Studies on the Vorticity Field of Harmonically Excited Bluff-Body Flames

This paper describes an experimental study of the effect of acoustic excitation on bluff-body stabilized flames. The Kelvin-Helmholtz (KH) instability of the shear layer is excited due to the incident acoustics. In turn, the KH instability imposes a convecting, harmonic excitation on the flame, which leads to spatially periodic flame wrinkling and heat-release oscillations. Understanding the factors influencing these heat release oscillations requires an understanding of the generation, convection, and dissipation of these vortical disturbances. Phase locked particle image velocimetry was carried out over a range of conditions to identify the frequency response of the shear layer and the rate of decay of the KHI-generated vorticity. The length over which the decay occurs scales with the length of the recirculation zone of the bluff body, i.e. the length over which the velocity profile transitions from shear layer to wake. The flame influences this decay process through increases in product viscosity and gas expansion. The latter mechanism leads to a reduction in velocity gradient in the shear layer by reducing the magnitude of negative velocities within the recirculation zone. These results also revealed cycle-to-cycle variation in the position of the rolled-up vortices that increases monotonically in the downstream direction. This leads to fairly significant differences between instantaneous and ensemble averaged flow fields and, in particular, the decay rate of the vorticity in downstream direction. I.