On the acoustics of a turning mid turbine frame with embedded design in a two-stage test-turbine

The paper deals with the investigation of the noise generation in the two-stage two-spool test turbine located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) at Graz University of Technology. The facility is a continuously operating cold-flow open-circuit plant which is driven by pressurized air. The flow path is formed by a transonic turbine stage (high pressure, HP) followed by a low pressure (LP) turbine stage consisting of a turning mid turbine frame and a counter-rotating LP rotor. Downstream of the low pressure turbine the measurement section is instrumented with acoustic sensors. The acquisition system consists of a fully circumferentially traversable microphone array located at the outer casing. Two configurations of turning mid turbine frames were tested. The baseline is an intermediate turbine duct with 16 turning struts. The second one is a new embedded concept for the turning mid turbine frame with two zero-lift splitters placed in the struts’ passages. In total 48 vanes (16 struts plus 32 splitter vanes) guide the flow from the HP rotor to the LP rotor. In order to determine the noise emission of both configurations the microphones signal spectra and the emitted sound power level are compared. The acoustic field is characterized by azimuthal and radial modes by means of a microphone array traversed over 360°. In the multi-splitter configuration, the overall sound power level depending on the blade passing frequency of the HP turbine is reduced by 7 dB and depending on the blade passing frequency of the LP turbine by 4 dB, respectively. The overall effect is a reduction of the acoustic emission for the turning mid turbine frame with embedded design.