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In order to reduce NOx emissions, modern gas turbines are often equipped with lean-burn combustion systems, where the high-velocity fuel-lean conditions that limit NOx formation in combustors also inhibit the ability of ignition, high altitude relight, and lean combustion stability. To face these issues, internally staged scheme of fuel injection is proposed. Primary and main fuel staging enable fuel distribution control, and multi-injections of main fuel lead to a fast and efficient mixing. A fuel-staged low emission combustor in the framework of lean-burn combustion is developed in the present study, i.e., the central pilot stage for low power conditions is swirl-cup prefilming atomization, the main stage is jet-in-crossflow multi-injection, and a combination of primary and main stage injection is provided for higher power output conditions. In lean-burn combustors, the swirling main air naturally tends to entrain the pilot flame and quench it at low power conditions, which is adverse to the operability specifications, such as ignition, lean blow-out (LBO), and high-altitude relight. In order to investigate the effects of the main swirl angle on combustion performances, the ignition and LBO performances were evaluated in a single dome rectangular combustor. Furthermore, the spray patterns and flow field are characterized by kerosene-planar laser induced fluorescence and particle image velocimetry (PIV) to provide insight into spray and combustion performances. Flow-flow interactions between pilot and main air streams, spray-flow interactions between pilot spray and main air streams, and flame-flow interactions between pilot flame and main air streams are comprehensively analyzed. The entrainment of recirculating main air streams on pilot air streams enhances with the increase of main swirl angle, because of the upward motion and increasing width of main recirculation zone. A small part of droplets are entrained by the recirculating main air streams at periphery of combustor and a majority of droplets concentrate near the centerline of combustor, making that entrainment of recirculating main air streams on pilot spray and quenching effects of recirculating main air streams on pilot flame are slight, and the extinguishing effects can be ignored. The contributions of main swirl strength to improvement of ignition and LBO performances are due to enhancement of air/fuel mixing by strengthening turbulence level in pilot zone. [ABSTRACT FROM AUTHOR]

It is widely recognized that the fuel-air mixing process is a critical factor in improving combustion efficiency and in minimizing pollutants such as NOx. Enhancement of fuel-air mixing can lead to lower pollutant emissions and greater efficiency. However, swirling flows in lean combustors play the role of fuel-air mixing and flame stability. The complex fluid dynamic phenomena encountered in swirling two-phase flow contribute to the difficulty in complete understanding of the different processes occurring in combustors. Fortunately, optical and laser-based visualization techniques available in our lab are important nonintrusive tools for visualizing flow process, especially for fuel injection and fuel-air mixing. To provide for a better understanding of effects of counter-rotating flow on droplets in atomization process, this study is a detailed characterization of the spray generated by an airblast atomizer by planar laser sheet imaging method. Optical facility for spray diagnostics with fuel planar laser induced fluorescence (fuel-PLIF) method for fuel distribution and particle image velocimetry (PIV) method for the velocity of droplets is used to evaluate the performance of an airblast atomizer. The results show that the performance of secondary atomization is influenced by swirling flow and primary atomization simultaneously; the swirling flow exhibits significant influence on the droplet size and space distribution relative to that of primary atomization. The primary swirling air reopens the spray cone generated by pressure-swirl atomizer, and the secondary swirling air affects the fuel distribution by forming the recirculation zone. The results provide critical information for the design and development of combustion chambers. [ABSTRACT FROM AUTHOR]