Intracavity laser absorption spectroscopy of NH₂ in methane/air flames doped with N₂O, NO, and NH₃

Intracavity laser absorption spectroscopy (ICLAS) was used to measure concentration profiles of NH₂ in low pressure (30 Torr) methane/oxygen/nitrogen flames doped with small amounts of N₂O, NO, and NH₃. The effective optical length of ICLAS, which dictates the sensitivity of the method, is controlled by the generation time of the quasi-cw-laser. The effective optical length of 0.87 km, reached at generation time of 75 μs, provides very high sensitivity: from 5×10¹⁰ molecules/cm³ at 500 K in the vicinity of burner up to 2×10¹¹ molecules/cm³ in the burned gas zone (T∼ 1800 K). The radial profile of NH₂, measured using a tomographic technique, indicates that the radical is located mainly inside a cylinder with diameter equal to the burner diameter. For the first time, the absolute mole fraction profiles of NH₂ were measured in hydrocarbon flames with different dopants and compared with one-dimensional calculations based on the GRI-3.0 mechanism. The dependence of the NH₂ concentrations on the equivalence ratio is in excellent agreement with the model prediction. The absolute NH₂ concentration values for different dopants are predicted very well by the calculations apart from the NO-doped flame where GRI-Mech 3.0 overpredicts the NH₂ concentration by a factor of 2. In the ammonia-doped flame, the calculations predict an additional concentration maximum located close to the burner. This maximum is not observable in the experiment. All the experimental profiles exhibit 1-2 mm shift further from the burner surface in comparison with the predicted ones. The reasons of those discrepancies are discussed.