Effect of Nonlinear Losses on the Design of Absorbers for Combustion Instabilities

Investigations were conducted to provide a basis for the design of multiple-resonator absorbers to control high amplitude combustion instabilities (screech) in high output combustors. An impedance tube technique was employed to determine the effect of high amplitude sound pressure waves ou the absorption characteristics of absorber configurations having open areas varying between 0.5 and 10 per cent. Frequencies were varied between 200 and 1200 cps. It was found tha t the acoustical impedance of the absorbers contains a nonlinear facing resistance which depends on the oscillating velocity and particle displacement in the absorber neck, and tha t the nonlinear resistance can generally be correlated with the amplitude of the incident pressure wave plotted on a decibel scale. A design procedure is presented in which the absorption characteristics of perforated liners for a particular sound wave amplitude can be calculated from known physical dimensions.