Symmetry-induced degeneracies and symmetry breaking

A symmetry-breaking dampers' pattern resolves the degeneracy of a thermoacoustic mode (star), but may cause the growth rate of one of the split thermoacoustic eigenvalues to become larger, an unwanted effect.

 

Combustion in gas turbines and aeroengines takes place in combustion chambers that have a nominal (discrete) rotational symmetry. The presence of symmetries induces some thermoacoustic eigenvalues to be degenerate with multiplicity two. The two-dimensional subspace spanned by the degenerate eigenvalues manifests experimentally in oscillations with different spatial structures: spinning waves, standing waves, or mixed modes. Breaking the symmetry via geometrical or structural asymmetries (e.g. a mean azimuthal flow or the presence of Helmholtz dampers) causes the degenerate thermoacoustic eigenvalues to split. This poses constraints in the optimal location of dampers, since the (optimal) splitting of the eigenvalues depends on the symmetry properties of the installed pattern of dampers.

• PI: Dr. Alessandro Orchini
• Funding: ITN ANNULIGhT, European Commission under Marie Sklodowska-Curie Actions
• Collaborators: Prof. Jonas Moeck (NTNU Trondheim), Prof. C.O. Paschereit, Sylvain Humbert

Related Publications

S. Humbert, J. Moeck, A. Orchini, C.O. Paschereit, "Effect of an Azimuthal Mean Flow on the Structure and Stability of Thermoacoustic Modes in an Annular Combustor Model With Electroacoustic Feedback" Journal of Engineering for Gas Turbines and Power, 2021, Vol. 143, pp. GTP-20-1401. [link]

G. Mensah, L. Magri, A. Orchini, J. Moeck, "Effects of asymmetry on thermoacoustic modes in annular combustors: a higher-order perturbation study" Journal of Engineering for Gas Turbines and Power, 2019, Vol. 141, pp. GTP-18-1358. [link]