Raman/Rayleigh/LIF measurements of nitric oxide formation in turbulent hydrogen jet flames

Abstract

Simultaneous, time-resolved measurements of NO, major species, mixture fraction, temperature, and OH are obtained in nonpremixed turbulent hydrogen jet flames, using the combination of spontaneous Raman scattering, Rayleigh scattering, and laser-induced fluorescence. Results are presented for an undiluted hydrogen flame at Reynolds number 10,000 and for flames with 20 percent and 40 percent helium dilution. Scatter plots and conditional averages of NO mole fraction show gradual increases with streamwise distance. The width in mixture fraction coordinates of the zone of high NO mole fractions decreases with increasing streamwise distance. At a given mixture fraction, the fluctuation in NO mole fraction relative to the mean is greatest near the nozzle and decreases toward the flame tip. These effects are related to the streamwise evolution of turbulent flame structure from thin reaction layers near the nozzle to broad, near-equilibrium zones at the flame tip. Dilution with helium significantly reduces the measured NO levels and causes a slight shift of the curves of conditional average NO mole fraction toward the fuel-lean side of the stoichiometric mixture fraction. Favre averages and ensemble averages show significant bias, relative to conditional averages, in the relationship between NO mole fraction and mixture fraction. This suggests that the “rich shift” observed in some sampling-probe studies may results from the averaging process itself.