Flame-Vortex Breakdown Interaction in Stable and Unstable Combustion

Abstract

Experimental combustion results with a lean direct-injection swirl stabilized fuel nozzle are presented, with focus on the interaction between the vortex breakdown occurring due to the sudden expansion of the swirling flow and the instantaneous flame position. Tests were run using the Triple Annular Research Swirler (TARS) which allows for a creation of a variety of flow fields and fuel distributions. Particle image velocimetry (PIV) was used to obtain the velocity field while OH* chemiluminescence was performed to identify the reaction region. Tests run for a single swirler configuration fitted with and without a premixing tube showed markedly different behavior. While a strong recirculation region was established for all cases without a mixing tube (except for very close to the lean blow-out point), the flame easily transitioned inside the mixing tube when it was present, even for moderate equivalence ratios. Proper orthogonal decomposition was also used to gain further insight on the main structures driving the flow. I. Introduction AS turbine engines have consistently improved performance regarding efficiency, durability, noise, and emissions. One of the latest trends is towards combustors operating in lean premixed or nearly premixed conditions due to the lower peak temperatures and better temperature profile, resulting in lowered NOx emissions and increased durability of downstream components. Among the drawbacks of this new approach is its propensity for unstable behavior due to the lack of near stoichiometric regions that anchor the flame, the more even mixture distribution that makes more regions inside the combustor sensitive to feedback mechanisms, and the increased solidity of the combustor walls which, having less cooling or dilution holes than conventional combustors, reflect more acoustic energy back into the flame region. This increases the probability of coupling between the heat-release rates and the acoustic field, resulting in instability growth. Research towards understanding the mechanisms that drive combustion instabilities is ongoing in order to enable the reliable operation of lean combustors. Swirl-stabilized flames are the norm in gas turbine combustors, with the combination of swirl and sudden expansion creating a recirculation zone that anchors the flame and transports fresh mixture to the reaction zone. The current study focuses on such injectors and the interaction of the recirculation zones with the flame region. Observations are to be made through PIV and chemiluminescence and will further extend previous efforts in this field of interest.