TEMPERATURE-DEPENDENCE OF LASER-INDUCED FLUORESCENCE OF NITRIC-OXIDE IN LAMINAR PREMIXED ATMOSPHERIC-PRESSURE FLAMES

The temperature dependence of laser-induced NO A (2) Sigma(+)-X (2) Pi fluorescence in the hot gases of natural gas-air flames, seeded with known quantities of NO, has been determined experimentally by means of a difference method. The flame temperature at three fixed equivalence ratios was changed when the mixture velocity was varied through a wat er-cooled, flat-flame burner and was measured by coherent anti-Stokes Raman spectroscopy. When the possible reburning of part of the seeded NO is allowed for, the results in the range 1700-2150 K are best descr ibed by the temperature dependence obtained from a model in which quen ching corrections are neglected, as in the case of a saturated two-lev el system, when millijoule pulse energies are used. Measurements of th e fluorescence intensity at constant seed concentration as a function of equivalence ratio between 0.75 and 1.3 also indicate that quenching corrections are unnecessary under these excitation conditions. Using the measured intensities of the seeded flame as a calibration factor, we determined the absolute NO concentrations as functions of the equiv alence ratio at 1 cm above the burner. The results indicate that, with the calibration method presented here, a relative accuracy of 5% shou ld be obtainable. (C) 1997 Optical Society of America.