Concentration measurements of OH· and equilibrium analysis in a laminar methane-air diffusion flame

Absolute concentration measurements of hydroxyl radical have been made in a laminar, co-flowing methane-air diffusion flame using laser absorption and laser-induced fluorescence methods to probe the A2Σ+ ← X2Π transition. The maximum OH· concentration is found to be 1.8 ± 0.2 × 1016 cm−3 (mole fraction = 5.0 × 10−3) at a temperature of 2080 K, which is twice the value calculated assuming local total equilibrium but less than half that predicted from partial equilibrium (O2 + H2 ⇄ 2OH·). Evidence is presemted that in general partial equilibrium does not exist for the fast, bimolecular chain branching and radical shuffle reactions in this flame. The overall chemical production-destruction rate profile of OH· has been determined, as well as the rates of hydrogen abstraction reactions by hydroxyl radical to form the vinyl and ethynyl radicals. These results are used to identify the most rapid routes for the production of radical species important in chemical growth processes.

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