The effect of airfoil clocking on noise generation and propagation in a two shaft test turbine

The paper deals with the investigation on the acoustics in a two-stage two-spool test turbine located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) of Graz University of Technology regarding six distinct relative positions between the HP stator and the struts of a Turning Mid Turbine Frame (TMTF). The facility is a continuously operating cold-flow open-circuit plant which is driven by pressurized air and which allows the acoustic measurements to be performed at engine relevant flow conditions (Mach number similarity). The flow path consists of a transonic turbine stage (HP) followed by a low pressure turbine stage consisting of the turning mid turbine frame (TMTF) and a counter-rotating low pressure rotor. Compared to the setup within the EU-Project DREAM, the rig was upgraded by fully circumferentially traversable measurement sections at the inlet of the TMTF as well as downstream of the LP-turbine. Due to the fact that noise emitted by aero engines became a very important issue especially during the last few years, acoustic measurements were carried out downstream of the low pressure turbine for six stator-stator clocking positions. The aim of these analyses was to find an ideal vane-vane position where sound emissions can be reduced compared to other positions and thereby finding a quite simple possibility for making aero engines more quiet respectively reducing the amplitudes of certain modes which not only influence acoustics but can also have a big impact on the aeroelastics of an aero engine. In order to evaluate the noise emission of the turbine the outflow duct of the facility was instrumented with a new acoustic measurement setup which uses a circumferentially traversable microphone array located at the outer casing. The acoustic field was characterized by azimuthal and radial modes determined by traversing the microphone array over 360 degrees and therefore the emitted sound power levels at certain significant radial modes are compared for different vane-vane positions.

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