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Show number for both methane and propane flames for the two nozzle diameters tested. Data are also shown for three different jet velocities for precessing methane flames. Figure 11 shows the effects of jet deflection angle $ on NOx emission indices. Several observations can be made from these figures, and we focus first on the general effect of Strouhal number (Fig. 10). For the propane flames, a monotonically decreasing trend of NOx emission index with Strouhal number is observed for both the 3-mm and 10-mm nozzles. This trend is wholly consistent with the increasing radiant fraction associated with these flames (Fig. 7), and hence, the lowering of characteristic temperatures with increasing Sip. In the low-Stp regime, the precessing jet NOx emission indices are higher than those from the corresponding simple flames, which, again, are indicated on the ordinate. This higher level of emissions is consistent with the relatively small radiant fractions of the precessing jet flames in this regime (cf. Fig. 7). However, as the precession Strouhal number is increased, and the flame becomes highly luminous, levels fall slightly below those of the simple jet flames. For example, at the highest Strouhal numbers, the emissions indices for the precessing jet flames are approximately 4% and 8% lower for the large and small nozzle diameters, respectively. Exact quantification of such differences is not possible, since the statistical uncertainty associated with the EINOx determinations is about 2.5% (standard deviation) and identical nozzle conditions are not obtained for both the precessing and simple jets. The effect of precession Strouhal number on the methane flame NOx emission indices is much more complex than that described above for the propane flames. Looking first at the de = 10 mm results, we see a mild general increase in NOx with Strouhal number for Stp > 0.01 for all three jet velocities investigated; while in the low-Stp regime (Stp < O.OI), the NOx data for highestvelocity case appear to rise as Stp approaches zero. A similar Strouhal number characteristics appears when the jet deflection angle $ is varied parametrically. In this case, no effect of deflection angle is observed in the high-Stp regime, while the smallest angle data ($ = 30°) show a slight upturn in NOx as Stp approaches zero. A possible explanation of the insensitivity of NOx emissions to jet velocity and deflection angle at high Strouhal numbers "and, conversely, the sensitivity at low Strouhal numbers, is the difference in the character of the mixing in these two regimes. In the high-Stp regime, the dominant scale of mixing is quite large. This large-scale turbulent mixing is generated when Stp and <t> are within the range that the low-pressure core and recirculation zone are established (Schneider et al., 1994). Provided this flow regime is established, the dominant scale of mixing is much larger than that of the reaction zone, and the influence of changes in Stp and $ are likely to be second order. At low Stp. however, the dominant scale of mixing may be comparable with that of the reaction zone thickness. The large-scale 11 |