Radical concentration profiles in a low-pressure methane-air flame measured by intracavity laser absorption and cavity ring-down spectroscopy

Abstract

Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) were used to measure temperature and concentration profiles in low-pressure (30 torr) methane/oxygen/nitrogen flames. Concentration profiles of the HCO and ¹CH₂ radicals were measured by ICLAS, whereas the OH radical profiles and temperature profiles were measured by CRDS. Flames with equivalence ratio =0.8, 1.0, and 1.2 were studied. HCO profiles and peak amounts agree well with model predictions based on the GRI-Mech 2.11 mechanism for the stoichiometric flame (=1.0). The absolute concentration of singlet CH₂ radical could not be determined accurately, because of large uncertainty in the absorption cross section of this radical. Nevertheless, the experimental singlet CH₂ concentration seems to be higher than predicted by model (by more than 20 times, based on the estimated cross-section data). The relative ¹CH₂ profile is very close to the prediction. The OH profiles agree well with model calculation: however, absolute experimental OH peak concentration is about two times lower than the predicted value.