Thermo-Acoustic Instabilities and Shape Bifurcations of a Pilot-Only Spray Flame in a Staged Burner

Abstract Aircraft engine manufacturers develop new combustion technologies to comply with stricter regulations on pollutant emissions such as nitrous oxides (NOx) or soots. Staged systems coupled with lean premixed combustion show promising prospects to reduce these emissions through correct mixing of the fuel ahead of the combustion chamber but are more prone to thermo-acoustic instabilities. Hence guaranteeing the stability over the entire operating range is more challenging. The thermo-acoustic behaviour of lean staged systems is investigated experimentally on the laboratory-scale BIMER combustor, an atmospheric pressure test rig composed of a two-stage swirled injector reminiscent of industrial systems and fuelled with liquid dodecane. It uses a simplex atomizer in the pilot stage, multipoint injection in the main stage and can be externally forced using a siren device located upstream of the injector. This study focuses on the thermo-acoustic behaviour of the pilot stage of the BIMER combustor. Flame response to acoustic perturbations at different pre-heating temperatures (ranging from 383 to 473K) is assessed. To this end, Flame Describing Functions (FDF) are measured using acoustic forcing. An acoustic network model of the combustor using the L-? model for the injector is used to compute acoustic velocity fluctuations at the base of the flame. The accuracy of the model prediction is discussed. Measurements also display flame shape bifurcations from V- to Tulip-shape flames generated by acoustic forcing at certain frequencies and amplitudes.