CH-Radical Concentration Measurements in Fuel-Rich CH4/O2/Ar and CH4/O2/NO/Ar Mixtures Behind Shock Waves

CH radicals are important precursors in prompt-NO formation and they also play a role in NO removal via reburning [1].There have been several direct studies of the reaction CH ϩ NO 3 products in hydrocarbon pyrolysis systems using laser absorption of CH in shock tubes, e.g.[2, 3], and flow reactors, e.g.[4].Little information exists, however, on the absolute concentrations of CH during fuel-rich methane oxidation and their dependence on NO concentrations.Here we report results of CH-radical measurements for such conditions, using a shock tube in combination with a sensitive CH absorption diagnostic.These measurements provide useful data for the further development of large-scale reaction mechanisms, such as the recently developed Gas Research Institute methane oxidation and NO reburning mechanism (GRI-Mech 2.11) [5].CH-radical concentration measurements were made behind reflected shock waves in gas mixtures of 80 ppm CH 4 ϩ 100 ppm O 2 ( ϭ 1.6) or 80 ppm CH 4 ϩ 100 ppm O 2 ϩ 400 ppm NO diluted in Ar using narrow-linewidth laser absorption at 431.131 nm.The shock tube and the experimental procedures are described elsewhere [6].The test gas mixtures were prepared manometrically from commercially available high-purity gases in a stainless steel cylinder.The mixture containing NO was used immediately after preparation to prevent conversion of NO to NO 2 due to the Bodenstein reaction.The conversion of CH absorption data into concentration-time histories was done using a spectroscopic model for the overlapping Q 1d (7) and Q 2c (7) rotational transitions in the A 2 ⌬ 4 X 2 ͟ (0, 0) band developed by Dean and Hanson [7].