Laser-induced Fluorescence Triple-integration Method Applied to Hydroxyl Concentration and Fluorescence Lifetime Measurements

We report quantitative hydroxyl concentrations obtained by using a new laser-induced fluorescence triple-integration method (LIFTIME), which is capable of rapid and continuous fluorescence lifetime measurements via a unique photon-counting technique. LIFTIME has been convolved with picosecond time-resolved laser-induced fluorescence to permit the rapid monitoring of instantaneous species concentrations in flames. Here, LIFTIME is used to measure hydroxyl concentrations and fluorescence lifetimes at a sampling rate of 1 Hz in eight premixed laminar flat flames and in one laminar opposed flow diffusion flame. Fluorescence lifetimes as a function of axial position are generally obtained with less than 5% uncertainty, while concentrations at the same locations are obtained with less than 10% uncertainty (95% confidence interval). The hydroxyl concentration measurements are shown to agree well with modeling predictions and with previous laser-saturated fluorescence measurements. The measurements are also compared with predictions based on existing quenching cross-section correlations.