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The remaining cases can be separated into two groups, those where the fuel is split between injection locations (1) and (2) and those where the fuel is split between injection locations (1) and (3). When 50% of the fuel is injected at location (2) the stability characteristics are observed to improve (case 2), however, when all of the fuel is injected at location (2) the combustor is unstable at all equivalence ratios (case 1). Since fuel injection at location (2) results in fuel lean conditions along the centerbody (see Figure 2) where the flame is stabilized, it is not surprising that in case 1 the combustor is more unstable. The improved stability in case 2, however, requires further study.

In cases 4 through 7, increasing amounts of fuel are injected at location (3). As shown in Figure 3, the fuel distribution is the same in cases 3 through 7, however, the stability characteristics change as the fraction of fuel injected at location (3) is increased.

In particular, the combustor becomes very unstable when 50% or more of the fuel is injected at location (3). One possible explanation for this is feed system coupling since injection location (3) is downstream of the choked inlet to the mixing section and therefore can be affected by pressure oscillations in the mixing section. This will be investigated in more detail during the next reporting period.

In summary, the stability results shown in Figure 4 clearly demonstrate the significant effect that the fuel distribution can have on the stability characteristics. These results suggest that by changing the fuel distribution one may actually be able to avoid instabilities. Stabilities are obviously not the only concern in lean premixed combustion systems. One also must understand the effect of the fuel distribution on emissions. Figure 5 shows the NOx emissions at the same operating conditions for which the stability data were presented. As expected there is a significant decrease in NOx emissions as the equivalence ratio decreases, but what is of greater interest is the effect of the fuel distribution among the seven fuel distribution cases. If one associates incomplete mixing with increased NOx emissions, one would expect the cases with a uniform fuel distribution , i.e., cases 3 through 7, to have the lowest NOx emissions for a given equivalence ratio. Figure 5 certainly shows that as the fuel distribution becomes less uniform, as it does in going from case 3 to case 2 to case 1, that the NOx emissions increase. The somewhat surprising result in Figure 5 is the fact that even though cases 3 through 7 all have a uniform fuel distribution, the NOx emissions first decrease in going from case 3 to 4 and then increase in going from cases 4 to 7, as an increasing amount of fuel is injected at location (3). This is likely to be related to the fact that the combustor becomes more unstable as a greater fraction of the fuel is introduced at location (3). This behavior will be studied more closely in the next reporting period.

Figure 5. NOx Emissions Map for a 5 m/s Inlet Velocity, 350°C Inlet Temperature, 30° Swirl Operating Condition and Fuel Split Cases 1 through 7.
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