The K-H Instability of Reacting, Acoustically Excited Bluff Body Shear Layers

Abstract

This paper describes an experimental study of the effect of acoustic excitation on bluffbody 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 therefore requires an understanding of the generation, convection, and dissipation of these vortical disturbances. The evolution of these vortical disturbances is strongly influenced by the presence of combustion due to enhanced diffusivity in the hot products, volume dilatation, and baroclinic torque. PIV measurements are reported of the decay of these vortices over a range of conditions, which suggest that the high product diffusivity controls the reduction in vorticity amplitude downstream. Of particular significance is the relative location of the flame and vortex sheet. If the vortex sheet is inside the hot products, it dissipates much more rapidly than if it lies in the reactants. In addition, experiments were performed with two bluff bodies, one with a triangular cross section and another with a circular cross section. The triangle has a well defined separation point, leading to phase locked and transversely symmetric vorticity and flame wrinkling. In contrast, while instantaneous images from the circular bluff body look similar to those of the triangle, overlays from cycle to cycle reveal a substantial amount of phase jitter in the vortex sheet, and therefore flame position. Vorticity fluctuations of comparable magnitudes are generated instantaneously for both bluff body shapes, but the spatial jitter leads to reduced ensemble averaged amplitudes for the circular bluff body. This phase jitter is reduced by increasing the amplitude of acoustic excitation.

Fuel and Equivalence Ratio Effects on Transfer Functions of Premixed Swirl Flames

“…In particular, Bunce et al [18] suggested that the closer the flame is to the shear layer near the nozzle tip, the weaker is the AGV. This could be attributed to the increase of the temperature in the shear layer due to its closer proximity to the flame, which could affect gas expansion, baroclinic generation of vorticity, and viscous diffusion, and thereby reduce the circulation of the AGV [18,62,[67][68][69][70][71][72][73]. Note that there is a small difference between the two fuels.…”