Acoustic characteristics of impingement cooling sheets; effect of bias-grazing flow interaction on the liner impedance in a thin annulus

This study investigates the acoustic characteristics of impingement sheets that are typically installed around can-combustors in stationary gas turbines for cooling purposes. From an acoustic point of view the impingement sheet is a perforated liner backed by the large compressor plenum of the gas turbine. The annular gap between the can and the liner is often only a few millimeters wide to allow for a strong jet impingement on the outer combustor wall. Due to the small cross sectional area of the annular gap, the grazing flow is strongly accelerated along the perforated liner. In this study, we present an acoustic model for impingement sheets, which accounts for bias-grazing flow effects. We experimentally investigate liners with four different porosities between 2% and 5%, in the frequency range between 60 Hz and 700 Hz. The bias flow Mach number is varied between 0.03 and 0.25, the axially varying grazing flow Mach number between 0 and 0.14 and setups with and without jet impingement on a solid center-body are investigated. Measured reflection coefficients agree well with predictions of the presented model. A significantly increased influence of grazing flow on aperture resistance, compared to existing studies without jet impingement, is noted. The acoustic damping potential of impingement sheets is elaborated and a condition of maximum damping in the low frequency limit is derived, which agrees well with the experimental observations. The results of this work provide the fundamentals for the acoustic analysis and acoustic design of impingement cooling sheets.