Central Recirculation Zone Visualization in Confined Swirl Combustors for Terrestrial Energy

This study characterizes the central recirculation zones formed under combustion conditions, with natural gas as fuel with different geometries and degrees of premixing using a swirl combustor firing into a confinement representative of gas turbine combustors. Phase-locked particle image velocimetry is used as the main method of characterization. The technique enables characterization of the time-dependent behavior of the central recirculation zone and a three-dimensional reconstruction of its boundaries. The central recirculation zone typically had an asymmetric lobed shape and precessed about the central axis. Partially premixed combustion at near-stoichiometric equivalence ratios reduced the coherence of the central recirculation zone and often caused it to nearly disappear, although a small remnant of recirculation could still be found in the three-dimensional space. Lower equivalence ratios strengthened the central recirculation zone considerably, for both non-premixed and partially premixed combustion. Although the central recirculation zone was asymmetric in shape and precessed about the central axis, the precessing vortex core, commonly found in these flows, was found to be significantly suppressed especially when central fuel injectors were used. Its occurrence then became intermittent and irregular. A quarl exit nozzle with a divergent lip was found to reduce flame attachment to the fuel injector.