Effects of Periodic Incoming Wakes on the Aerodynamics of a High-Speed Low-Pressure Turbine Cascade

Authors

Loris Simonassi  - von Karman Institute for Fluid Dynamics, Belgium
Gustavo Lopes - von Karman Institute for Fluid Dynamics, Belgium
Sergio Lavagnoli - von Karman Institute for Fluid Dynamics, Belgium

Abstract

The geared engine architecture allows for increased low-pressure turbine pressure ratio, associated with improved efficiency and a reduction in stage count and weight. Therefore, the design space of modern high-speed LPTs is pushed towards transonic exit Mach numbers (M2 > 0.8) and low-Reynolds numbers. This combination of operating conditions impacts the transition and separation processes as well as the loss mechanisms. Consequently, the influence of the unsteady wakes incoming from the upstream stages assumes high relevance for its potential to trigger transition and influence the separation of the boundary layer on the blade suction side at low Reynolds numbers typical of this application. The aim of this paper is the experimental characterisation of the influence of incoming wakes on the 2D aerodynamics of a high speed LPT cascade operating at low Reynolds and transonic exit Mach number. A detailed analysis of the status of the flow along the blade under investigation and its impact on the profile losses are presented for a range of Mach number from 0.70 to 0.95 and Reynolds number from 70000 to 120000 under steady and unsteady inflow conditions. Test have been conducted at on- and off-design engine realistic conditions in the VKI S1-C transonic cascade. The wakes incoming from an upstream blade row have been replicated using a set of rotating bars, which shed wakes at the test section inlet characterised by engine representative Strouhal (St = 0.95) and flow coefficient (Φ = 0.97). A set of densely instrumented traversable blades have been used to sample steady and unsteady surface pressure distributions. The development of the boundary layers along the blade suction and pressure sides have been examined through quasi-wall shear stress obtained with surface-mounted hot-film sensors. Wake traverses have been carried out downstream of the cascade with a miniaturised L-shaped 5-hole probe to characterise the blade losses. The introduction of periodic incoming wakes promotes variations in the flow topology over the blade. All tested cases show higher velocity peak under unsteady inflow, while the beneficial effect on the suction side separation bubble is shown to depend on the exit flow conditions. At low Mach numbers the incoming wakes determine a reduction in the size of the bubble, in contrast this effect is less significant as the exit Mach number increases. Consistently, a high dependence of the unsteady wakes effect on the profile losses on the exit Reynolds number and Mach number is demonstrated. The large variety of tested conditions under steady and unsteady engine realistic conditions represents also a novel experimental dataset concerning the effects of unsteady wakes at high Mach number and low Reynolds number, providing references for the validation of models and CFD computations.