Methane laminar flamespeed using openFoam

How to use Chemkin OpenFoam converter to simulate methane flames

Have you ever tried using the chemkin openFoam converter (chemkinToFoam)?

From searching online people have had problems with both the conversion steps and interpreting the simulation results. This post demonstrates how to use chemkinToFoam to convert the GRI-Mech 3.0 mechanism to openFoam format. A video showing the process is given below, followed by a description of some of the input format problems and assumptions.

Video showing how to use chemkin openFoam converter (chemkinToFoam)

Methane Flame Test Case

The test case used in the video is a one-dimensional methane flame. The equivalence ratio is 0.8 and the initial conditions are computed using Cantera. This solution is interpolated onto the openFoam mesh and stored in the \0 folder. A future post will explain this process in more detail for those who are interested.

Download the test case using this link:

Test Case: Methane Flame With 0.8 Equivalence Ratio

Copy the files and use the command “tar -xvzf Chemkin-OpenFoam-Methane-Flame-Simulation.tar.gz” to unzip the folder

This test case uses the Gri-Mech 3.0 kinetic mechanism. This detailed mechanism contains 325 reactions and 53 species. The chemistry file (grimech30.dat) and thermodynamics file (thermo30.dat) can be retrieved from the Gri-Mech 3.0 Website.

Input File Format Problems

In order to use the chemkin openFoam converter, two changes to the input format are needed:

  1. Replace THERMO with THERMO ALL in thermo30.dat
  2. Search and replace CH2(S) to CH2S in both files

I am not sure why chemkinToFoam is expecting THERMO ALL in the header, but this change is needed for the files to convert. The parser does not like brackets in the species names, so these must be changed as well. After fixing the format issues use the command “chemkinToFoam grimech30.dat thermo30.dat grimech30_foam.dat thermo30_foam.dat” to convert the files.

Input Format Problems Chemkin OpenFoam Converter chemkinToFoam
Input format problems using GRI-Mech 3.0 mechanism with chemkin openFoam converter (chemkinToFoam)

Other Problems and Assumptions

There are two other things to keep in mind when using files converted with chemkinToFoam:

  1. Not all chemkin files are in the correct format for the converter
  2. Viscosity, thermal conductivity, and diffusivity properties are not taken from the thermodynamics file

Some reaction mechanisms given in chemkin format do not actually contain the reaction rate constants (e.g., Lu and Law 19). These files require a second converter and cannot be used directly with the openFoam converter. Also, there are complaints online about converting files with older chemkin formating rules. I have not tested any of these to date.

The second problem involves the treatment of thermodynamic properties in openFoam. The following figure shows the converted openFoam thermo file. Note that the Sutherland parameters are the same for each specie. These parameters appear to be hard-coded in the converter and are not taken from the input thermodynamic data file.

The Sutherland parameters are used to calculate fluid viscosity in openFoam. This parameter is then used to calculate thermal conductivity and diffusivity. Both the constant values used for calculating viscosity, and the assumptions used to determine conductivity and diffusivity will impact the accuracy of the flame simulation.

Constant Sutherland parameters assumed using chemkin openFoam conveter
Constant Sutherland parameters assumed using chemkinToFoam converter

Typical Methane Burning Velocity Results with OpenFoam

Methane flame simulations were completed using the chemistry mechansims Gri-mech 3.0, Lu and Law 19, BFER2, MP1, and DRM19 found at the following links: GRI-Mech 3.0, Lu and Law Reduced Mechanisms, Cantera User Guide, and Kazakov and Frenklach. In all cases the therm30.dat thermodynamics file from the gri-mech 3.0 mechanism was used. It appears that this file is sufficient to use for most reaction mechanisms, although this has not been tested extensively.

Typical flame speed results are summarized in the following plots. The cantera simulation results are shown on the left and openFoam results with the same mechanisms on the right. Cantera uses an adaptive meshing procedure, while the openFoam mesh varied between 50 and 25 micron resolution.

Methane Laminar-Flamespeed Using-Chemkin OpenFoam-Converter chemkinToFoam
Unstreatched laminar burning velocity for methane using detailed and global mechanisms. The experimental data was extracted from the plots of Hu et al., 2015 [1].

The differences in flame speed between Cantera and OpenFoam appear to be due to the thermal conductivity and mass diffusivity assumptions. These will be reviewed further in another post, but the intersted reader can refere to the CFD-online post here: “Difficulties with premixed laminar methane flame in openFoam“.

If you have any questions or would like more information on the problem setup and results, leave a message below or reach out to me on the contact page. You can also email me direct at:

Email: chris@myDustExplosionResearch.com.

[1] E. Hu, X. Li, X. Meng, Y. Chen, Y. Cheng, Y. Xie, and Z. Huang, “Laminar flame speeds and ignition delay times of methane-air mixtures at elevated temperatures and pressures,” Fuel, vol. 158, p. 1—-10, 2015.
[Bibtex]
@ARTICLE{Hu2015,
title={Laminar flame speeds and ignition delay times of methane-air mixtures at elevated temperatures and pressures},
author={Hu, E. and Li, X. and Meng, X. and Chen, Y. and Cheng, Y. and Xie, Y. and Huang, Z.},
journal={Fuel},
volume={158},
pages={1—-10},
year={2015},
}

2 Comments

  • Sun JInguo

    April 20, 2017

    Hello Chris.
    I’m a student in Tsinghua University, China! I am using OpenFOAM4.0 and found the help docs says: chemkinToFoam can convert transporfFile to Foam-format. So I hope you can help me about how to do it? (I am sure my command was right) could you show me a case of OpenFoam4.0
    I would really appreciate this!

    Reply
    • Chris Cloney

      April 20, 2017

      Hello Sun, That would be no problem!

      Can you email me the transport file you are trying to convert and maybe a screen capture of the error message you are getting? My email is chris@myDustExplosionResearch.com. I should be able to convert it and show you how!

      Reply

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