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Show and increased input swirl strength (0.72), condition 5, yields a dramatically degraded value of the performance index. One reason for this degraded performance is the reduced recirculated mass, which tends to reduce NOx and thereby increase performance. The level of recirculated mass, however, cannot be the only factor affecting performance. For example, the counter-swirl injector in condition 6 has a similar level of recirculated mass as condition 7 but significantly degraded performance. Since CO emission for the counter-swirl injector is low throughout the entire regime interrogated (see Figure 6), this difference is attributed to the more vigorous fuel and air mixing in the air annulus that likely accompanies the higher excess air level (condition 7). Clearly, factors associated with the fuel and air mixing are giving rise to this difference and exerting a significant impact on the NOx emission under these high efficiency conditions. SUMMARY AND CONCLUSIONS The results reveal that the burner performance is sensitive to the fuel and air interaction in a complex manner that has yet to be fully delineated and understood. The three fuel injection strategies investigated (co-swirl, radial, and counter-swirl) represent a varying amount of swirl disruption and mixing agitation in the air annulus. Emissions for the three injectors, measured over their stability ranges, reveal dramatically different responses in terms of NOx and CO emissions. A performance function was defined and used to identify conditions of interest. High and low performance, excess air, and swirl conditions were selected for detailed laser anemometIy measurements. The conclusions are as follows: • At a fixed excess air level and given load, NOx emissions in this model industrial burner decrease with increasing recirculated mass. • A reduction in NOx emission is typically accompanied by a decrease in combustion efficiency. • This notwithstanding, by employing select fuel and air mixing strategies, it is possible to achieve a reduction in NOx emission without sacrificing combustion efficiency. • Such conditions are limited and occur (in the present example) near the stability limit of the burner. • The data suggest that the successful accomplishment of reduced NOx emission and high combustion efficiency is complex and associated with the near-field fuel and air mixing processes. 13 |