Modification of eigenmodes in a cold-flow rocket combustion chamber by acoustic resonators

The influence of an absorber ring on the acoustics of a rocket combustion chamber is studied numerically and experimentally under nonreacting conditions. The transverse split modes that 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 of an eigenvalue analysis of the linearized Euler equations in frequency space. Good agreement is obtained for the predicted eigenfrequencies in which the absorber model modification significantly improves the trends of the eigenfrequency development via absorber length.