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 Ã2A1–2B1
electronic transition of NH2
and (100)–(000) and
(011)–(000) bands of the Ã1A″–
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.