For a gas species to absorb or emit infrared energy, its molecular structure must allow rotational and vibrational transitions and change in dipole moment. Accordingly, all monatomic species, such as O and H, and homonuclear diatomic species, such as O2, H2, and N2, are "transparent" with respect to thermal radiation transfer. Most species that are actively absorbing and emitting infrared energy in flames can be neglected in the radiation calculation because they either have very low concentrations (for example, NO and NO2) or exist in cooler regions of the flow domain (for example, hydrocarbon fuel species and CH3 OH). The most important radiating species in hydrocarbon flames are CO2 and H2 O. The next important gas species are CO and CH4, which emit about 1/10 as much infrared energy as the two dominant species.
The Planck mean absorption coefficients a for CO2, H2 O, CO, and CH4 can be calculated by a narrow-band absorption coefficient model such as RADCAL [3]. Since narrow-band calculations require integrating absorption-line intensities over all wavelengths, direct integration of a narrow-band model into the radiation model would greatly decrease the computational performance. To incorporate good absorption coefficient data without sacrificing performance, polynomials are fitted to temperature to represent the absorption coefficient data for individual gas species [4].
Two types of temperature polynomials are accepted by the radiation model for gas species:
The curve-fitting parameters c j are entered as optional-data lines in the thermodynamic data file or in the thermodynamic data section of the mechanism input file. The absorption coefficient data line is formatted as
![_ AbsorptionCoefficient=" polynomial_form Tmin Tmax c0 c1 c2 c3 c4 c5 c6 " _]
AbsorptionCoefficient is the tag indicating the data are for species
absorption coefficient calculation. polynomial_form indicates the form
of polynomial used to fit the absorption coefficient. This must have a value of 1.0 or 2.0,
corresponding to Equation 2–1 or Equation 2–2, respectively.
Tmin is the lower temperature bound in [K] for the fitted curve to be
valid. Tmax is the upper temperature bound in [K] for the fitted curve
to be valid. cj are the seven parameters of the polynomial.
This data line must appear before the thermodynamic data of the associated gas species. Different gas species can use a different form of polynomial. Each gas species can have as many as two absorption-coefficient data lines, which allows two sets of parameters of different temperature ranges. If there are two tag lines for a given gas species, they must be of the same type of polynomial and the low-temperature line should appear before the high-temperature line. The gas radiation model will not be activated if no absorption-coefficient data are included in the thermodynamic data file.
For example, the absorption coefficient data of CO are fitted to two type-1 polynomials over temperature ranges of 300–750 K and 750–2500 K, with the coefficients included in the optional-data lines as follows:
! [_ AbsorptionCoefficients=" 1 300 750 4.8E0 -6.95E-2 2.96E-4 -4.26E-7 2.03E-10 0 0 " _] ! [_ AbsorptionCoefficients=" 1 750 2500 10. -1.2E-2 4.78E-6 -5.87E-10 -2.53E-14 0 0 " _] CO TPIS79C 1O 1 00 00G 200.000 3500.000 1000.000 1 2.71518561E+00 2.06252743E-03-9.98825771E-07 2.30053008E-10-2.03647716E-14 2 -1.41518724E+04 7.81868772E+00 3.57953347E+00-6.10353680E-04 1.01681433E-06 3 9.07005884E-10-9.04424499E-13-1.43440860E+04 3.50840928E+00 8.67100000E+03 4