Cross sections for quenching of CH a ²Δ, v' = 0, by N₂ and H₂O from 1740 to 2160 K

Measurements of time-resolved CH fluorescence are reported for 77 methane-based counterflow diffusion flames at atmospheric pressure. Seven oxidizer mixtures, with variations in O₂/N₂/Ar mole fractions, are used with each of 11 CH₄/N₂/Ar fuel mixtures to vary the resulting nitrogen mole fraction without altering the flame temperature. A wide range of fluorescence lifetimes is observed at the locations of peak CH concentration. A procedure for extracting single-collider thermally averaged cross sections for nitrogen in these flames is developed despite the complicated collisional environments. Because of linear relationships between nitrogen and other collider mole fractions in the 7 oxidizer mixtures, the effects of quenching from species other than nitrogen can be subtracted without knowledge of their quenching rate coefficients. The 11 fuel mixtures are used to parametrically assess the N₂ cross-section dependence on temperature from 1740 to 2160 K. The results show that N₂ quenching of CH A ²Δ, v' =0, at flame temperatures is less efficient by a factor of 3 than the correlation of Tamura et al., which relies on extrapolation from a single measurement at 1300 K. Using the improved nitrogen cross-section correlation of σ_q N₂ = 1.53 × 10^-4 T¹²³ exp(-522.1/T), water is found to be an equally important quencher of CH in flames. The collision environment for the 77 diffusion flames is predicted using OPPDIF with GRI-Mech 3.0 kinetics, and the resulting data are used to suggest improvements to the H₂O cross section. The present data are consistent with a constant H₂O thermally averaged cross section versus temperature as opposed to a constant quenching rate coefficient.