Low-Order Acoustic Modelling for Annular Combustors: Validation and Inclusion of Modal Coupling

Abstract

A two-dimensional low-order model of a generic annular premixed combustor, comprising an annular combustion chamber connected to an annular plenum via a finite number of burners, is developed and validated. The shapes and frequencies of the eigenmodes as well as the stability of the combustor against self-excited oscillations can be predicted with such a model. The dynamical characteristics of each burner is described mathematically in terms of its transfer matrix. The case where the transfer matrices of individual burners differ from each other can be handled by the model formulation presented. This is important in situations where non-identical burners are used in an annular combustor as a means of passive control, or where nonlinear effects lead to non-identical burner behaviour. The resulting loss of axisymmetry enhances the coupling between nonplane acoustic modes of different order. This modal coupling is accounted for by the model. The eigenmode shapes and frequencies predicted by the low-order model are validated by comparison with the results of a three-dimensional finite element acoustic model of a generic annular combustor configuration.