Interaction Between the Acoustic Pressure Fluctuations and the Unsteady Flow Field Through Circular Holes

Gas turbine combustion systems are prone to thermo-acoustic instabilities, and this is particularly the case for new, low emission, lean burn type systems. The presence of such instabilities is basically a function of the unsteady heat release within the system (i.e. both magnitude and phase), and the amount of damping. This paper is concerned with this latter process and the potential damping provided by perforated liners and other circular apertures found within gas turbine combustion systems. In particular the paper outlines experimental measurements that characterise the flow field within the near field region of circular apertures when being subjected to incident acoustic pressure fluctuations. In this way the fundamental process by which acoustic energy is converted into kinetic energy of the velocity field can be investigated. Experimental results are presented for a single orifice located in an isothermal duct at ambient test conditions. Attached to the duct are two loudspeakers that provide pressure fluctuations incident onto the orifice. Unsteady pressure measurements enable the acoustic power absorbed by the orifice to be determined. This was undertaken for a range of excitation amplitudes and mean flows through the orifice. In this way regimes where both linear and non-linear absorption occur along with the transition between these regimes can be investigated. The key to designing efficient passive dampers is to understand the interaction between the unsteady velocity field, generated at the orifice, and the acoustic pressure fluctuations. Hence experimental techniques are also presented that enable such detailed measurements of the flow field to be made using PIV. These measurements were obtained for conditions at which linear and non-linear absorption was observed. Furthermore, Proper Orthogonal Decomposition was used as a novel analysis technique for investigating the unsteady coherent structures responsible for the absorption of energy from the acoustic field.