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Show fraction data from conventional straight jet flames are also indicated on the ordinate of Fig. 7 for comparison with the precessing jet results. These straight jet data, which are also given in Table 2, were selected to match as closely as possible the jet exit conditions of the precessing flames. Since data for identical nozzle diameters are not available, we chose to match jet Reynolds numbers. For the propane flames (de = 3 mm or 3.86 mm), radiant fractions are much less for the low-Stp regime precessing jet flames than those for the simple jet flames, presumably as a result of the total radiation being reduced as a consequence of less in-flame soot in this regime. At the largest Strouhal numbers in the high-Stp regime (Stp > 0.015), values of propane flame radiant fractions exceed that of the simple jet flame. For the methane flames, the radiation trends appear to be less closely linked to flame luminosity (in-flame soot) and generally follow trends in global residence times, as might be expected, if the flame radiation is dominated by the gas-molecular component (Turns and Myhr, 1991). For the larger precessing methane flames (de = 10 mm), radiant fractions generally exceed the simple jet flame values, while radiant fractions for the smaller precessing jet flames (de = 3 mm) are less than those of the conventional jet flames. CO Emissions and N02-to-NOx Ratios The CO and NOx emissions from the precessing-jet flames exhibit interesting characteristics as the precession Strouhal number is varied. Figure 8 shows CO emission indices as functions of Stp, and Fig. 9 shows corresponding N(h-to-NOx ratios. From these figures, we see that, for conditions within the low-Strouhal-number regime (i. e., Stp < 0.005), CO emissions are quite high, compared to both the bigh-Stp regime precessing-jet flames and simple jet flames. Similarly, nearly all of the NOx emitted from the low-Stp flames is N(h, while for-the simple jetflames, N(h-to-NOx ratios are about 10-30%. At the higher Strouhal numbers, CO emissions fall, but never reach the levels measured in simple jet flames. Similarly, N02-to-NOx ratios fall with Stp for the CRt flames, but remain quite high for the C3Hg flames. In most diffusion flames, N02-to-NOx ratios are relatively low; however, Turns & Brooks (1994) have observed . simultaneously high CO emissions and N02-to-NOx ratios, similar to those described above, when injecting air jets transversely into a simple jet flame. Driscoll and co-workers (1992) also found that increasing the quantity of coflow air around a confined methane jet flame resulted in increased N~-to-NOx ratios, while the total NOx emissions were unaffected. Hori et ale (1992) have shown that unburned hydrocarbons can promote the conversion of NO to N02 when hot combustion products are mixed with air. It seems likely that hydrocarbonenhanced NO-to-N02 conversion is responsible for the high N02-to-NOx ratios observed in the 9 |