Absorption spectroscopy diagnostics of amidogen in ammonia-doped methane/air flames

The NH₂ absolute concentration profiles were measured by intracavity laser absorption spectroscopy in methane/air laminar flat flames doped with trace amounts of ammonia (∼0.1% of the total gas flow) at a pressure of 30 Torr. The radial distribution of NH₂ in the flame was obtained using tomographic reconstruction. The obtained results are compared with one-dimensional model predictions. The temperature profiles, experimentally measured by OH and NO laser-induced fluorescence, were used as an input for the model. Several mechanisms were used. GRI-Mech 3.0 predicts the appearance of an additional maximum for the NH₂ profile in the vicinity of the burner surface, which is not observed in the experiment. The effect of different reactions of the NH₂ radical on its concentration profile at close distances from the burner is discussed. All the mechanisms successfully predict the peak concentration of amidogen in ammonia-doped flames, which is almost unaffected by the equivalence ratio in contrast to flames doped with nitrogen oxides. The maxima of the experimental profiles are located ∼1 mm farther from the burner surface than the simulated ones, which is probably caused by applying a 1-D model for describing the real 3-D flame.