Setup of a test rig for the characterization of devices for acoustic measurements in hot flow

The installation of passive noise control systems, i.e. acoustic liners, at the exit of the low pressure turbine (LPT) of a modern turbofan engine, represents a promising way to reduce LPT noise emission without affecting aerodynamic performances. In order to evaluate the noise reduction effectiveness of such systems, i.e. the acoustic impedance of the liner, grazing flow rigs are used. The impedance shall be estimated under test conditions representative of actual turbine operations, at this aim test rigs operating at high flow speed and high temperature shall be used. Within such context, the need to develop acoustic measurement devices withstanding with high temperature flows arose. At the Department of Industrial Engineering of the University of Florence the development of a measurement technique suitable for hot flow acoustic measurements is on-going. It consists of a microphone installed at the end of a cylindrical cavity fully or partly occupied by a sound absorbing material that, at the same time, isolates the microphone from the hot flow and damps the resonances excited in the cavity. The frequency response function (FRF) of such device has to be experimentally estimated, hence a dedicated test rig has been realized. This paper describes the test rig and the design solutions deployed for its realization. As the rig is expected to provide an almost anechoic environment and at the same time to ensure the proper estimation of the acoustic FRFs, a series of qualification tests has been performed. The tests demonstrated both the goodness of the design of the anechoic termination and the accuracy of the measurement system by using a rigid termination as a test case, results are showed in the paper. The influence of different variables, e.g. temperature and positioning error of the sensors, on the measured acoustic parameters has been investigated. The measurement technique and the good construction of the rig have been verified by measuring the frequency responses of simple air cavities and comparing them against well-known analytical solutions and FEM calculations.