Design Guidelines for Swirl-Venturi Fuel-Air Mixers for Lean Direct Injection Combustors
Design guidelines for aircraft gas turbine combustors are provided based on the swirl-venturi lean direction injection (SVLDI) scheme. The guidelines are developed through a combination of literature review and internal research at NASA GRC that sought to develop an understanding of how LDI geometry affects NOx emissions, minimum stable f/a, and combustion dynamics. Key objectives are to improve low power operability by reducing the fuel/air ratio achievable with the SV-LDI concept and to reduce NOx emissions for both the subsonic landing-takeoff cycle and supersonic cruise conditions. The design of any combustion system will be a set of compromises between flame stability and decreasing combustion emissions. This document is to serve as a guide for selecting the best options given the overall mission. This document is organized as follows. In section 1, we introduce and describe the SV-LDI concept as a way to reduce NOx emissions. Section 2 provides descriptions of the individual components that make up a single swirl-venturi LDI element — the venturi, swirler, and fuel injector and their primary purposes. Considerable focus is applied to how changes on the air side components affect the flow field. Section 3 examines variations within a single SV-LDI element swirler: blade thickness, presence of a diffuser section, position of fuel nozzle, et cetera. We include examples from the literature to emphasize key aspects of these changes. In Section 4 we consider the interaction of a single element with the combustion chamber and with neighboring elements. In Section 5, we look at fuel staging effects on emissions, combustion efficiency, and combustion dynamics. Throughout these sections, we will highlight any findings we consider universal, and in Section 6 we consolidate those findings to provide guidelines for use in SV-LDI designs.