[vinerm]

I would recommend not to look at the residuals for convergence. As long as those are smaller than 1, they are OK. Look at some of the monitors, such as the pressure monitors you have setup. Furthermore, if the monitors are setup for a vertex, these will oscillate as well. Standard practice is to monitor area-weighted or mass-weighted averages over boundaries. If these settle down to plausible, constant values, then simulation is numerically converged.

[the_sixth_hokage]

Originally Posted by Biagio

Hi, i-m simulating LOx-CH4 Combustion in supercritical condition. I'm using real gas Soave Redlich Kwong equation of state and Eddy Dissipation model dfor the combustion, and i'm struggling with this error: Temperature is below the spinodal point in xxx cells", has anyone ever experienced this? Any solution?

hey...I am also doing the same simulation (Test case G2) and hope you could shed some light on some problems I'm facing...

I am using

- a 4 step Jones-Lindstedt mechanism and Grimech 3.0 Thermo
and Transport files..
- EDC model with k-epsilon and srk equations..

I am also facing liquid solution only possible error and dasac failure at temperature= xxxx... errors

Hope you see this... Can you give some tips on solving this test case G2...

[vinerm]

DASAC is an ode solver that directly integrates the chemical source term over time. It can show failure in the beginning if the initialization is not very good, however, it should improve as the simulation proceeds. However, if Fluent keeps on showing the error, then it could be due to any reason ranging from bad initialization to material properties or reaction mechanism. Since specific heat is very important for calculation of enthalpy values, if it is given as a UDF or some function, do check that it is compatible with enthalpy calculation, otherwise, this may lead to failure of source term integration. You mentioned GRI Mechanism, however, if you are using GRI Mechanism for reactions kinetics and species as well, do check the units in the file.

Furthermore, you may try with Chemistry Agglomeration. This has potential to improve the situation.

[the_sixth_hokage]

Originally Posted by vinerm

DASAC is an ode solver that directly integrates the chemical source term over time. It can show failure in the beginning if the initialization is not very good, however, it should improve as the simulation proceeds. However, if Fluent keeps on showing the error, then it could be due to any reason ranging from bad initialization to material properties or reaction mechanism. Since specific heat is very important for calculation of enthalpy values, if it is given as a UDF or some function, do check that it is compatible with enthalpy calculation, otherwise, this may lead to failure of source term integration. You mentioned GRI Mechanism, however, if you are using GRI Mechanism for reactions kinetics and species as well, do check the units in the file.

Furthermore, you may try with Chemistry Agglomeration. This has potential to improve the situation.

what monitor should I use to monitor the convegence in combustion flows... I am not using any udfs for specific heat..im using the polynomial fit from thermo files..

[vinerm]

Ensure that the coefficients for the polynomial are correct.

As far as monitoring is concerned, you may monitor temperature or enthalpy and mass fractions of species.

[the_sixth_hokage]

Originally Posted by vinerm

Ensure that the coefficients for the polynomial are correct.

As far as monitoring is concerned, you may monitor temperature or enthalpy and mass fractions of species.

I have check the units of in chem.out file and was same as the given units in input file...

The chemistry agglomeration made the simulation faster...

The errors I'm currently getting are:

Mass_diffusivity: invalid (zero) diffusivity

Divergence detected in AMG solver: temperature
Divergence detected in AMG solver: epsilon
Divergence detected in AMG solver: species-0
Divergence detected in AMG solver: speices-1....etc

WARNING: Invalid cp (0.000000e+00 J/kgK ) for chemkin import at temperature 2370.470703 K


Will the grimech 3.0 Thermo polynomials be enough or should I create one for myself at 5.6Mpa pressure using data from NIST...

[vinerm]

You should check the accuracy of polynomial coefficients by plotting it over the whole temperature range. As you observed, Fluent is calculating 0 value for a certain temperature. This implies there is something wrong with the polynomials. Use any plotting tool and plot polynomial with the given coefficients over the temperature range, ranging from 1 K to at least 5000 K. Sometimes the polynomial is not valid outside a certain range; this would be given within the database.

[the_sixth_hokage]

Originally Posted by vinerm

You should check the accuracy of polynomial coefficients by plotting it over the whole temperature range. As you observed, Fluent is calculating 0 value for a certain temperature. This implies there is something wrong with the polynomials. Use any plotting tool and plot polynomial with the given coefficients over the temperature range, ranging from 1 K to at least 5000 K. Sometimes the polynomial is not valid outside a certain range; this would be given within the database.

I can get data from NIST database for my operating pressure (5.6 MPa) and using some curve fitting tool I can generate polynomials...but I don't know how to implement these into Fluent....I tried to change the default coefficient values and tried it...but I ran into some errors and crashed...not sure whether I did it correct...

[vinerm]

Before changing the coefficients, do check their validity by plotting. The plot doesn't have to be continuous but valid over the range being used. If the mixture or specie is using thermodynamic database for specific heat, then you have to go through the CHEMKIN manual for understanding the format of thermo.db to change it.