Experimental Investigation of Purge and Secondary Flow Effects on the Aerodynamics of a Transonic Low-Pressure Turbine Cascade

The Ultra-high bypass ratio geared turbofan (GTF) has been identified as a key engine concept to reduce emissions and noise footprint of the next generation of civil aircrafts. The increase in rotational speed of the low-pressure turbine (LPT) enabled by the geared architecture offers potential benefits in efficiency and considerable savings in engine weight, overall dimension and cost. However, the higher pressure ratio implies that the LPT operates at transonic Mach number at the exit (M_is ~ 1), in combination with low Reynolds numbers.

For this reason, maintaining or increasing the high efficiency levels of novel high-speed turbines with respect to conventional direct drive LPTs represents a challenge for the turbomachinery designers: Due to the operating conditions, secondary flow effects have a different impact and structure compared to a conventional turbine. The secondary flows are further affected by the hub cavity-purge and tip-leakage flows. It is evident that there is a high demand for developing new engineering knowledge and reliable tools to design highly efficient and compact high-speed LPTs. However, detailed aerodynamic and performance measurements in high-speed LPTs operating at engine representative conditions are in short supply. This shortage of relevant experimental data also concerns the impact of leakage and purge flows on the secondary flow structures development, and consequently on the high-speed LPTs loss mechanisms.

The current research targets the investigation and understanding of the unsteady aerodynamics of the next-generation of high-speed LPTs and is part of a large EU-funded program named SPLEEN (SECONDARY AND LEAKAGE FLOW EFFECTS IN HIGH-SPEED LOW-PRESSURE TURBINES) led by the von Karman Institute in collaboration with Safran Aircraft Engines.

The study focuses on the experimental characterization of the secondary flow structures and the interaction of cavity purge and leakage flows with the mainstream by experimental data of high-speed turbine geometries tested at engine-scaled conditions. The cavity flow effects will be investigated in the VKI S-1/C large-scale high-speed linear cascade, operating at engine-representative exit isentropic Mach and Reynolds numbers, and inflow conditions (free-stream turbulence and periodic wakes). Three cavity-airfoil arrangements will be investigated, representative of the leakage flow emerging from the stator-rotor hub cavity and of those leakage flows established between the casing and the tip of a shrouded rotor. The secondary air flow rates will be also be varied to understand the impact of purge streams on the LPT airfoil performance and 3D flow structure.

A dense test matrix is envisaged to measure the aerodynamics of the LPT cascade in a time-averaged and time-resolved manner. Time-resolved total pressure and turbulence measurements will be performed to characterize the cascade inlet flow conditions, obtain boundary layer integral parameters and determine freestream turbulence intensity as well as length scales. The inlet and outlet three-dimensional and unsteady flows will be characterized by velocity, pressure and angle measurements thanks to time-averaged and time-resolved pitch and spanwise probe traverses. Quasi-shear stress measurements will characterize the boundary layer status on the blade pressure and suction sides as well as cascade endwall by means of time-resolved surface mounted hot-films. Additionally, the cascade central blade and passage endwall will also be instrumented with pressure taps to investigate the time-averaged and time-resolved blade loading and endwall aerodynamics under simulated unsteady wakes and cavity flows, respectively. Surface pressure measurements using Pressure sensitive paint (PSP) will be performed on the blade suction side and cascade hub endwall to visualize the purge flow entrainment in the turbine passage. This research will introduce a novel technique for traversing an instrumented blade along its span, hence allowing to map with high resolution the static pressure and quasi-shear stress along its span.

This output of this research work will consist in a high-fidelity database that will aid the development, tuning and validation of analytical and numerical prediction models used in the early design stages of future high efficiency and compact high-speed LPTs.