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Speedup combustion reduced mechanism by TDAC to calc NOx

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Old   September 16, 2021, 10:06
Default Speedup combustion reduced mechanism by TDAC to calc NOx
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Hallo,

I am trying to find the setups for a complex combustion model which can be used to determine also the Nox emissions in complex geometries.

For this I use for example the GRI3.0 model with the additional NOx equations, or the scalatal model with 65 equations by Sun et. Al. (Reduced Methane Combustion Mechanism and Verificationů)

The problem is of course that the GRI model is quite slow, the 65eq model is ca. 20 times faster simulating the Sandia cases.

I am trying to speedup the simulations using reduced equation mechanisms. By using the TDAC implemented already by OpenFOAM I saw an additional speedup of ca. 5 times with the Sun model. The problem is the simulation using TDAC is more unstable, the flames are oscillating in some cases where the standard solving has no problem.

But the temperature profiles and the main combustion compounds are computed well. But I want to calculate especially the Nox emissions. Here I have very high uncertainties, the results are fluctuating very high.
My main setup conditions are: OpenFOAM 5, reactingFoam, kEpsilon turbulence model, PaSR combustion model

Has anybody an idea? I have seen that some changes in newer OF versions have been done. I tried also with OpenFOAM 8, but the results were similar.

Here is the chemistryProperties file. I changed only minor parts in the file. (important species NO,CO).

Thank you.

chemistryType
{
chemistrySolver ode;//EulerImplicit;//ode;
chemistryThermo psi;
TDAC on;
}

EulerImplicitCoeffs
{
cTauChem 1;
equilibriumRateLimiter off;
}

chemistry on;

importantSpecies
{
CH4;
H2O;
O2;
CO2;
H2;
CO;
NO;
}

initialChemicalTimeStep 1e-04;

odeCoeffs
{
solver seulex; // Rosenbrock34, seulex or rodas23
absTol 1e-12;
relTol 1e-2;
}

reduction
{
// Activate reduction
active on;

// Switch logging of the reduction statistics and performance
log on;

// Tolerance depends on the reduction method, see details for each method
tolerance 1e-4;

// Available methods: DRG, DAC, DRGEP, PFA, EFA
method DRG;//DAC;

// Search initiating set (SIS) of species, needed for most methods
initialSet
{
CO;
CH4;
HO2;
H2;
NO;
}

// For DAC, option to automatically change the SIS switch from HO2 to H2O
// and CO to CO2, + disable fuel
automaticSIS off;

// When automaticSIS, the method needs to know the fuel
fuelSpecies
{
CH4 1;
}
}

tabulation
{
// Activate tabulation
active on;

// Switch logging of the tabulation statistics and performance
log on;

printProportion off;

printNumRetrieve off;

// Tolerance used for retrieve and grow
tolerance 0.003;

// ISAT is the only method currently available
method ISAT;

// Scale factors used in the definition of the ellipsoid of accuracy
scaleFactor
{
otherSpecies 1;
Temperature 10000;
Pressure 1e15;
deltaT 1;
}

// Maximum number of leafs stored in the binary tree
maxNLeafs 5000;

// Maximum life time of the leafs (in time steps) used in unsteady
// simulations to force renewal of the stored chemPoints and keep the tree
// small
chPMaxLifeTime 1000;

// Maximum number of growth allowed on a chemPoint to avoid distorted
// chemPoints
maxGrowth 100;

// Number of time steps between analysis of the tree to remove old
// chemPoints or try to balance it
checkEntireTreeInterval 500;

// Parameters used to decide whether to balance or not if the tree's depth
// is larger than maxDepthFactor*log2(nLeafs) then balance the tree
maxDepthFactor 2;

// Try to balance the tree only if the size of the tree is greater
minBalanceThreshold 30;

// Activate the use of a MRU (most recently used) list
MRURetrieve false;

// Maximum size of the MRU list
maxMRUSize 0;

// Allow to grow points
growPoints true;

// When mechanism reduction is used, new dimensions might be added
// maxNumNewDim set the maximum number of new dimensions added during a
// growth
maxNumNewDim 10;
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