Vibrational level relaxation effects on laser-induced fluorescence measurements of hydroxide number density in a methane–air flame

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Abstract
The laser-induced fluorescence technique was investigated via detailed rate equation modeling of hydroxide in a simulated premixed atmospheric methane–air flame environment. The extent of deviation from the simple two-level model, due to buildup of population in the vibrational bath levels from quenching and vibrational exchange collisions, was addressed as were the effects of variation in the magnitude of the collisional energy exchange rate constants. Typical results show a breakdown in the two-level model on a nanosecond time scale and indicate that OH number density measurements with accuracies better than an order of magnitude will require (1) better information on detailed quenching rates and (2) laboratory measurements which address the time history of the fluorescent signal on a nanosecond time scale.