Quenching-corrected Saturated Fluorescence Measurements of the Hydroxyl Radical in Laminar High-pressure C2H6/02/ N2Flames

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Abstract
We have employed the TOPLIF (two-optical paths laser-induced fluorescence) method for making quenching-corrected laser-saturated fluorescence measurements of hydroxyl concentration in high-pressure laminar C2H6/02/N2 flames. With this technique, which requires detection of fluorescence along two optical paths, the ratio of the two fluorescence signals accounts for the spatial dependence of the laser irradiance and can be used to determine the influence of the excitation and quenching rate coefficients on the fluorescence signals. In an atmospheric flame, we have calibrated the fluorescence signals with absorption measurements and have generated a quenching correction function from the corresponding saturation curves. With this function, the fluorescence voltages from flames at 3·1, 6·1, and 9·2 aim have been adjusted for changes in quenching and laser power. We have thus obtained absolute concentration profiles of OH for these high-pressure flames. The TOPLIF technique is still affected by the inaccuracy of the balanced cross-rate model: at 6·1 atm, the disparity between fluorescence and absorption measurements indicates ˜25% depletion of the laser-coupled levels. Moreover, the method is sensitive to scattered and reflected laser radiation. Nonetheless. TOPLIF could be particularly useful in complex combustion environments where local variations in the fluorescence quenching rates can adversely affect the accuracy of concentrations derived from laser-induced fluorescence.