Impact of the heat release distribution on high-frequency transverse thermoacoustic driving in premixed swirl flames

Self-excited, high-frequency first transversal thermoacoustic instabilities in a cylindrical combustion chamber equipped with a premixed swirl-stabilized flame are investigated. Phase-locked image analysis of the phenomena shows the displacement of the flame and a higher burning rate in the region of elevated pressure. The impact of diffuser angle and fuel composition on the stability limits and the flame position is investigated. The Rayleigh-Index is computed for a three-dimensional domain based on analytical flame transfer functions for experimentally obtained data of OH*-chemiluminescence as measure for the spatial heat release. Two models from different sources are applied, which describe the interaction between flame and acoustic locally. The axial dependence of the amplitude of the transversal mode is computed by a numerical model, which takes the temperature distribution inside the combustion chamber into account. The comparison of the Rayleigh-Index of different operation points shows a correlation with the stability limits for some, but not for all investigated configurations.

Experimental Study of the Local Response of a Low Swirl Flame to Acoustic Perturbation

A low swirl burner is used to examine thermo-acoustic instability under different perturbation level. For a constant forcing frequency of 125 Hz, a low swirl flame with the velocity of 5 m/s is used to examine the effect of the driving pressure amplitude (0.045–0.649 % of atmospheric pressure) on the global and local flame response. Rayleigh Index is used in current research to show the local and global coupling level between the heat release and the pressure oscillation. The toroidal structures found in the Rayleigh Index map indicate that the coupling strength increases with the perturbation level and then stays stable. At the same time, the coupling location doesn’t change much with the forcing amplitude. In addition, the root mean square of the Rayleigh index (RRMS) exhibits a linear region and a nonlinear region when perturbation magnitude rises. Besides the global response, the local RRMS and locally-weighted RRMS are studied. The contributions of the positive and negative toroidal structures on the global RRMS are similar in current case. Locally phase-averaged OH-PLIF data are used to investigate the local heat release variation trend. Phase analysis shows that the local phase is not sensitive to the perturbation level, which induce that the Rayleigh Index is more dominated by the oscillation magnitude of the heat release.

Numerical Investigation of Combustion Instability in a V-Gutter Stabilized Combustor

The stability of the combustion process in a V-gutter stabilized combustor is numerically investigated. To this end, 3D compressible turbulent and unsteady reacting flow calculations have been carried out using LES. The time history of the pressure at several locations is used to determine the frequency and amplitude of the oscillations along with the mode shapes. A shift in the dominant mode of the frequency spectra from the acoustic mode to the hydrodynamic mode is observed. A POD analysis of pressure time histories on the symmetry plane also corroborates this trend. The computational domain is divided into several subvolumes in the wake region of the V-gutter and the time histories of pressure, temperature, and heat release are collected in the individual volumes. It is seen that the fluctuation of pressure and heat release tend to oscillate from being in phase to out of phase over a time period. Unstable regions predicted by the Rayleigh index across a plane are shown to be different from those predicted in a volume owing to the three dimensionality of the flame. Quite interestingly, the calculated values of the indices show the combustor to be most unstable for an equivalence ratio of 0.1665 which is not the leanest one considered here. The global Rayleigh index is shown to correlate well with the amplitude of the dominant mode.