Helical-mode excitation of lifted flames using piezoelectric actuators

Experiments of helical excitation using piezoelectric actuators on jet flow s and lifted flames are performed to enhance the understanding of the effec ts of vortical structures of various instability modes on the stabilization mechanism of the lifted flame. In addition to the common ring and braid st ructures, five or seven azimuthal fingers (or lobes) can be identified in t he transverse image of the jet near field. Excitation with various helical modes enhances the azimuthal structures and entrainment in the near field. When helically excited with the asymmetric m = 1 mode, one of the fingers i s enhanced and may evolve into a strong streamwise vortex. The streamwise v ortices generated in the braid region between the adjacent ring vortices ma y enhance fuel-air mixing due to additional azimuthal entrainment upstream of a lifted flame when helically excited with the m = 1 mode. Therefore, th e streamwise vortex serves as an additional path of high probability of pre mixed flammable layer for the upstream propagation of the lifted flame so t hat the flame base on one side of the lifted flame may extend farther upstr eam and the flame base is inclined. In addition to the inclined flame base, multiple-legs phenomenon is also observed in the flame base, which is stro ngly associated with fingers of the helical modes of the jet flow.