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Show fact, the 1.2 ms residence time is smaller than the 2.6 ms value for the air-diluted flame. The increased residence time for the Case 3 (uo = 50 cm/s) fuel-diluted flame can be understood as a result of the fuel dilution causing the flame to move closer to the stagnation plane, i.e., to move into a region of lower velocity. This is principally a flow phenomenon, since we saw previously for the nonreacting flow (cf. Fig. 2) that location of the stoichiometric fuel mole fraction moves into a lower velocity region when the fuel stream is diluted. For the Case 4 fuel-diluted flame, however, the decreased residence time is a consequence of the generally much greater velocities occurring in this flame as shown in Fig. 5 and Table II. T ABLE II. Residence Times, Peak Flame Temperatures, and Velocity and N 2 Mole Fractions at Peak Flame Temperature (Cf4-air, T = 300 K) Parameter Case No. Residence Time forT>IS00 K (ms) Temperature at Flame (K) Velocity at Flame (cm/s) N2 Mole Fraction at flame ---uo = SO cm/s --No Dilution Air Dilution Fuel Dilution (Z = 0) (Z = O.IS) (Z = O.IS) 1 3.2 1927 -SS.S 0.683 2 2.6 1802 -S4.4 0.726 3 3.6 1762 -31.0 0.737 - uo = 128 cm/s - Fuel Dilution (Z=O.IS) 4 1.2 1672 -6S.6 0.737 Also presented in Table II are the peak flame temperatures and the N2 mole fraction at the peak-temperature location. Here we see that fuel dilution is more effective than air dilution in reducing the peak temperatures (cf. Cases 2, 3, and 4), with the fixed fuel mass flux case having the lowest temperature (1672 K) of the three. The N2 mole fractions are not significantly different among the three flames, but the small differences are consistent with the differences in flame temperature, i.e., the lowest temperature coincides with the highest N2 concentration, and vice-versa. Nitric Oxide Emissions Table III compares the NO emission indices for the four cases discussed above. Here we see that the addition of the N2 diluent decreases NO emissions for either air- or fuel-side dilution as expected. Comparing the results for fixed fuel velocity (Cases 2 and 3), we see that, for these 7 |