Effect of Strong Azimuthal Swirl on Ignition and Light-Around in an Annular Combustor

Abstract

In this paper we investigate the effect of strong azimuthal swirl on ignition dynamics in a laboratory-scale annular combustor. Bulk azimuthal swirl was produced by a novel angled injector configuration, producing swirling jet flames oriented downwards towards the combustor backplane and in the azimuthal direction, replicating a simplified version of the SAFRAN spinning combustor concept. To provide more realistic flow conditions, the design included Rich-Quench-Lean (RQL) staging via a circumferential distribution of dilution ports and an effusion cooled combustor backplane. High-speed imaging and an azimuthal array of photomultipliers to measure OH* chemiluminescence were used to characterise the ignition dynamics for different injector velocities and global equivalence ratios. The mass flows through the injectors, dilution ports, and effusion cooled backplane were independently metered so that the injector equivalence ratio and global equivalence ratio could be separately controlled. The light-around times were found to have no clear correlation with the injector velocity since the rich injector equivalence ratio meant the flame burned in a non-premixed mode even though the global equivalence ratio was lean due to the RQL staging. However, it was found that lower injector velocities extended the lean ignition limit based on the global equivalence ratio. The ignition sequence during light-around (order in which the injectors are ignited) was found to be highly repeatable, igniting each consecutive injector in the anticlockwise direction (the direction of bulk swirl). In rare cases, the ignition sequence was observed to branch in both directions. Finally, in an effort to extend the lean ignition limit, the effect of azimuthal staging was investigated. Two configurations were tested. In the first configuration, the injectors on one half of the annulus were operated at a fixed equivalence ratio whereas the other half of the annulus was operated at a different equivalence ratio. In the second configuration, every second injector had the same equivalence ratio. Both configurations extended the lean extinction limit but the first configuration was the most effective.