The formation of NO and NO2 are sensitive indicators of both temperature and residence time. In this work, the NOx emission index is measured in an unsteady counterflow diffusion flame for methane, propane and ethylene, as a function of average strain rate and amplitude and frequency of imposed sinusoidal oscillation. The flames studied vary from non-sooting to high soot loading, and from low average strain rate to near extinction. Due to the relatively long time scales associated with NOx formation, the effect of unsteadiness on emission index is weaker than on either temperature or soot volume fraction. Time average global measurements were taken using a California Analytical Instruments Model 400 HCLD NO/NOx analyzer. Results are compared with unsteady calculations using a modified OPPDIF code included in the Chemkin package.

Keywords: Counterflow diffusion flames Nox.