Absolute, spatially resolved, in situ CO profiles in atmospheric laminar counter-flow diffusion flames using 2.3 μm TDLAS

Abstract

We developed a new, spatially traversing, direct tunable diode laser absorption spectrometer (TDLAS) for quantitative, calibration-free, and spatially resolved in situ measurements of CO profiles in atmospheric, laminar, non-premixed CH4/air model flames stabilized at a Tsuji counter-flow burner. The spectrometer employed a carefully characterized, room temperature distributed feedback diode laser to detect the R20 line of CO near 2,313 nm (4,324.4 cm−1), which allows to minimize spectral CH4 interference and detect CO even in very fuel-rich zones of the flame. The burner head was traversed through the 0.5 mm diameter laser beam in order to derive spatially resolved CO profiles in the only 60-mm wide CH4/air flame. Our multiple Voigt line Levenberg–Marquardt fitting algorithm and the use of highly efficient optical disturbance correction algorithms for treating transmission and background emission fluctuations as well as careful fringe interference suppression permitted to achieve a fractional optical resolution of up to 2.4 × 10−4 OD (1σ) in the flame (T up to 1,965 K). Highly accurate, spatially resolved, absolute gas temperature profiles, needed to compute mole fraction and correct for spectroscopic temperature dependencies, were determined with a spatial resolution of 65 μm using ro-vibrational N2-CARS (Coherent anti-Stokes Raman spectroscopy). With this setup we achieved temperature-dependent CO detection limits at the R20 line of 250–2,000 ppmv at peak CO concentrations of up to 4 vol.%. This permitted local CO detection with signal to noise ratios of more than 77. The CO TDLAS spectrometer was then used to determine absolute, spatially resolved in situ CO concentrations in the Tsuji flame, investigate the strain dependence of the CO Profiles and favorably compare the results to a new flame-chemistry model.