TDLAS-based in situ measurement of absolute acetylene concentrations in laminar 2D diffusion flames

We report the first quantitative and calibration-free in situ C2H2 measurement in a flame environment using direct Tunable Diode Laser Absorption Spectroscopy(TDLAS). Utilizing a fiber-coupled Distributed Feedback diode laser near 1535 nm we measured spatially resolved, absolute C2H2 concentration profiles in a laminar non-premixed CH4/air flame supported on a modified Wolfhard-Parker slot burner with N2 purge slots to minimize end flames. We developed a wavelength tuning scheme combining laser temperature and current modulation to record with a single diode laser a mesh of 37 overlapping spectral windows and generate an ∼7 nm (30 cm−1) wide, high-resolution absorption spectrum centered at 1538 nm. Experimental C2H2 spectra in a reference cell showed excellent agreement with simulations using HITRAN2004 data. The enhanced wavelength coverage was needed to establish correct C2H2 line identification and selection in the very congested high temperature flame spectra and led to the P17e line as the only candidate for in situ detection of C2H2 in the flame. We used highly efficient optical disturbance correction algorithms for treating transmission and background emission fluctuations in combination with a multiple Voigt line Levenberg-Marquardt fitting algorithm and Pt/Rh thermocouple measurements to achieve fractional optical resolutions of up to 4 × 10−5 OD (1σ) in the flame (T up to 2000 K). Temperature dependent C2H2 detection limits for the P17e line were 60 to 480 ppm. By translating the burner through the laser beam with a DC motor we obtained spatially resolved, absolute C2H2 concentration profiles along the flame sheet with 0.5 mm spatial resolution as measured with a knife edge technique. The TDLAS-based, transverse C2H2 concentration profiles without any scaling are in excellent agreement with published mass spectrometric C2H2 data for the same flame supported on a similar burner, thus validating our calibration-free TDLAS measurements.