Experimental Aerodynamics of a High-Speed Low-Pressure Turbine

Recent progress in propulsive system’s architectures allows a migration toward greener aeronautical engines. Between the most effective evolutions, ultra-high bypass ratio Geared Turbofans (GTF) cover a focal point, leading to higher propulsion efficiencies. In GTF engines, low-pressure turbine’s (LPT) rotational speed is considerably increased, allowing improvements on performances and weight saving. This novel architecture will require the turbine to work at transonic exit conditions and low Reynolds numbers.

Modern turbine’s design efforts are focused on further increasing the already high performance of low-pressure turbines by mitigating secondary endwall flows and the degrading interaction with hub cavity-purge and tip-leakage flows.

The interest in the High-Speed Low-Pressure Turbine and its aerodynamics is now rising although a lack of experimental data is found. Experimental investigations on LPT fluid-dynamics in an engine representative (multi-row rotating) environment at engine-realistic condition is needed to establish new design knowledge, focusing on endwall flows interaction with cavity/leakage flows at the novel operative conditions.

The von Karman Institute aims at filling up this gap with an extensive experimental undertaking in the project “Secondary and Leakage Flow Effects in High-Speed Low-Pressure Turbines” (SPLEEN) within the framework of the Clean Sky 2 research program.

The doctoral program framed in the project SPLEEN has the objective to investigate the unsteady aerodynamics of high-speed LP turbines of geared-fan propulsion systems. Tests will be performed in a laboratory-reproduced turbine environment at engine scaled conditions on a one-and-a-half turbine stage. The experimental campaign planned on the annular rotating axial turbine rig will demonstrate a TRL of 5 for a fully-featured multi-row high-speed LPT stage.

The project aims at designing an open-access turbine hardware that is suitable for research while being representative of a high-speed turbine component and experimentally investigate phenomena of large technological interest occurring in high-speed LPT. The unsteady interaction of purge and leakage flows with secondary flows will be assessed investigating cavity’s geometry and mass flow rate impact. For this purpose, changes of shape for both stator-rotor hub cavity and rotor shroud will be performed. In the same way, the effect of different cavity mass flow rate will be assessed as well as the impact of the varying rotor clearance. Finally, the turbine off-design operation will be investigated at nominal purge and leakage flow rates.

Computational fluid dynamics (CFD) will be used to design the experiment including the aerothermodynamic scaling of the rig to fit the facility’s constraints and capabilities. Preliminary RANS CFD will be used to optimize the measurements location and set the instrumentation range and bandwidth.

The test section of the rotating turbine rig will be heavily instrumented to measure the time-averaged performance and the unsteady aerodynamics of the turbine. Detailed characterization of total quantities and freestream conditions will be performed at the rig inlet. Quasi-shear stress measurements are envisaged to measure the time-resolved behavior of the boundary layer on the airfoil profiles under the effect of the unsteady interactions between leakage flows, secondary flows and wakes. Measurement of flow angles and total pressures will be carried out traversing rakes of fast-response pressure sensors within the turbine stages to quantify secondary flows, stage losses and unsteady aerodynamics at the exit of the rotor and of the second vane’s row. The stator hub and rotor endwalls unsteady aerodynamics will be quantified by means of time resolved and time averaged pressure measurements. The impact of geometry variation on the interaction between rotating and non-rotating lips will be evaluated by quasi-shear stress measurement performed on the shroud and rotor-stator cavity.

The effort of this study and the project SPLEEN will mark a fundamental contribution to the progress of high-speed low-pressure turbines, characterizing the impact in an engine-representative of cavity-purge and leakage flows unsteady aerodynamics. The output of the project will consist in a highly space and time resolved interaction and performance database at representative, engine-like operating conditions of the test.