OCR Text |
Show For a given excess air regime (high or low), NOx decreases as the amount of recirculated mass increases. This is attributed to the increased internal product gas recirculation and the resultant decrease in reaction temperature. The lower NOx levels for the high excess air compared to the low excess air regime is attributed to the more aggressive mixing of fuel and air within the air annulus. In the low excess air regime, an "outlying" point is conspicuous: the counter-swirl injector at high swirl, S'=0.92 (condition 6). Although the counter-swirl injector at high swirl has essentially the same amount of recirculated mass as the radial injector at 5% excess air and S'=0.62 (condition 3), the counter-swirl injector yields much lower NOx. Thus, given the same excess air level, the percent recirculated mass alone cannot be responsible for the NOx emissions characteristics, and other factors (e.g., the fuel-air distribution) must playa role as well. The lowest NOx emission is recorded for condition 2, the co-swirl injector at 20% excess air. This, however, does not take into account any degradation in combustion efficiency. In Figure 10, the performance index is plotted against percent recirculated mass. Note that condition 2 has the most degraded performance overall. The low emission of NOx is at the sacrifice of a high combustion efficiency. -s- x Q) ~ .E CD 0 c as E '- 0 "t:: Q) a. 0.75~--------------------------------------------~ 0.70- 0.65- 0.60- 0.55- 0.50- A'- cx-swirl200/0 0.66 (J) A .- radial 150/0 0.62 ® ~.- cx-swirl 50/0 0.92 ® ~ ~.- radial 50/0 0.62 @ ~CX-SWirl50/0 0.72 @ ~ ~ 0) co-swirl 5% 0.40 0.45 - ~ Low Excess Air A High Excess Air A'-' co-swirl 200/0 0.52 ® 0.40~~--~1~~1--~~1~~-r-1~-T1--~~1--~~1r-~-r-1~-1 00/0 1% 2% 3% 4% 5% 6% 7% 8% Recirculated Mass (0/0) Figure 10: Performance Index vs. Recirculated Mass (0) Denotes condition identified in Table 2) The highest performance is reflected for condition 7, the counter-swirl injector at 20% excess air and input swirl strength of 0.66. Note that the same injector at a reduced excess air level (5%) 12 |