| OCR Text |
Show free fraction of the diluted-fuel flame increases dramatically with increasing diluent fraction, while the increase is much more modest with air-dilution. At the highest diluent fraction tested (Z = 0.2), the flame with fuel dilution is entirely blue with no visual evidence of any in-flame soot. We turn now to the actual temperature measurements. Figure 11 shows the axial temperature distribution along the centerline for undiluted and diluted (Z = 0.15) flames, while Figs. 12 and 13 present radial profiles for the same flames at a height of 5 mm and near the flame tip, respectively. The flame tips, where the centerline temperatures reach a maximum (cf. Fig. 11), nominally correspond to heights of 50, 55, and 60 mm for the undiluted, fuel-diluted, and air-diluted flames, respectively. Since thennal NO is fonned in the highest-temperature regions of the flame, we focus our attention to the peak temperatures in each of these figures. Peak temperatures are presented in Table V for several axial locations in the flames. TABLE V. Maximum Temperatures Measured in U ndiluted (Tad = 2275 K) and Air- and Fuel-Diluted (Tad = 2128 K) C~-Air Flames Distance from burner exit, x (mm) 5 25 50 55 60 Maximum Flame Temperature (K) No dilution (Z = 0) 2175 2206 2031 Air Fuel dilution dilution (Z = 0.15) (Z = 0.15) 1974 2026 1955 1893 1938 2052 2006 1956 From Table V, we see that, low in the flame (x = 5 mm), peak temperatures for all three flames are substantially less than their respective adiabatic values, with the greatest difference resulting for the fuel-diluted condition. At this location, kinetic limitations and departures from equilibrium likely occur. This explanation is consistent with the fuel-diluted case having the lowest temperature here since fuel dilution causes a dramatic decrease in fuel concentration relative to the undiluted flame (unity to 0.39), compared to air dilution, which results in a relatively modest reduction in the 02 concentration (0.21 - 0.18). Further downstream, kinetic limitations become less important, but radiation heat losses now begin to influence the temperature, as evidenced by the monotonic decrease in peak temperatures beyond x = 25 mm for all flames. We note also that the fuel-diluted flame is about 50-60 K hotter than the air- 13 |
