Experimental study of temperature and CH radical location in partially premixed CH₄/air coflow flames

As part of an ongoing investigation of an exhaust NO(x) emission index minimum measured for partially premixed fames, radial temperature profiles and CH radical locations were measured in atmospheric-pressure, partially premixed, coflow, methane/air flames with fuel-side equivalence ratios of 1.6, 2.0, and 3.5, at three axial heights above the burner. The work was undertaken because of the importance of temperature and CH radical behavior in NO formation chemistry. Thin-filament pyrometry was found to be more appropriate than thermocouple thermometry for temperature measurements in partially premixed flames. Results demonstrated that the 1.6-equivalence- ratio flame exhibited classical double-flame structure, the 2.0-equivalence- ratio flame was a merged flame, and the 3.5-equivalence-ratio flame exhibited diffusion-flame structure. Signals from CH* chemiluminescence and CH laser- induced fluorescence provide evidence that, for the present measurement locations, double flames exhibit single CH peaks which can be associated with their premixed component flames. Double CH radical peaks, which were predicted to occur in low-strain-rate flames, were not found for the limited number of flame conditions and locations studied. In the near-burner region, the premixed and nonpremixed component flames of the Φ(B) = 1.6 double flame diverge radially with increasing downstream distance and merge together for larger values of Φ(B).