15.2.8 Importing a Surface Kinetic Mechanism in CHEMKIN Format

Importing surface kinetic mechanisms in CHEMKIN format (Section  15.1.9) requires that the gas-phase mechanism file accompany the surface mechanism file for full compatibility with CHEMKIN. If the gas-phase mechanism file is not available, then you will need to create one that you will import along with the surface mechanism file. The mechanism files are imported into ANSYS FLUENT using the CHEMKIN Mechanism Import dialog box (Figure  15.2.1).

File $\rightarrow$ Import $\rightarrow$ CHEMKIN Mechanism...

Figure 15.2.1: The CHEMKIN Mechanism Import Dialog Box for Surface Kinetics

In the CHEMKIN Mechanism Import dialog box

1.   Enter a name for the chemical mechanism under Material Name.

2.   Enable Import Surface CHEMKIN Mechanism.

3.   Enter the path to the Gas-Phase CHEMKIN Mechanism File (e.g., path/ gas-file.che) and the Surface CHEMKIN Mechanism File (e.g., path/ surface-file.che).

4.   Specify the location of the Gas-Phase and Surface Thermodynamic Database File. The thermodynamic database format is detailed in the CHEMKIN User's Guide [ 39]. The default thermo.db file supplied with ANSYS FLUENT has only gas-phase species available. You will need to supply a surface thermo.db file for your surface species if this thermo information is not in the mechanism file.

Note that ANSYS FLUENT will initially search for the thermodynamic data in the Surface CHEMKIN Mechanism File. If the data does not exist in the mechanism file, then ANSYS FLUENT will search for the thermodynamic data in the specified Surface Thermodynamic Database File.

5.   (Optional). Import transport properties by enabling the Import Transport Property Database and entering the path to that file.

To read in KINetics transport properties, go to Section  15.1.9 for detailed information.

6.   Click the Import button.

ANSYS FLUENT will create a material with the specified name, which will contain the CHEMKIN data for the species and reactions, and add it to the list of Fluent Mixture Materials. You can view all of the reactions by clicking the Edit... button to the right of Mechanism, under Properties in the Create/Edit Materials dialog box.

Note that for surface reaction mechanisms, the surface reaction rate constant can be expressed in terms of a sticking coefficient. ANSYS FLUENT will convert this sticking coefficient form to the Arrhenius rate expression [ 38].

The CHEMKIN import facility does not provide full compatibility with all CHEMKIN rate formulations. To access more complete functionality, you should consider the KINetics module option described in Section  15.1.2.

Compatibility and Limitations for Gas Phase Reactions

ANSYS FLUENT will allow for the following reaction types:

• Arrhenius reactions with arbitrary reaction order, third-body efficiencies and non-integer stoichiometric coefficients.

• Pressure-dependent reactions (Lindemann, Troe and SRI forms)

• Arbitrary reaction units

• Duplicate reactions (keyword DUP)

ANSYS FLUENT will not allow for the following reaction types:

• Landau-Teller reactions (keyword LT)

• Reverse Landau-Teller reactions (keyword RLT)

• Janev reactions (keyword JAN)

• Exponential modified power series reactions(keyword FIT1)

• Radiation reactions (keyword HV)

• Energy loss reactions (keyword EXCI)

• Multi-fluid temperature dependence reactions (keyword TDEP)

• Electron momentum transfer collision frequency (keyword MOME)

• Arbitrary reverse reaction (keyword REV)

Note that the reaction types that ANSYS FLUENT will not allow are mostly applicable to plasmas.

Compatibility and Limitations for Surface Reactions

ANSYS FLUENT will allow for the following reaction types:

• Arrhenius reactions with arbitrary reaction order, third-body efficiencies and non-integer stoichiometric coefficients.

• Sticking coefficients (keyword STICK). Fluent converts these to an equivalent Arrhenius expression.

• Arbitrary reaction units

• Duplicate reactions (keyword DUP)

• Surface coverage modification (keyword COV)

ANSYS FLUENT will not allow for the following reaction types:

• Ion-Energy Dependent reaction (keyword ENRGDEP)

• Bohm rate expressions (keyword BOHM)

• Ion-Enhanced reaction

• Motz-Wise correction (keywords MWON and MWOFF)

ANSYS FLUENT will warn you of any incompatibilities.

For a detailed description of the keywords, see [ 38].