On the design, realization, and preliminary run of a novel grazing flow test rig for acoustic liners’ assessment under high temperature condition

The installation of acoustic liners can represent an effective strategy for noise reduction in aero engines. The performance of an acoustic liner as a sound absorbing device is expressed by the acoustic impedance which mainly depends on its geometry and on the peculiar flow conditions acting on its surface. Several parameters are involved in the estimation of the impedance through a non-trivial dependence; this is even more complex when such devices are installed within a high-speed and high-temperature flow field such the one characteristic of the core nozzle of an aeroengine. Within such a scenario the experimental investigation is fundamental in order to assess the liner performance accurately. The most reliable way to determine the acoustic impedance is through a grazing flow rig which allows to experimentally investigate the acoustic liner performances under different flow conditions by using a controlled acoustic signal as excitation. At the Department of Industrial Engineering in Florence, a novel grazing flow rig for testing liners at flow conditions representative of the Low-Pressure Turbine (LPT) exit, i.e. at high speed and high temperature, has been designed and realized. The present work describes the design of the test rig and the preliminary experimental outcomes achieved for assessing the rig performances. The design of all the main components has been addressed following a comprehensive approach in order to meet the acoustic and aerodynamic requirements and by considering the thermo-structural effects as well. The design process has been supported by the use of Finite Element Analyses, the main results and the design solutions achieved are shown. As the rig is supposed to work under high temperature conditions, the sensors to be used for collecting the acoustic data shall withstand the hot flow as well. Hence, an experimental analysis has been carried out in order to compare the main market solutions available. As a result, the most suitable measurement device was identified. Once the design and the construction of the rig was finished, a preliminary set of tests was performed to prove the eligibility of the rig for the designed purpose. The uniformity of the flow field into the test section has been verified through a preliminary measurement of the flow profile. From the acoustic point of view, the reflection coefficient was measured at the duct ends and the results demonstrated very low reflections proving the effectiveness of the designed anechoic terminations. In addition, a first test on an acoustic liner sample was performed, i.e. the transmission loss was measured at cold flow condition. The overall results demonstrate that the rig is properly designed for the desired purposes.