Unsteady Investigation of Intermediate Turbine Ducts Under the Influence of Purge Flows

The aerodynamic flow field at the outlet of a high-pressure turbine (HPT) stage in a modern turbofan engine is highly unsteady and strongly influenced by periodic fluctuations from the HPT rotor, also interacting with the HPT stationary vanes. Downstream of the last HPT stage, the flow enters the intermediate turbine duct (ITD), where new unsteady components are generated, due to the interaction between the incoming flow and the struts supporting the ITD. This paper illustrates the differences between the unsteady flow behavior in two state-of-the-art ITD configurations for turbofan engines: the turbine center frame (TCF), with symmetric strut fairings (typically adopted in conventional dual-spool engines), and the turbine vane frame (TVF), which is commonly used in geared turbofan engines and incorporates the first low-pressure turbine (LPT) vane row inside the duct. The experimental results discussed in this work are conducted in the Transonic Test Turbine Facility, situated at Graz University of Technology. To achieve engine-representative operating conditions at the ITD inlet and outlet sections, the tested setups include not only the ducts (TCF or TVF), but also the last HPT stage and the first LPT stage row. Additionally, the test rig features a secondary air supply system, capable of feeding purge air to every stator-rotor cavity in the test vehicle, with independent mass flow and temperature control. Unsteady measurement data at the inlet and outlet of the two duct configurations are acquired with fast-response aerodynamic pressure probes, for two purge flow conditions. Azimuthal mode decomposition is applied to phase-averaged data at selected radial positions, to isolate and quantify the contribution of different interaction modes to the overall unsteady flow field.