Computation of azimuthal combustion instabilities in an helicopter combustion chamber

This work presents a detailed study of the first azimuthal mode of an annular helicopter combustor. First it is shown that the casing and geometrical details such as primary holes have to be accounted for to produce correctly the acoustics of the combustor. Then a methodology to compute azimuthal combustion instabilities is presented. It requires a thermoacoustic model using a n − τ formulation for the coupling between acoutics and combustion. The parameters n and τ are computed from a LES of a single sector of the combustor. The fields of n − τ model is extended for this combustor under the Independence Sector Assumption in Annular Combustor (ISAAC). Then the stability of the first azimuthal mode is investigated.