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.