Theoretical and Experimental Investigations on the Acoustic Absorptions of Baffled Injectors

Abstract

Baffled injectors are widely used to restrain transverse combustion instabilities in liquid rocket engines. However, there is still a lack of theoretical research on the attenuation mechanisms. In this work, a theoretical model accounting for the viscothermal effect has been constructed upon the narrow gap between two neighboring injectors. The acoustic wave equation for the pressure perturbation is derived, and the perturbation method is then used to derive its analytical solutions. Preliminary validation of the analytical solutions is performed by comparing them to the results by numerically resolving the wave equation. An experimental rig to measure the acoustic absorption coefficient of one modeled baffled injector unit is constructed in order to further validate the proposed theoretical models and analytical solutions. Good agreements have been observed between the analytical and experimental results. The normalized minimum gap between neighboring injectors of and the normalized frequency of are proposed for the analyses, where , , , and are the minimum gap, the viscous boundary-layer thickness, the angular frequency, the injector diameter, and the speed of sound, respectively. As the Strouhal number exceeds 0.1, the optimal ratio of to achieve the maximum acoustic absorption approximately equals 1.7 and does not rely on the forcing frequency, injector diameter, ambient pressure, and temperature.