Modification of eigenmodes in a rocket combustion chamber by acoustic resonators under non-reacting conditions

The influence of an absorber ring on the acoustics of a rocket combustion chamber is studied numerically and experimentally under non-reacting conditions. The split modes which occur upon application of the absorber ring are evaluated in terms of their eigenfrequencies, damping rates and mode shapes from numerical calculations. Thereby a systematic variation of the resonators’ impedance is carried out to capture the structure of the eigensolutions. An analytic model for the absorber ring impedance is used to obtain predictions for certain absorber specifications. Based on numerical single resonator simulations, a modification of the absorber model is proposed. The results are validated with experimental data for different absorber lengths. The numerical approach consists in an eigenvalue analysis of the Linearized Euler Equations in frequency space. Good agreement is obtained for the predicted eigenfrequencies in which the absorber modification significantly improves the trends of the eigenfrequency develpoment via absorber length.