Local OH concentration measurement in atmospheric pressure flames by a laser-saturated fluorescence method: two-optical path laser-induced fluorescence.

The first (to our knowledge) measurements of number density of OH in flames at atmospheric pressure by TOPLIF are reported. TOPLIF (acronym for two optical paths laser-induced fluorescence) improves the accuracy of LIF measurements by taking into account both the spatial profile of the exciting laser intensity and the collisional transfer rate. The method is based on simultaneously recording the LIF signals from focal volumes of two different shapes. The ratio of the signals is a measure of the saturation parameter (which depends on the laser intensity and the quenching) using which accurate determination of the species number density can be deduced from the fluorescence signals. The method is valid as far as at least partial saturation is reached. First, experimental verification of the theoretical basis of the method is reported. The population of a single rovibronic level is measured as it is in most of the spectroscopic methods. TOPLIF measures this population relative to this level's population in a chosen reference flame. Absolute value can therefore be obtained if the value in the reference flame is known or measured. Absolute [OH] profiles obtained in flat flames burning at 60 and 1000 mb are presented and compared to laser absorption measurements.