This paper deals with the investigation on the acoustics of different turning mid turbine frames (TMTF) in the two-stage two-spool test turbine located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) of Graz University of Technology. The facility is a continuously operating cold-flow open-circuit plant which is driven by pressurized air. The flow path consists of a transonic turbine stage (HP) followed by a low pressure turbine stage made of a turning mid turbine frame (TMTF) and a counter-rotating low pressure rotor. Downstream of the low pressure turbine a measurement section is instrumented with acoustic sensors.
Three TMTF setups have been investigated at engine like flow conditions. The first configuration (C1) consists of 16 highly 3D-shaped turning struts. The goal of the second design (C2) was to reduce the length of the TMTF by 10% without increasing the losses and providing comparable inflow to the LP turbine rotor. This was achieved by applying 3D-contoured endwalls at the hub. The third one (C3) is a new embedded concept for the turning mid turbine frame with two zero-lift splitters placed into the strut passages. In total 48 vanes (16 struts plus 32 splitter vanes) guide the flow from the HP rotor to the LP rotor.
The comparison in terms of noise generation and propagation of the turbines is done by the microphones signal spectra, the emitted sound pressure and sound power level of each TMTF setup. Therefore the acoustic field is characterized by azimuthal and radial modes by means of a microphone array at the outer casing traversed over 360 degrees.
By comparing the first two setups (C1 and C2) in terms of noise generation the propagating modes due to the HP turbine were found to be the same, while a difference of 5 dB in amplitude of the modes related to the LP turbine was found due to the different axial spacing between both setups. In the multi-splitter configuration (C3), the overall sound power level depending on the blade passing frequency (BPF) of the HP turbine is reduced by 7 dB and depending on the BPF of the LP turbine by 4 dB compared to C1, respectively. The overall effect is a reduction of the noise emission for the HP turbine due to the embedded design as well as for the LP turbine due to increasing the axial spacing between the TMTF Vanes and LP Blades on the one hand and considerably due to the embedded design on the other hand.