Influence of the Rotor-driven Stator Flow Perturbation on the Unsteady Aerodynamics of a HP Turbine Stage

Stator - rotor interaction studies are often focused on the unsteady rotor aerodynamics promoted by the periodic perturbations induced by the stator potential field and by the convection of stator wake and secondary flows. Such structures modify periodically the rotor loading and the inlet conditions, thus triggering a periodic fluctuation of the rotor wake and secondary flows, ultimately affecting the rotor aerodynamic forcing and the stage efficiency. However, the rotor itself acts periodically on the flow released by the stator, basically due to the rotor potential field propagating upstream of the rotor leading edge. In this paper, time-resolved experimental results obtained by applying a Fast Response Aerodynamic Pressure Probe (FRAPP) in the HP turbine stage of the Laboratory of Fluid-Machines of the Politecnico di Milano are presented and compared to time-accurate Computational Fluid Dynamics (CFD) simulations of the flow inside the stage. The turbine stage is representative of a modern uncooled high pressure axial turbine, operated in high-subsonic conditions. The FRAPP was traversed both inside the stator-rotor axial gap and downstream of the rotor. Simulations were performed applying an experimentally validated CFD model, based on the commercial code CFX, making use of the time-inclined technique to handle the exact number of stator and rotor blade. Even though the static pressure perturbation induced by the rotor has a null average contribution, the sweeping of the rotor blade periodically pressurizes the flow in the rear region of the stator, introducing a periodic perturbation in both the stator wake and the stator secondary flows, as well as in the total pressure field upstream of the rotor. In particular, the stator viscous structures are periodically bowed before being chopped by the rotor leading edge: they undergo a fluctuation in flow direction of some degrees, their turbulence level is periodically enhanced, and the related losses are periodically modified as well. As for the pressure field, the rotor action leads to a rotating perturbation that locally enhances both the static and the total pressure. As such, the pressure perturbation interacts with the stator flow structures and periodically modifies the total to static expansion ratio across the rotor with respect to the time-mean condition; moreover, the rotor periodically blocks the stator discharge section, thus making the flow rate per channel fluctuate. Both these effects lead to a periodic pulsation of the entire rotor flow, having a considerable impact on the unsteady flow released by the stage.