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

Cavity Ring-Down Spectroscopy and Intracavity Laser Absorption Spectroscopy were used to measure the concentration profiles of NH and NH₂ at 30 torr methane/oxygen/nitrogen flames doped with small amount of N₂O. These absorption spectroscopy methods displayed high sensitivity (4 x 10¹⁰ cc for NH and 8 x 10¹⁰ cc for NH₂). The absolute NH concentrations were in agreement with those predicted by GRI-Mech 2.11 mechanism and one-dimensional PREMIX code. The absolute NH₂ concentrations in the lean flame also agreed well with calculations, but the observed concentrations in the stoichiometric and rich flames were about twice the calculations. In the rich flame, the calculations underpredicted the NH concentrations at large distances from the burner. The strong spectra of NH₂ were observed even at 40 nm above the burner where the model calculations predicted very insignificant NH₂ concentration. The discrepancy was attributed to limitations of both the chemical mechanism and one-dimensional PREMIX code used for calculations.