Absoluteconcentration profiles of NH2 and HNO have been measured in low-pressure methane/airflat flames doped with small amounts of NO and N2O. Addition of a small amountof nitrogen oxides does not alter significantly the flame speeds, temperature profilesand other parameters of the relatively well-understood methane/air flames. Intracavitylaser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS)are high-sensitivity techniques used to measure absolute concentrations of minor speciesin flames. In this work ICLAS is used to monitor NH2 and HNO, whereas CRDS is usedfor temperature measurements using OH spectra in the UV range. The (090)–(000) and(080)–(000) bands of the (Equation presented) electronic transition of NH2and (100)–(000) and(011)–(000) bands of the (Equation presented) transition of HNO are used. Methane flames of different equivalenceratios are used. NH2 and HNO are observed in the flame as well as in the zonesurrounding the flame, closer to the walls of the low-pressure chamber where the burner islocated. An absorption originating from the species in this zone can affect substantially theresults of line-of-sight experiments. A slow flow of nitrogen through the optical windowholders was added in order to separate the spectra of HNO originating fromthe central flame zone. Calculations based on the commonly used GRI-Mech chemicalmechanism predict two maxima in the HNO concentration profile in the NO doped flames. The firstis located in the vicinity of the burner, and the second is closer to the luminescenceflame zone. We were able to observe the first maximum, and its measured locationagrees well with prediction. On the other hand, GRI-Mech strongly underpredictsthe observed absolute concentration of HNO in this maximum. The measuredabsolute concentrations ofNH2 arein reasonable agreement with the GRI-Mech predictions.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry