Analysis and control of combustion instabilities by adaptive reflection coefficients

With the growing development of lean premixed combustion, prediction of combustion instabilities becomes a key issue. Amongst the numerous problems to overcome, the influence of the acoustic boundary conditions on the development of instabilities remains a difficult issue. These boundaries are usually not known and difficult to measure in reacting systems. However, they are a key element in the coupling since they strongly influence both the unsteady pressure and flow fields inside the combustion chamber. In the present study, an original passive control method of the upstream acoustic boundary condition is proposed. Our aim is to provide a tunable and predictable acoustic boundary condition, where the reflection coefficient can be adjusted from0 to 1 at each frequency in the range below 1 kHz to study the influence of the acoustic boundary condition on self-sustained oscillations, without any external flow forcing devices. Acoustic properties of this system were first tested in a cold gas setup. Results for the reflection coefficient measured with a three microphones technique are in good agreement with predictions. This system was then integrated in a combustor and it was shown that combustion instabilities could be strongly damped using the adapted set of parameters.

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