Identification of unstable coherent modes in reacting swirling flows and their control

Abstract

The identification of multiple interacting coherent structures in the flow of a swirlstabilized combustor is discussed in this article. We focus on the interaction of axisymmetric flow structures that are generated by acoustic forcing and helical structures that are self-excited by the flow field. Within the context of swirl-stabilized combustion, the helical structure is known as the precessing vortex core (PVC). The interaction of these two structures is of mayor importance for the understanding of thermoacoustic instability of swirl flames. In the present article, the interaction of the PCV with the forced axisymmetric structures is investigated experimentally. Special focus is put on the identification of the coherent structures in PIV data. We consider time-resolved PIV data at an operating condition, where neither classic proper orthogonal decomposition nor Fourier mode decomposition provide useful insight into the flow dynamics. We then apply Spectral Proper Orthogonal Decomposition (SPOD), a novel method that provides a spectral constraint to the POD modes. It is show that SPOD is superior to the classic methods. The SPOD modes can be unambiguously assigned to the actuation and the PVC over the entire range of forcing amplitudes. The SPOD clearly shows that with increasing forcing amplitudes, the PVC dynamics are first weakened and then modulated by the forcing. Classic POD and Fourier decomposition suffer from the fact that the PVC dynamics of the forced flow are relatively weak and spread over multiple frequencies.