In-Situ Quantification of NH₂ Radicals in Low-Pressure NH3/O2/N2 Flames Implementing Cavity Ring-Down Spectroscopy: Challenges & Mitigation

Quantitative measurements of NH 2 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 NH 2 radicals in three premixed NH 3 /O 2 /N 2 flat flames (ϕ = 0.87–1.30; P=75 Torr) using cavity ring-down spectroscopy (CRDS). The widely used A ˜ 2 A 1 (0,9,0) ← X ˜ 2 B 1 (0,0,0) P Q 1,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 A ˜ 2 A 1 (0,9,0) ← X ˜ 2 B 1 (0,0,0) band, P P₄,₅(8) at 16764.97 cm⁻¹, combined with localized N 2 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 NH 2 mole fractions reached approximately 5500, 4200, and 7000 ppm for ϕ = 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 NH 2 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.