Thermoacoustic instabilities of lean disc flames

Abstract

The present work evaluates impinging lean disc flames, stabilized on a Helmholtz resonator, under thermoacoustic oscillations. These unsteady flames were found to oscillate with either one single frequency (f) or two simultaneous frequencies (f,2f). The resonant frequencies f or 2f are coupled to the acoustic response of the burner (acting as Helmholtz resonator) through the interaction between the flame and the vortices, controlled by the Kelvin–Helmholtz instabilities of the jet shear layer. As a result of the flame–vortex interaction, the bottom part of the flame presents a “S” shape, verified by high speed imaging. Spectral analysis of the reconstitution of the tomographic images showed that any unsteady disc flame responds with a lower frequency f associated with the whole flame, and a higher frequency 2f associated with the bottom part (flame–vortex interaction region). Based on the experimental data and the above mathematical model, it was found that the system (burner–flame) oscillates with one or two frequencies depending on the H/U0 ratio. For the flame–burner resonance with a single frequency f, the coupling is controlled by the entire flame. On the other hand, when it oscillates with f and 2f, the coupling is controlled by the bottom part of the flame.