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

As part of an ongoing investigation of an exhaust NOx emission index minimu m measured for partially premixed flames, radial temperature profiles and C H radical locations were measured in atmospheric-pressure, partially premix ed, 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 undertake n because of the importance of temperature and CH radical behavior in NO fo rmation chemistry. Thin-filament pyrometry was found to be more appropriate than thermocouple thermometry for temperature measurements in partially pr emixed flames. Results demonstrated that the 1.6-equivalence-ratio flame ex hibited classical double-flame structure, the 2.0-ecluivalence-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 f luorescence provide evidence that, for the present measurement locations, d ouble flames exhibit single CH peaks which can be associated with their pre mixed component flames. Double CH radical peaks, which were predicted to oc cur in low-strain-rate flames, were not found for the limited number of fla me conditions and locations studied. In the near-burner region, the premixe d and nonpremixed component flames of the Phi(B) = 1.6 double flame diverge radially with increasing downstream distance and merge together for larger values of Phi(B). (C) 1999 by The Combustion Institute.