The paper deals with the investigation on the aerodynamics and the acoustics of two 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.
The two TMTF setups have been investigated at engine like flow conditions. The first configuration consists of 16 highly 3D-shaped turning struts. The goal of the second design 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.
To estimate the pressure loss over the duct aerodynamic measurements are performed at the inlet and the outlet of both turning mid turbine frames by using 5-hole probes (FHP) and total pressure rakes. The FHP-measurements at the inlet of the TMTF were performed in three different ways to obtain the influence of probe positioning and traversing on the results. While the 5-hole probe was traversed only in a sector the rakes were traversed over the full circumference. The comparison between the two turning mid turbine frame setups shows from an aerodynamic point of view that it is possible to reduce the engine weight by designing a 10% shorter TMTF with endwall contouring providing the same pressure loss and comparable inflow conditions for the LP turbine rotor.
Due to the fact that noise becomes more and more an issue additional acoustic measurements were carried out downstream of the low pressure turbine. By comparing the two setups in terms of noise generation the propagating modes due to the HP turbine were found to be the same, while an increase of 10dB in amplitude of the modes related to the LP turbine was found in the 10% shorter setup. This is in good accordance with previous studies, where reducing the distance between stator and rotor increases the emitted sound.