The project SPLEEN is characterized by two large work packages in which the aerodynamics of high-speed low-pressure turbines is measured in a linear cascade environment (WP1) and in a rotating turbine stage (WP2).
In WP1 we have investigated the effects of cavity geometry and secondary-air purge and leakage flows in a large-scale high-speed linear cascade at engine-representative Mach and Reynolds numbers.
A new high-speed turbine airfoil was designed by Safran Aircraft Engines. A new test article was designed, expanding the traditional 2D measurement capability of the facility, to introduce cavity flow effects and measure turbine 3D flows under controlled inflow conditions. The cascade was densely instrumented with pneumatic and fast-response pressure taps and high-bandwidth hot film sensors. A sliding instrumented blade was introduced to translate the sensor arrays from the endwall to mid-span. A set of probes was used to interrogate the time-resolved flow at several planes upstream and downstream of the cascade. The linear cascade measurements were conducted in the high-speed, low-Reynolds facility S-1/C of the von Karman Institute.
The test campaign was divided into three major phases. In the first phase, the baseline aerodynamics of the SPLEEN C1 cascade was measured with a flat endwall and steady. Measurements were performed to detail the cascade boundary conditions, to assess the flow quality and the test repeatability as well as the interaction effects of the instrumentation. The cascade performance was measured over a wide range of turbine exit Mach numbers (0.70 to 0.95) and Reynolds numbers (65,000 to 125,000). A second set of measurements was performed on the cascade equipped with an inlet wake generator. Extensive measurements were taken to characterize the unsteady wake-boundary layer interactions, the unsteady evolution of secondary flow structures, and the time-averaged turbine loss under unsteady inlet flows.
The final phase of the WP1 test campaign assessed the effect of cavity geometries and purge (leakage) air rates on the local flow features and turbine performance. Five cavity configurations were investigated. The cavity variants were tested with the wake generator on, at design operating exit Mach (0.90) and Reynolds number (70,000) at two different injection rates.
In the second part of SPLEEN, a high-speed low-pressure turbine stage was tested at scale one in the rotating turbine rig of the von Karman Institute. The stage was designed by Safran Aircraft Engines and the von Karman Institute with the target of replicating the flow-field and the loss mechanisms found in high-speed low-pressure turbines. Significant efforts were dedicated to instrumenting the test section. The final turbine test vehicle featured more than 800 measurement points including a set of aero-thermal probes traversed at the turbine inlet and upstream/downstream of the rotor row. The experimental campaign focused on the measurement of the flow structures, turbine global performance and the unsteady leakage/purge interactions with the turbine main flow. Turbine experiments were carried out at a nominal operating point and at off-design purge rates at the upstream hub rotor cavity.
The project SPLEEN also investigated the introduction of an innovative technology aimed at controlling the interactions of the cavity purge with the rotor flow. The novel concept was designed and numerically tested using Navier-Stokes CFD simulations in a turbine environment. The results showed gains in turbine efficiency that justify further optimization studies and maturation in future testing programs.
The extensive databases have been made available to Safran Aircraft Engines and the results discussed and analyzed during several meetings. The outcomes of SPLEEN are pivotal to advance procedures and guidelines for the design of high-speed LP turbines, validate numerical models, and reduce design safety margins.
The measurements and the flow analysis were the subjects of technical presentations and journal publications. More articles and technical presentations are planned for international conferences, symposia, and peer-reviewed journals.
The extensive experimental datasets collected through the linear cascade test campaign were published in an open-access database. The release of a second open database of the turbine stage experiments is planned for 2023.
