Helical-mode excitation of lifted flames using piezoelectric actuators

 Experiments of helical excitation using piezoelectric actuators on jet flows and lifted flames are performed to enhance the understanding of the effects of vortical structures of various instability modes on the stabilization mechanism of the lifted flame. In addition to the common ring and braid structures, five or seven azimuthal fingers (or lobes) can be identified in the 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 is enhanced and may evolve into a strong streamwise vortex. The streamwise vortices generated in the braid region between the adjacent ring vortices may enhance fuel-air mixing due to additional azimuthal entrainment upstream of a lifted flame when helically excited with the m=1 mode. Therefore, the streamwise vortex serves as an additional path of high probability of premixed flammable layer for the upstream propagation of the lifted flame so that the flame base on one side of the lifted flame may extend farther upstream 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 strongly associated with fingers of the helical modes of the jet flow. Received: 21 August 1997/Accepted: 24 January 1999