Quantitative measurement of atomic hydrogen in low-pressure methane flames using two-photon LIF calibrated by krypton

Abstract

Abstract In this work, the two-photon laser-induced fluorescence technique (TPLIF) was applied to measure the concentration profile of atomic hydrogen in low-pressure laminar premixed flames. Excitation of H-atoms was performed by two-photon absorption at 205\xa0nm and collecting the fluorescence at 656.3\xa0nm. For the first time in flames, the TPLIF signals from the H-atom have been calibrated using the TPLIF signal from krypton, directly seeded in the flame, excited at 204.13\xa0nm and collecting the fluorescence at 826.5\xa0nm. This method was previously demonstrated in plasma environments and recently applied in our group to calibrate O-atom TPLIF signals using xenon as a standard gas. The calibration requires the measurements of TPLIF signals of H and Kr atoms in a flame, where the quenching rates can be determined from time-resolved LIF measurements. Given the short fluorescence lifetime of H-atom, this last task was particularly challenging. A calibration flame was chosen to minimize collisions and the response time of the detection was determined using a deconvolution method. We found that the quenching rate is fairly constant around 4.6\xa0×\xa0108\xa0s−1 at 5.3\xa0kPa in a large portion of the flame. The calculated quenching rate overestimates the measured value from 35% to 500% depending on the chosen assumption on the dependence of the quenching coefficient with temperature. The quantitative measurement of H-atom mole fraction was carried out in three nitrogen-diluted low-pressure methane flames. The experimental profiles were compared with the calculated ones using chemical modeling. The variation in the experimental H-atom mole fraction in the range of equivalence ratios agrees well with the simulated values. Quantitatively, the calculated mole fractions agree within 30% with the experimental ones. The method is robust but its accuracy is limited by the uncertainty in the knowledge of the ratio of the two-photon cross-sections of Kr and H atoms. Application of this calibration method to atmospheric is discussed.