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 C_p (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.