Investigation of the boundary layer development on a highly loaded low pressure turbine cascade under the influence of unsteady flow conditions

The flow in turbomachines is characterised by high turbulence levels as well as by strong non uniformity and non stationarity. Each blade row generates wakes with lowered velocities and higher turbulence levels. The relative motion between rotor and stator rows gives rise to a non-uniform and unsteady flow impinging on the downstream blade rows. This phenomenon strongly influences the boundary layer behaviour on the blade surfaces and hence the losses and the loading of the profile. Measurements of profiles in cascade wind tunnels with homogeneous inlet conditions have often revealed inferior aerodynamic performance compared with that obtained in a real turbomachine. The High-Speed Cascade Wind Tunnel of the Universitat der Bundeswehr Munchen has been equipped with a wake generator that can simulate a moving blade row in front of the fixed linear cascade mounted in the test section. Measurements were performed on a highly loaded low pressure turbine cascade varying the velocity and the pitch and hence the redzced frequency of the wake-generating bars. The loss measurements showed an optimal reduced frequency, at which the profile losses were reduced to 77% of those observed under stationary inlet flow conditions. Hot-film measurements on the other hand were able to give a deeper insight into the physical basis of the complex phenomenon of unsteady boundary layer transition.