Experimental characterization of the acoustic response of cavity-backed perforated plates to control thermo-acoustic instabilities in gas turbines

Prediction and control of thermoacoustic instabilities is a major challenge in the development of modern power generation gas turbines and aeroengines. Such instabilities arise from the coupling between flame dynamics and combustor acoustic modes, resulting in severe oscillations that can lead to premature aging of combustor components and structural damage. In many combustors, passive dampers are implemented to increase the acoustic energy dissipation of the system and prevent the onset of these harmful flame-acoustic interactions. In the present study, passive damping systems based on a cavity-backed perforated plate are experimentally analyzed, with a focus on studying the impact of bias flow on the reflection coefficient over a wide range of frequencies. Tests are carried out on two cavity-backed perforated plates characterized by the same porosity but a different number of holes 25 and 49, namely P25 and P49, respectively. It is observed that, for a given plate geometry, a higher bias flow leads to an increase in the plates absorption capacity over a wider range of frequency. This is more pronounced in the P49 plate configuration. For both tested configurations, comparing the experimental results with Scarpato model proposed in the literature, a good match it is observed only for low values of bias flow. The model instead is not able to correctly capture the behavior of the damping systems when higher dissipation is reached.