Disturbance Field Characteristics of a Transversely Excited Burner

Transverse acoustic instabilities in premixed, swirl-stabilized flames are an important problem in low NOx combustors. Transverse excitation of swirling flames involves complex interactions between acoustic waves and fluid mechanic instabilities. This paper presents high-speed PIV characterization of the flow field characteristics of a swirling, annular jet under reacting and nonreacting conditions. These data show that the flame response to transverse acoustic excitation is a superposition of acoustic and vortical disturbances that fluctuate in both the longitudinal and transverse direction. In the nozzle near-field region, the disturbance field is a complex superposition of short wavelength and convecting vortical disturbances, as well as longer wavelength transverse and longitudinal acoustic disturbances. Very near the nozzle, distinct vortical structures are evident that are associated with the separating inner and outer annulus shear layers. Their relative phasing on the left and right side of the burner annulus changes by 180° under conditions where the burner centerline is nominally at a transverse acoustic velocity node and antinode. These suggest that the dominant excited instability mode of the annular jet changes from axisymmetric to helical as the structure of the acoustic mode shape changes. Farther downstream, these structures disappear rapidly and the disturbance field is dominated by the longer wavelength, transverse acoustic field.