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Show 7 to the stability limits. The counter-swirl nozzle provides the best performance at 22.5% excess arr and 0.68 swirl intensity, where NOx is 17.5 ppm and combustion efficiency is 99.97%. 35 Co-Swl 30 25 ?f?- :c~;: 20 en en CD u 15 x W 10 5 0 0.3 0.4 ------ - - - -- - ---- - -- I I I I 0.5 0.6 0.7 0.8 Swirl, S' 10.80+ 0.69 to 0.80 0.57 to 0.69 0.46 to 0.57 ~~ 0.34 to 0.46 ~{m 0.23 to 0.34 ~Ir 0.11 to 0.23 . 0.00 to 0.11 Counter-SWlrl 0.9 Figure 6: Performance Index (1) Maps for Co-swirl, Radial, and Counter-swirl Nozzles NOx and Efficiency equally weighted DISCUSSION The counter-swirl nozzle exhibits the best performance and, as shown in Figure 4, high combustion efficiencies over its entire operating range. This behavior is attributed to the extent of fuel and air mixing. With the fuel introduced counter-swirling to the air stream, an "opposed jets" scenario arises which mixes the fuel and air quickly and uniformly. The complete combustion of intermediate species, such as CO and hydrocarbon radicals, continues even with increased swirl and excess air levels. As a result, the counter-swirl nozzle is able to maintain high combustion efficiencies even when increased entrainment and higher air flows tend to quench and convectively cool the reaction, thereby lowering the temperatures and NOx emissions. The end effect is better overall performance for this nozzle, with optimized performance occurring in the regions close to the high excess air stability limits where the NOx emissions are reduced. As the fuel jets become less opposed to the air flow, i.e., from radial to co-swirling, the mixing, efficiencies, and therefore performance decrease, as evidenced in Figure 6. The advantages of the performance function and the optimization of a single variable are readily seen from these results, namely the identification of high performance regions within a burner's |