In-situ quantification of NH₂ radicals in low-pressure NH3/O2/N2 flames implementing cavity ring-down spectroscopy: Challenges & mitigation
Year: 2026
Type: article
Abstract: Quantitative measurements of NH2 radicals are essential for validating ammonia combustion kinetics but remain experimentally challenging in low-pressure, confined environments. This current study presents the first spatially resolved, absorption-based quantification of ground-state NH2 radicals in three premixed NH3/O2/N2 flat flames (ϕ = 0.87–1.30; P=75 Torr) using cavity ring-down spectroscopy (CRDS). The widely used A2A1 (0,9,0) ← X2B1 (0,0,0) PQ1,N(7) transition at 16739.9 cm⁻¹ was found unreliable under these conditions due to strong H₂O broadband absorption and multi-exponential time-decay of CRDS signal from excessive optical depth. A weaker transition within the A2A1(0,9,0) ← X2B1(0,0,0) band, PP₄,₅(8) at 16764.97 cm⁻¹, combined with localized N2 purging, ensured optically thin absorption and clean mono-exponential time-decays. Absolute mole fractions were obtained through oscillator-strength anchoring at a validated reference location, simulation-derived effective path lengths, and Boltzmann correction from independently measured temperatures. Peak NH2 mole fractions reached approximately 5500, 4200, and 7000 ppm for f = 0.87, 1.10, and 1.33 respectively, with peak locations shifting downstream and profiles broadening as the mixture became richer. Simulations reproduce peak positions well but systematically underpredict mole fraction magnitudes, particularly in the rich flame. The reported NH2 profiles provide new benchmark data for ammonia oxidation and NOₓ formation mechanisms, and the methodology offers a transferable diagnostic framework for radical quantification in systems with presence of broadband absorbers.
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,910
Funder: Agence Nationale de la Recherche
OpenAlex ID: W7172423490