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

Abstract: Intracavity laser absorption spectroscopy (ICLAS) was used to measure concentration profiles of NH₂ in low pressure (30Torr) 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 0.87km, reached at generation time of 75μs, provides very high sensitivity: from 5×10¹⁰ molecules/cm³ at 500K in the vicinity of burner up to 2×10¹¹ molecules/cm³ in the burned gas zone (T ∼1800K). 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 1–2mm shift further from the burner surface in comparison with the predicted ones. The reasons of those discrepancies are discussed. [Copyright &y& Elsevier]