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Show diluted flame at or near the flame tip. These results are consistent with both the lower luminosity observed and the higher NOx emissions measured for the fuel-diluted flame. An additional experiment was run to detennine if the differential effect on NOx of fuel versus air dilution was principally a result of fuel dilution decreasing the soot fonned and, hence, decreasing radiation losses. In this experiment, a nonluminous flame, created by fuel dilution to Z = 0.20, was established as a base case, and additional diluent was added to either the fuel or air stream. All flame conditions, of course, were nonluminous with no evidence of in-flame soot. Diluent was added until the total diluent fraction reached 0.30. NOx emission indices fell from about 3.1 (Z = 0.20) to 2.65 g/kg (Z = 0.30), independent of how the diluent was added. Measurements of the temperature fields in the Z = 0.30 flames showed no significant differences between the peak temperatures occurring in the air- and fuel-diluted flames. These results offer further evidence that, for laminar jet flames, any differential effectiveness in NOx reduction between air and fuel dilution is a result of differences in flame temperatures related to in situ soot production and its effect on radiant heat losses. Other measurements in the present investigation include molar N02-to-NOx ratios and CO emission indices. Dilution was found to increase slightly the N02 proportion of the total NOx. Differences between air and fuel dilution were small and within the statistical uncertainty associated with the measurements. The N02-to-NOx ratios ranged from about 0.16 to 0.23. These values are similar to measurements from turbulent jet flames [16]. Carbon monoxide emission indices increased with added dilution from about 0.4 to 0.8 g/kg for 298 K reactants and from about 0.3 to 0.5 g/kg for 400 K reactants. No consistent difference in CO emissions between air and fuel dilution were seen for the unheated reactants, while, for 400 K reactants, fuel dilution result in slightly lower CO emission indices, e.g., 0.48 g/kg for fuel dilution versus 0.55 g/kg for air dilution at Z = 0.20. The CO levels observed also were within the same range as those measured in turbulent CH4-air jet flames [17]. SUMMARY AND CONCLUSIONS Numerical Modeling Nonreacting CH4-air-N2 counterflows and counterflow flames were simulated. In these calculations, either the air stream or the fuel stream was diluted with N2 as a means of simulating some of the possible chemical and molecular transport effects related to the observed increased 14 |
