2.4. Input in 9-Coefficient Format (NASA9)

The format of the thermodynamic data fitted using a 9-coefficient polynomial is given in Appendix C of https://ntrs.nasa.gov/citations/20020085330 [2]. Specifically, the non-dimensional specific heat at constant pressure is given as

(2–5)

The corresponding enthalpy and entropy are obtained by the corresponding integrations and therefore produce two additional integration constants. The data obtained from the reference above are provided with Chemkin in a file named therm_NASA.dat and this file is located in the \installDir\reaction\data, where installDir is the installation directory for Ansys software. The following set of rules and restrictions apply:

  1. The data must be contained within a block that starts with THERMO NASA9 and ends with END.

  2. Comments may be added before or after the data block or between the data for species. However, there can be no comments within the data (within the fit coefficients) characters. The comment lines must begin with character "!".

  3. Although arbitrary temperature exponents for the fitting polynomial are allowed by the NASA9 format, this is not true for Ansys Chemkin in this release. Specifically, the temperature exponents must be {−2.0, −1.0, 0.0, 1.0, 2.0, 3.0, 4.0}. The data at the reference website include two species, P4010(cr) and P4010(L), that do not have the standard fit. These species are not included in the file therm_NASA9.dat that is distributed with the installation.

  4. It is possible to use data in both 7-coefficent polynomial format and this 9-polynomial form.

    1. In order to do so, both input data-blocks should be in the same file. One data block would be contained between THERMO NASA9 ..... END and the other would be within THERMO ..... END.

    2. The two data-blocks can be in any order.

    3. There can be comments in-between the two blocks.

    4. The data in 9-coefficient form take precedence. That is, when the data are included in both formats for a given species, the version in 9-coefficient form will get preprocessed and utilized for that species.

  5. Some additional data input such as SMILES-strings or absorption coefficients may also be specified (with appropriately tagged comment lines) within the NASA9 data. Since it is also possible to include blocks of data in both formats, you may copy-paste the existing (7-coefficient) data that includes such additional data and append (or prepend) NASA9 data in the same file. Ansys Chemkin will then use the 9-coefficient form to compute the thermodynamic quantities while the other additional data will come from the 7-coefficient block. This capability may be useful, as it avoids copying a lot of lines and inserting them in the proper location in the NASA9 input data.

  6. The number of elements in a species is restricted to 5. Since data in both formats can be given in the input file, it is possible to specify very large molecules with an unlimited number of elements in 7-coefficient form, as mentioned in Alphabetical Listing of Keywords [T-Z], along with NASA9 data for other species.

  7. Note for some common gas-phase species names/symbols in combustion related mechanisms: A pair of classic examples of this are: (a) Argon is declared as Ar (and not AR); (b) for C2H2, there are two different species, namely, acetylene and vinylidene. Therefore, you may have to make appropriate changes to the included therm_NASA9.dat file if you are using it with existing mechanisms. This situation underscores the need for an informed usage of the thermodynamic data. Note that, in general, a reaction mechanism and thermodynamic data should be considered as one consistent unit. Therefore, even though changing the data to NASA9 format should not change or invalidate the results obtained using an existing reaction mechanism, you should take care to verify the outcome.

  8. The NASA9 data can also be included in the reaction-mechanism input file. When doing so, follow these guidelines:

    1. It must be contained within the THERMO NASA9 and END.

    2. This block must be located before the REACTIONS block.

    3. When given in the reaction mechanism file, it is treated as if all the data for the gas-phase species are included in that block. That is, there would neither be a separate data-input file for the gas-phase thermodynamic data nor would there be the 7-coefficient format input (between THERMO ... END) block.