Azimuthally forced combustion instabilities in an annular combustor across different operating conditions and fuels

Gas turbines running in low-emissions configurations (using alternative fuels in lean premixed mixtures) are known to exhibit self-excited azimuthal combustion instabilities. Previous work by Nygård et al. (2019 & 2021) has shown that these instabilities can be created and sustained in a lab-scale annular combustor by using a circular array of speakers. Furthermore, Nygård et al. developed the Azimuthal Flame Describing Function (AFDF) formalism to characterise azimuthal instability modes in a quantitative manner. Although their work covered a large range of instability mode types, it only focused on a single fuel mixture of ethylene operating at a single mean bulk velocity.

In the present study, AFDFs are measured in the same combustor using the same acoustic forcing procedure as Nygård et al. (2021), but for ethylene at various equivalence ratios and for various bulk velocities. The same is also done for hydrogen-methane mixtures, with the addition of varying the hydrogen power fraction. This parameter study is done to test and improve the current understanding of AFDFs and methods for their measurement. Such a parameter study, by showing which instability modes are preferred under which conditions, offers a better understanding of the operational stability of gas turbines.