Figure 1 Schematic Drawing of Laboratory Dump Combustor Illustrating Various Fuel Injection Locations

During this reporting period the effect of changing the inlet fuel distribution, by using various combinations of injection locations (1), (2) and (3), on the stability and emissions characteristics of the combustor was investigated. The operating conditions over which these tests were conducted are summarized in Table 1. The inlet velocity given in Table 1 refers to the bulk velocity in the combustor at the indicated combustor pressure and inlet temperature. The combustor to mixing section area ratio is 13.4, therefore the velocities in the mixing section are 13.4 times greater than the indicated combustor inlet velocities.

pressure

1 atm

inlet temperature

300°, 350°C

inlet velocity

3.5, 5.0, 6.5 and 8.0 m/s

swirl

30°

equivalence ratio

LBO to 0.8

fuel

natural gas

power

40 - 180 kW

fuel distribution

see figure 2

Table 1. Operating Conditions

Table 1. Operating Conditions

The fuel distribution at the inlet to the combustor, i.e., the annular exit of the mixing section, was varied by changing the fuel split between injection locations (1), (2) and (3). For the tests conducted during this reporting period the 7 fuel split cases listed in Table 2 were used. For each of these cases the actual fuel distribution across the exit of the annular mixing section was measured using laser induced fluorescence where the fuel was doped with a small amount of acetone vapor which served as a fluorescence tracer. Fuel distribution measurements for fuel split cases 1, 2 and 3 are presented in Figure 2 for an inlet velocity of 6.5 m/s, an inlet temperature of 100°C and an equivalence ratio of 0.55. As expected, case 3, where all of the fuel is introduced at location (1), results in a uniform fuel distribution. The results for cases 2 and 1 show that fuel injected through the centerbody at location (2) penetrates to the outer wall of the mixing tube, resulting in a skewed fuel distribution. In case 1, where all of the fuel is injected at location (2), the equivalence ratio appears to vary almost linearly from about 0.25 to about 0.85 from the inner to outer diameter of the mixing section. Figure 3 shows the effect of fuel injected at location (3) on the fuel distribution. As shown, there appears to be little effect on the fuel distribution, even when all of the fuel is injected at location (3), i.e., case 7. Although the fuel distribution is the same for injection locations (1) and (3), there is an important difference between these two injection locations. Since location (1) is upstream of the choked inlet to the mixing section it is isolated from the effect of pressure fluctuations in the mixing section, i.e., so-called feed system coupling. Injection location (3), however, will be affected by pressure fluctuations in the mixing section and therefore susceptible to feed system coupling. This may have a significant effect on the stability characteristics.

Fuel Distribution

Injection Location (1)

Injection Location (2)

Injection Location

(3)

Case 1

0%

100%

0%

Case 2

50%

50%

0%

Case 3

100%

0%

0%

Case 4

75%

0%

25%

Case 5

50%

0%

50%

Case 6

25%

0%

75%

Case 7

0%

0%

100%

Table 2. Fuel Splits Cases

Table 2. Fuel Splits Cases

distance from centerbody [mm]

Figure 2. Laser Induced Fluorescence Fuel Distribution Measurements for Fuel Split Cases 1, 2 and 3.

distance from centerbody [mm]

Figure 3. Laser Induced Fluorescence Fuel Distribution Measurements for Fuel Split Cases 3, 5 and 7.

distance from centerbody [mm]

Figure 3. Laser Induced Fluorescence Fuel Distribution Measurements for Fuel Split Cases 3, 5 and 7.

At each test condition the pressures in the mixing section, the fuel line and at several locations in the combustor were measured with high frequency response pressure transducers. In addition, the exhaust gases were sampled and analyzed with a chemiluminescence NOx analyzer. The results are then plotted in terms of stability and emissions maps, where stability is expressed in terms of the peak-to-peak pressure fluctuation and emissions are expressed in terms of the NOx concentration.

The stability map, based on the combustor pressure measured at the dump plane, for one operating condition (6.0 m/s inlet velocity, 350°C and 30° swirl), is shown in Figure 4. Fuel split case 3 can be considered the reference case. In case 3 all of the fuel is injected and mixed with the air upstream of the choked inlet to the mixing section, which as shown in Figure 2 results in a uniform fuel distribution. In this case the combustor is stable for equivalence ratios between 0.6 and 0.75, but becomes unstable at equivalence ratios below 0.6.

dBVp-p dBVp-p

0 0

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