Relationships among nitric oxide, temperature, and mixture fraction in hydrogen jet flames

Simultaneous point measurements of NO, the 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% and 40% helium dilution, with emphasis on the relationships among nitric oxide, mixture fraction, and temperature. Conditional statistics are used to extract information on the coupling between reaction zone structure and measured NO levels. Fluctuations of the NO mole fraction conditional on mixture fraction are greatest near the base of the flame, where NO formation rates and concentrations are sensitive to local strain. Similarly, when NO levels are averaged conditional on both mixture fraction and temperature, the temperature dependence is strongest near the flame base, where reaction zones are thin. The radial dependence of the conditional mean NO mole fraction (conditional on mixture fraction) is shown to be small at upstream locations and negligible at the downstream locations that contribute most to the overall NO emission. These results combined with previously reported data on the present hydrogen flames provide a detailed basis for evaluation and refinement of turbulent combustion models for thermal NOx formation in jet flames.