Azimuthal Modes Measurement in an Intake Duct for a Turbofan Engine

The paper describes the acoustic mode measurements in a turbofan engine duct, performed with a sparse circumferential 100 - microphone array. The array was initially tested in the laboratory conditions in the anechoic chamber of Perm National Research Polytechnic University (PNRPU) with a mode synthesizer. It has been demonstrated that the measurement system correctly determines the synthesized modes in the duct. Then the array was installed on a turbofan engine, and the azimuthal structure of sound field in the engine intake duct was measured during static engine tests. The dominant modes were consistently determined for different engine runs albeit their amplitude changed from run to run, probably due to the absence of a turbulence control screen during the tests. As a result, it has been concluded that the measurement system can be used for measuring azimuthal structure of sound field in the ducts of new turbofan engines and thus can be instrumental in enhancement of the acoustic liner performance in turbofan engine noise reduction. A feasibility study on determining azimuthal modes in the engine duct with an array of microphones located in a limited angle range has been performed.