Tuned Passive Control of Acoustic Damping of Perforated Liners

To suppress combustion instabilities, perforated liners can be fitted along the bounding walls of a combustor to provide acoustic damping. These liners are typically subjected to a low-Mach-number bias flow (a cooling flow through perforated holes), and they tend to be effective only over narrow frequency ranges. To investigate the damping effect of perforated liners on plane acoustic waves and to increase their effective frequency range, experiments and numerical simulations are carried out. An acoustically driven pipe system containing a lined section was designed and experimentally tested. The length of the pipe system, along with the bias flow rate, could be varied. The experimental results showed that the liner damping depended on both the pipe length and the bias flow rate, in agreement with predictions from the numerical model presented by (Eldredge, J. D., and Dowling, A. P., "The Absorption of Axial Acoustic Waves by a Perforated Liner with Bias Flow," Journal of Fluid Mechanics, Vol. 485, No., 2003, pp. 307-335.). To maintain the acoustic damping of the liner in the presence of large frequency changes (corresponding to instability frequency changes in a combustor), real-time tuning of perforated liners was experimentally investigated. Both a pipe length parameter and the bias flow rate were sequentially tuned using a multiple-parameter tuning scheme. The scheme required two algorithms to be developed: one for characterizing the liner's acoustic damping in real time and another for sequentially determining the two optimum actuation signals for the damper tuning. The former involved developing a real-time version of the two-microphone technique for resolving the two plane acoustic wave strengths, which is widely applicable. On implementing these algorithms in the pipe system, optimal damping of the liner was achieved and maintained over a broad frequency range.