Absorption spectroscopy measurements of NH and NH₂ absolute concentrations in methane/air flames doped with N₂O

Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) were used to measure concentration profiles of NH₂ and NH in low-pressure (30 torr) methane/oxygen/nitrogen flames doped with a small amount of N₂O. High sensitivity of these absorption spectroscopy methods was demonstrated (4 × 10¹⁰ cm^-3 for NH and 8 × 10¹⁰ cm^-3 for NH₂). The absolute NH concentrations agree well with those predicted by the GRI-Mech 2.11 mechanism and one-dimensional PREMIX code. The absolute NH₂ concentrations in the lean flame are fitted well by the calculations, but in the stoichiometric and the rich flames, the observed concentrations are about twice that predicted. In the rich flame (ψ = 1.2), the calculations underpredict the NH concentrations at large distances from the burner. The strongest difference was found for the NH₂ radical at large distances above the burner. The strong spectra of NH₂ were observed even at 40 mm above the burner where the model calculations predict very insignificant concentration of NH₂. The observed discrepancy can be caused by limitations of both the chemical mechanism and one-dimensional PREMIX code used for calculations.