Laser absorption spectroscopy diagnostics of nitrogen-containing radicals in low-pressure hydrocarbon flames doped with nitrogen oxides

Abstract

Absolute concentration profiles of NH₂ and HNO have been measured in low-pressure methane/air flat flames doped with small amounts of NO and N₂O. Addition of a small amount of nitrogen oxides does not alter significantly the flame speeds, temperature profiles and other parameters of the relatively well-understood methane/air flames. Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) are high-sensitivity techniques used to measure absolute concentrations of minor species in flames. In this work ICLAS is used to monitor NH₂ and HNO, whereas CRDS is used for temperature measurements using OH spectra in the UV range. The (090)–(000) and (080)–(000) bands of the òA₁–²B₁ electronic transition of NH₂ and (100)–(000) and (011)–(000) bands of the ùA″–¹A′ transition of HNO are used. Methane flames of different equivalence ratios are used. NH₂ and HNO are observed in the flame as well as in the zone surrounding the flame, closer to the walls of the low-pressure chamber where the burner is located. An absorption originating from the species in this zone can affect substantially the results of line-of-sight experiments. A slow flow of nitrogen through the optical window holders was added in order to separate the spectra of HNO originating from the central flame zone. Calculations based on the commonly used GRI-Mech chemical mechanism predict two maxima in the HNO concentration profile in the NO doped flames. The first is located in the vicinity of the burner, and the second is closer to the luminescence flame zone. We were able to observe the first maximum, and its measured location agrees well with prediction. On the other hand, GRI-Mech strongly underpredicts the observed absolute concentration of HNO in this maximum. The measured absolute concentrations of NH₂ are in reasonable agreement with the GRI-Mech predictions.