combustion modeling strategies fluent
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© Fluent Inc. 6/23/20051 / 7
Fluent User Services Center
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Fluent Software TrainingCombustion Apr 2005
Combustion Modeling Strategiesin
FLUENT
© Fluent Inc. 6/23/20052 / 7
Fluent User Services Center
www.fluentusers.com
Fluent Software TrainingCombustion Apr 2005
Combustion GuidelinesStart in 2D
Determine applicability of model physicsMesh resolution requirements (resolve shear layers)Solution parameters and convergence settings
Boundary conditionsCombustion is often very sensitive to inlet boundary conditions
Correct velocity and scalar profiles can be criticalWall heat transfer is challenging to predict; if known, specify wall temperature instead of external convection/radiation BC
Initial conditionsWhile steady-state solution is independent of the IC, poor IC may cause divergence due to the number and nonlinearity of the transport equations Cold flow solution, then gas combustion, then particles, then radiationFor strongly swirling flows, increase the swirl gradually
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Fluent Software TrainingCombustion Apr 2005
Combustion GuidelinesUnderrelaxation factors
The effect of underrelaxation is highly non-linearDecrease the diverging residual URF in increments of 0.1Underrelax density when using the mixture-fraction PDF model (0.5)Underrelax velocity for high buoyancy flowsUnderrelax pressure for high speed flows
Once solution is stable, attempt to increase all URF’s to as close to defaults as possible (and at least 0.9 for T, P-1, swirl and species)
DiscretizationStart with first order, then converge with second order to improve accuracySecond order discretization especially important for tri/tet meshes
Discrete Phase Model - to increase stabilityIncrease number of stochastic tracks (or use particle cloud model)Decrease DPM URF and increase number of gas phase iterations per DPM
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Fluent Software TrainingCombustion Apr 2005
Combustion GuidelinesSpray Model
If particles are generated from atomizer use one of appropriate atomizer model. This will greatly speed up the problem setup process.
Magnussen modelDefaults to finite-rate/eddy-dissipation (Arrhenius/Magnussen)
For nonpremixed (diffusion) flames turn off finite-ratePremixed flames require Arrhenius term so that reactants don’t burn prematurelyMay require a high temperature initialization/patch
Use temperature dependent cp’s to reduce unrealistically high temperatures
Mixture fraction PDF modelModel of choice if underlying assumptions are validUse adequate numbers of discrete points in look-up tables to ensure accurate interpolation (no effect on run-time expense)Use beta PDF shape
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Fluent Software TrainingCombustion Apr 2005
Combustion GuidelinesDetailed Chemistry Models (Laminar, EDC, PDF-transport)
should be used when you are interested in simulating finite-rate chemical kinetic effects in turbulent reacting flowsWith an appropriate chemical mechanism, kinetically-controlled species such as CO and NOx, as well as flame extinction and ignition, can be predicted.start your modeling with small grids, and preferably in 2D.
Use ISAT to speed up the calculation
RadiationEnable WSGGM (domain based) for hydrocarbon combustionIf optical thickness aL > 1, consider P-1 for most cost-effective radiation modeling
Plot contours of absorption coefficient a to determine aL
© Fluent Inc. 6/23/20056 / 7
Fluent User Services Center
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Fluent Software TrainingCombustion Apr 2005
Combustion GuidelinesTurbulence
Start with standard k-εSwitch to RNG k-ε , Realizable k-ε or RSM to obtain better agreement with data and/or to analyze sensitivity to the turbulence model
Judging ConvergenceResiduals should be less than 10-3 except for T and P-1 and species, which should be less than 10-6
The mass and energy flux reports must balanceMonitor variables of interest (e.g. mean temperature at the outlet)Ensure contour plots of field variables are smooth, realistic and steady
© Fluent Inc. 6/23/20057 / 7
Fluent User Services Center
www.fluentusers.com
Fluent Software TrainingCombustion Apr 2005
Summary
Combustion modeling is considered a priority at Fluent, Inc.We have tremendous in-house expertise in combustion modeling.
Industrial experienceWorld-class developers and consultants
The range of physical models for combustion applications continues to grow.We are dedicated to providing better service to our customers; we appreciate and encourage your feedback!