Quantitative C2H2 measurements in sooty flames using mid-infrared polarization spectroscopy

Quantitative measurements of acetylene (C₂H₂) molecules as a combustion intermediate species in a series of rich premixed C₂H₄/air flames were non-intrusively performed, spatially resolved, using mid-infrared polarization spectroscopy (IRPS), by probing its fundamental ro-vibrational transitions. The flat sooty C₂H₄/air premixed flames with different equivalence ratios varying from 1.25 to 2.50 were produced on a 6 cm diameter porous-plug McKenna type burner at atmospheric pressure, and all measurements were performed at a height of 8.5 mm above the burner surface. IRPS excitation scans in different flame conditions were performed and rotational line-resolved spectra were recorded. Spectral features of acetylene molecules were readily recognized in the spectral ranges selected, with special attention to avoid the spectral interference from the large amount of coexisting hot water and other hydrocarbon molecules. On-line calibration of the optical system was performed in a laminar C₂H₂/N₂ gas flow at ambient conditions. Using the flame temperatures measured by coherent anti-Stokes Raman spectroscopy in a previous work, C₂H₂ mole fractions in different flames were evaluated with collision effects and spectral overlap between molecular line and laser source being analyzed and taken into account. C₂H₂ IRPS signals in two different buffering gases, N₂ and CO₂, had been investigated in a tube furnace in order to estimate the spectral overlap coefficients and collision effects at different temperatures. The soot-volume fractions (SVF) in the studied flames were measured using a He–Ne laser-extinction method, and no obvious degrading of the IRPS technique due to the sooty environment has been observed in the flame with SVF up to ～2×10^?7. With the increase of flame equivalence ratios not only the SVF but also the C₂H₂ mole fractions increased.