Experimental investigation on effusion liner geometries for aero-engine combustors: evaluation of global acoustic parameters

In new generation aero-engines based on the innovative lean combustion technology, thermoacoustic instabilities are one of the most important issues and their prevention and reduction are challenging goals. To achieve these targets, the use of multi-perforated liners, that have to primarily provide an efficient liner cooling, is very attractive because they are efficient passive dampers of pressure fluctuations, especially with bias flow. The design of multi-perforated liners for both thermal and acoustic purposes can be accomplished by selecting liner parameters, such as hole diameter, pattern and inclination, main and bias Mach numbers, fulfilling both requirements; this procedure requires to assess the effect of both geometrical and fluid-dynamic features. Thus, a specific research project is ongoing on the acoustic and thermal experimental characterization of selected multi-perforated liner geometries. In this paper, the complete experimental campaign on the acoustic behavior of the aforementioned liners has been carried out in the planar wave field range, that is of main concern in aero-engines. For this purpose, an innovative modular test rig has been designed to characterize test cases at ambient conditions, changing bias and main flows up to operating engine conditions. Liner geometries account for 3 different hole diameters, 5 different patterns and 2 hole inclinations, ranging within typical cooling system values; tests were performed with the two-source multi-microphone technique to evaluate global acoustic parameters independently from test rig boundary conditions. The acoustic performances of liners are discussed in terms of the energy dissipation coefficient.