Response of an annular burner nozzle to transverse acoustic excitation

Transverse combustion instabilities are increasingly problematic in land based and aerogas turbine engines. A common problem in rockets and augmenters, these transverse modes have a circumferential and/or radial structure and are often associated with high frequency oscillations. Depending on its location in the combustor, a flame experiences different parts of a standing circumferential acoustic wave, or in the case of a spinning waveform, a flame sees different parts of the wave structure over time. The current study investigates the response of an annular, swirling nozzle to two limits of the standing wave transverse excitation structure, associated with a pressure node and antinode at the nozzle centerline. The resulting disturbance field is significantly different for these two cases. This paper presents results showing how the disturbance field is composed of long wavelength, multi-dimensional acoustic disturbances and shorter wavelength convecting vortical disturbances. The flow vorticity originates in the separating boundary layers of the inner and outer annulus and rolls up into larger structures inside and outside of the annular jet. These structures, in turn, merge downstream in a staggered fashion into a single, larger vortex that convects downstream in the annular jet centerline. This paper quantifies the relative strengths and phasing of these disturbances, and discusses the key features of the disturbance field exciting the flame.