using particle
transport modeling can assist design by
showing the complex trajectories of
particles bouncing off an object such as
a valve as well as where the valve might
be eroded due to the impact of the
particles.
One of the important features of particle transport modeling in ANSYS CFX
software is that it can be run in parallel. The parallelization concept used for
the particle solver is based on the same domain decomposition approach
used by the fluid solver. This means that each partition solves only particles
in its own control volumes. Particles crossing partition boundaries are
communicated between the partitions. Results obtained from a serial and
parallel run are identical, even if particle tracks are perturbed by turbulent
dispersion. Special effort has been dedicated to the development of a novel
and intelligent load management procedure in order to maintain excellent
parallel performance with the particle transport model — a significant
achievement for the rather difficult combination of a particle solver and a
domain decomposition parallelization approach. Thus, particle transport
models and complex Eulerian models running together can be parallelized
with high efficiency.
The ANSYS CFX particle transport capability delivers the ability to track the
flow of discrete phases in a variety of industries including power
generation, aerospace, HVAC, automotive and many others. This enables
the engineer to accurately model the interaction between the phases both
to obtain greater insight into the physical processes taking place and to
achieve better overall predictions of their flows. Moreover, because of the
parallel algorithm within ANSYS CFX software, this can all be done very
efficiently within simulations on as fine a grid as required for the continuous
phase solution.
www.ansys.com
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