A commonly used technique for mitigating thermoacoustic instability in an enclosed combustion environment is removing more acoustic energy from the combustor, at the frequency corresponding to the acoustic mode(s) of the combustor which are sympathetic to such instability. This approach is based on adding tuned acoustic damping to the combustion environment. By incorporating in-situ adjustability into acoustic damping devices, they can change their mechanical attributes, e.g., mass and/or stiffness, and adapt themselves in a semi-active manner to the varying instability frequency. Adaptive-passive thermoacoustic mitigation solutions have less weight penalty than the alternative active solutions mainly because the adaptation is done in a semi-active way, at slow pace, with a small and less power-hungry actuation mechanisms. Moreover, the flexibility they offer make them highly desirable for land and marine instability mitigation applications. In this work, semi-active adjustment of a novel tuned acoustic damper, namely an acoustic radiator, is explored. The paper describes the inner working of a semi-active (adaptive-passive) acoustic radiator and the relevant control schemes to adapt them to the instability frequency on hand. The damping effectiveness of the proposed damper, is demonstrated experimentally. It should be mentioned that the semi-active control strategies developed for acoustic radiators can also be used, with minor modifications, for semi-active control of other acoustic damping mechanisms such as Helmholtz resonators and quarter-wave tubes.
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