Nitric oxide formation and differential diffusion in a turbulent methane-hydrogen diffusion flame

Temperature and composition measurements are reported for a turbulent, methane-hydrogen jet diffusion flame. Mixture fraction measurements compare well with predictions from a flamelet-based, turbulent jet flame model with an assumed mixture fraction pdf. A fuel mixture with components of disparate mass diffusivities (Lewis numbers), provides strong clues to molecular scale behavior. Molecular separation of fuel components, hydrogen and methane, and hydrogen and carbon elements was inferred to be much less than predicted from the steady-state, counterflow flamelet model. Supporting calculations of time-dependent diffusion in a shear layer and striated fuel-air mixing layers suggest that within the context of flamelet modeling, deviations from the predictions of a steady counterflow model, such as a weaker differential diffusion effect, can be attributed to transient behavior and flamelet interactions. Mean temperature and species measurements are in accord with observation. It is concluded that transient behavior and flamelet interactions must be considered in describing flame behavior. Contrary to predictions, measured nitric oxide profiles show little dependence on downstream position. Measured concentrations are well below predicted values, and the profiles are rich-shifted. No conclusive explanation was found for the rich shift; the possibility of an appreciable apparent shift due to chemical reactions within the probe is considered unlikely. Nearly identical results are obtained with both stainless steel and quartz probes. By default, the shift and deviations from the model predictions are attributed largely to premixing and homogeneous reaction effects, transients, and flamelet interactions.