High-Temperature Anharmonic Effect on Thermodynamic Properties of Methane Combustion-Related Species

Anharmonic
effects can significantly influence thermodynamic properties
at high temperatures and, consequently, the predictive performance
of combustion mechanisms. While anharmonicity associated with low-frequency
internal rotations is often considered, contributions from other vibrational
modes are frequently neglected. This study fully investigates the
impact of anharmonic corrections on the thermodynamic properties of
24 species within a methane combustion mechanism. Torsional anharmonicity
is treated using both hindered- and free-rotor models, and their applicability
is discussed in the context of high-temperature combustion conditions.
The anharmonic effects from other vibrational modes were estimated
by three schemes: the uncoupled rigid-rotor anharmonic oscillator
(uncoupled RRAO), the coupled rigid-rotor anharmonic oscillator (coupled
RRAO), and the nonrigid-rotor anharmonic oscillator (NRRAO). The results
for H2O reveal that the NRRAO reduces the relative deviation
of Cp (2000 K) from 3.41% in the RRHO
to 0.56%. For CH4, this reduction is from 7.67 to 2.90%.
By combining corrections from both torsional and nontorsional anharmonic
effects, thermodynamic properties of methane combustion-related species
were calculated, and ignition delay time simulations and theoretical
adiabatic flame temperature calculations were conducted by incorporating
the revised thermodynamic parameters into the methane combustion mechanism.
The simulated results indicate that including the anharmonic effect
in thermodynamic calculations significantly improves the performance
of reaction mechanisms.