Combustion zone - Acoustic cavity interactions in rocket combustors

Analytical and numerical models are employed to assess the stabilizing impact of acoustic absorbers on liquid propellant rocket engine intrinsic stability. The tuning and stability behavior of cavities in which two-dimensionalflow and temperature variations exist are predicted through the application of an iterative-integral, Green's function method of analysis. Results of stability calculations performed for a variety of cavity geometries and temperature gradients indicate that adding a large open area absorber to a combustion chamber significantly alters the spatial shape of the oscillation. This alteration consequently affects the driving and damping mechanisms in the chamber. Because of this interaction, the maximal stabilizing effect of a cavity with a given open area does not in general occur for a design tuned in the acoustic sense to the frequency of the oscillation, but rather for a design that is not tuned but which tends to maximize the influence of the damping mechanisms in the chamber relative to the driving mechanisms. Additionally, absorbers with large backing cavities are shown to perform better over a wider range of operating conditions than absorbers with small backing cavities. Calculations also reveal that large temperature gradients in an absorber tend to reduce its broadband effectiveness.