Wavelength-modulation spectroscopy with a standard commercial 1.55-µm distributed-feedback diode laser was applied to in situ quantitative measurements of OH radical concentration in combustion environments. The second-harmonic (2f) signal was generated from absorption by the P11.5 (ν′, ν") = (2, 0) overtone vibrational transition of OH at 6421.354 cm-1. The absorption occurred in the postflame region of a two-dimensional laminar counterflow burner (Tsuji burner) with a 60-mm line-of-sight path length. The postflame region lies between propane–air premixed twin flames stabilized in the Tsuji burner at various equivalence ratios (ϕ = 0.65–1.0). The OH concentrations were determined by least-squares fitting of theoretical 2f line shapes to the experimental counterparts. The measured OH concentrations were in general agreement with adiabatic chemical equilibrium predictions. The lower limit of OH detectivity by multiline deconvolution was limited by ubiquitous unidentified high-temperature H2O transitions.
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