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By Sergio A. Vasquez and Liz Marshall, Fluent Inc.
View the pdf of this article
Waves breaking near an off-shore oil rig, with static pressure
contours on the supports
The multiphase modeling capabilities in FLUENT have expanded
and matured over the years to pave the way for reliable simulations
ranging from cavitating propellers to cyclone separators.
Several models are available for the various classes of multiphase flow.
These models have been validated for basic functionality and industrial
strength applicability. Never before has the range of possibilities for
numerical multiphase modeling in FLUENT been so extensive.
Contours of vapor volume fraction illustrate cavitation in an
automotive fuel pump
The Eulerian multiphase model is the most general purpose offering.
Making use of separate sets of momentum equations for each
phase, it can be applied to mixtures with individual phase concentrations
from 0 to 100%. A special formulation is available for granular
materials, such as sand, and even mixtures with gas, liquid, and granular
constituents can be modeled. Heat and mass transfer between
phases can take place, as can species transport and homogeneous and
heterogeneous reactions.
Iso-surfaces of the volume fraction of a granular phase
suspended in a stirred tank
Another general purpose, yet more economical, treatment of multiphase
systems makes use of the mixture model. With this model,
only one set of momentum equations is used for the mixture. Fluid
properties are defined for all of the constituents, and depending on
the make-up of each cell, weighted average properties are computed
and used. The mixture model has enjoyed success with gas-liquid and
liquid-granular mixtures of all types. It forms the basis of the cavitation
model, which allows for mass transfer due to pressure tension
between liquid and gaseous phases. Add-on modules in FLUENT 6.2
allow for size-distribution modeling, based on population balance theory,
in Eulerian and mixture multiphase simulations. Unlike any other
commercial software, FLUENT offers three well-known approaches for
population balance: the sectional method, the method of moments,
and the quadrature method of moments.
While the Eulerian and mixture models are appropriate for fluid
mixtures, they are not the most suitable offering for immiscible fluids.
For this class of problems, the shape and evolution of the free surface
is often of interest. FLUENT’s volume of fluid (VOF) model has a
proven history of success for free surface flows. A choice of algorithms
is available for surface tracking, so that the most efficient methodology
can be applied to any given application. In addition, the VOF
model now benefits from the non-iterative time advancement solver
available in FLUENT 6.2, which cuts solution time significantly.
For phases that occupy 10% or less by volume, the discrete phase
model (DPM) is available. The longest-running multiphase model in
FLUENT, DPM allows for heat and mass transfer between a droplet or
particle phase and the background fluid. In addition to built-in flexibility
for the definition of size distribution and spray geometry, new
spray break-up models have been added that are governed by local
flow conditions. The widely-used DPM is a workhorse that has been
used for applications ranging from coal furnaces to automotive fuel
injectors.
The wealth of multiphase models, highlighted in the articles that
follow, touch applications in many industries. For more information on
these capabilities, please call your local Fluent office or distributor.
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