Laser-Induced Fluorescence of CH · in a Laminar CH4/Air Diffusion Flame: Implications for Diagnostic Measurements and Analysis of Chemical Rates

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Abstract
Relative CH radical concentration profiles have been measured in a laminar, co-flowing methane/air diffusion flame using laser-induced fluorescence from the (0,0) band of the A2Δ → X2П transition. CH· has the narrowest profile of any species yet measured in this flame. The CH· peak does not coincide with the position of the OH· and temperature maxima, but appears on the fuel-rich side, between the peak concentrations of hydrogen atoms and methyl radicals. If CH· fluorescence is detected with a narrow bandpass dielectric filter, rather than using a monochromator, severe interference attributed to fluorescence from polycyclic aromatic hydrocarbons (PAH) is observed. The CH· concentration decreases, while PAH fluorescence interference increases rapidly with height above the burner. Implications for CH· imaging experiments in turbulent diffusion flames are discussed. The net chemical production/destruction rate of CH· is determined as a function of flame position at a height H of 9 mm above the burner. This CH· production rate profile is used to derive a relative mole fraction profile for triplet methylene 3CH2:. The concentration ratio [3CH2:]max/[CH·] max is found to be 13-300, where the major source of uncertainty is the reaction rate of 3CH2: + H·→ CH· + H2. Maximum concentrations of the CH2 radicals are not well established in this flame. However, if the derived [3CH2:]max/[CH·] max ratio is combined with the calculated value of 25ppm for [3CH2:]max from Puri et al. (1987), one estimates that the peak CH· mole fraction lies between 0.08 and 2ppm at H = 9mm.