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By Fritz Bedford and Shaoping Shi, Fluent Inc.
The simulation of internal combustion engines
with moving pistons and valves is one of the
premier applications of the dynamic mesh
model in FLUENT. By breaking up the model into
different zones, it is possible to apply different mesh
motion types to different regions in a single simulation.
For example, Figure 1 shows a four-valve
engine where local smoothing and remeshing are
used in the upper part of the combustion chamber,
and dynamic layering is used in the lower
part of the combustion chamber adjacent to the
piston, and in the region above the valves. The
use of the unstructured smoothing and remeshing
approaches in the upper combustion chamber
greatly facilitates the simulated motion of the
valves. If only traditional structured approaches
were available, it would be difficult to generate
topologies that could accommodate the full range
of valve motion in this region. Typically, such structured
moving mesh approaches require special
pre-processing tools and involve significant manual
work. These tools and procedures are not required
for the dynamic mesh model in FLUENT, where
only the initial mesh and description of the boundary
are required. In the lower part of the combustion
chamber, it is more natural to use layered
elements, since the piston motion is linear and
there is no interaction with the moving valves.
Layered elements are also used above the valve,
allowing better resolution of the valve seat gap.
Although not required for the engine shown in
Figure 1, FLUENT 6.1 also includes tools for treating
arbitrarily complicated piston shapes.
The surface grid for a four-valve engine
Swirl patterns for a high-swirl research engine
A qualitative assessment of the predictive capability
of FLUENT’s hybrid approach for moving and
deforming mesh is offered in Figure 2. These images
are taken from a simulation of a high-swirl research
engine.1 The swirl patterns at the end of the intake
stroke at three different positions along the cylinder
axis are illustrated. The FLUENT dynamic mesh
model results are in excellent agreement with experimental
measurements and calculations performed
by an in-house code at General Motors,
as reported in Ref. 1. In particular, the FLUENT predictions
accurately capture the location of the swirl
center at each axial position. The FLUENT results
for the flow field at the point in the cycle when
the piston is in the top dead center position also
agree well with data. This result is important because
in this position, the combustion chamber is completely
filled with tetrahedral elements. Qualitative
comparisons such as these support the trio of remeshing
schemes in FLUENT for use in other in-cylinder
flow applications.
To simulate the complex physics that are fundamental to internal combustion
engines, the dynamic mesh capability is fully compatible with FLUENT’s
suite of spray and combustion models. Figure 3 shows a snapshot of a fuel
spray in a diesel engine2 several milliseconds after the start of injection.
The breakup of the fuel spray is governed by the KHRT (Kelvin-Helmholtz
Rayleigh-Taylor) spray breakup model. After the spray evaporates, FLUENT’s
eddy-dissipation model is used to simulate mixing controlled combustion,
and the resulting fuel iso-surfaces are shown, colored by temperature.
This simulation also takes advantage of the IC-specific crevice model,
which accounts for unresolved crevice volumes as well as blow-by past
the piston rings. The spray models are also important for sparkignited,
gasoline direct injection engines. In fact, CFD is a useful tool for modeling
stratified- charge (non-homogenous) spark-ignited engines in general,
where it is necessary to determine if vaporized fuel is delivered to the
spark plug electrode at the instant of ignition. If the spark is not immersed
in a combustible mixture, a misfire will occur.
Looks like a fancy dessert, but it is really liquid fuel spray and an
isosurface of vaporized, yet unreacted fuel concentration, colored by
temperature in a diesel engine. The fuel is being consumed by combustion
but is getting replenished by the fuel spray. The competing effects have
reached a standoff in terms of the motion of the iso-surface. The simulation
predicts that there will be a significant amount of fuel vapor at high
temperature, which leads to the formation of soot.
The geometry of the DOE/NETL natural gas engine
Fluent’s dynamic mesh capability has also been used in spark ignition
natural-gas engines. A joint effort between Fluent Inc. and the Department
of Energy’s National Energy Technology Laboratory (DOE-NETL) is currently
underway to study simulation techniques for these engines. One case studied
recently is the experimental DOE-NETL stationary engine shown in Figure
4. Natural gas is premixed with air upstream of the intake port in this
engine. The mixture is compressed in the combustion chamber and ignited
by an electric spark. The evolution of the flame-front after ignition
is shown in Figure 5. Because of the homogeneous nature of the mixture,
FLUENT’s premixed combustion model can be used.This model determines
a turbulent flame speed based on the local turbulent kinetic energy and
dissipation rate. The turbulent flame speed is then used to determine
the location of the flame front on either side of the burned and unburned
mixtures. Rather than solve for multiple species, it is therefore only
necessary to solve for a single progress variable. Since port fuel injected
and carbureted gasoline engines also involve the spark ignition of a homogeneous
premixture, the same approach can be used in these engines as well. In
cases where the airfuel mixture is not perfectly premixed, FLUENT’s
partially premixed model may be used.
Temperature contours track ignition in the cylinder (top view)
Planning and development are currently
underway to extend the existing spray and
combustion capabilities. For example, diesel
auto-ignition models are a high-priority item
currently being implemented. These will extend
the scope of diesel combustion applications
that can currently be solved by FLUENT. A
wall-film model is also being developed. This
model is necessary for direct-injection and
port-fuel injected gasoline engines, and some
small-bore diesel applications. Other advanced
combustion capabilities are also in the planning
stage, including unsteady flamelet
approaches and multicomponent vaporization.
When combined with the flexibility of
the dynamic mesh model, these options will
allow for the most comprehensive suite of
internal combustion modeling tools available
in commercial software today.
References
1 Khalighi, Bahram, Haworth, Daniel, and Huebler, Mark, “Multidimensional
Port-and-in-Cylinder Flow Calculations and Flow Visualization Study in
an Internal Combustion Engine with Different Intake Configurations,”
SAE 941871, 1994.
2 Dec, John E., “A Conceptual Model of DI Diesel Combustion Based
on Laser-Sheet Imaging,”
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