Research on Hot Stream Liners for Acoustic Damping at DLR

Thermoacoustic instabilities remain of great concern for the design of modern gas turbine combustors, especially due to the application of lean burn concepts, the need for fuel flexibility, and a wide operating range with respective safety margins. Combustor oscillations can cause severe damage to the combustor itself and adjacent components. Thus, gas turbine manufacturers – which includes both aero-engines and industrial gas turbines – have developed a range of concepts and devices in order to avoid, suppress, or reduce these instabilities. One of those concepts applicable for gas turbine combustors relies on the damping of the occurring pressure fluctuations in order to weaken the coupling with the unsteady heat release to suppress the instability growth. Since most of the current combustors need some dilution and cooling air somewhere downstream of the primary combustion zone, the feeding system for this secondary air could be designed in a way to provide appropriate damping for the pressure fluctuations as well. Here, the research activity of the s at the German Aerospace Center (DLR) in the field of damping liners, especially for hot stream application, is presented. The results of extensive experimental studies on the influence of the various liner parameters like for instance perforation ratio, bias flow and cavity design will be shown. Since the liner design for real engine application requires also the damping dependencies on pressure and temperature, the DLR Department of Engine Acoustics built up a new test facility, the Hot-Acoustic-Testrig (HAT). This testrig allows the investigation of damping characteristics and cooling performance of hot stream liner segments. The features of this test facility will be explained in detail