A comprehensive investigation of the secondary flow in a low-pressure turbine cascade under periodic inflow conditions is conducted. Initial analysis reveals some uncertainties regarding the inlet endwall boundary layer due to leakage flow through the gap of a bar wake generator. Therefore, an improved cascade design is presented based on CFD studies. The main design feature is an adjustable two-part flat plate, integrated at part-span and acting as a cascade endwall. This enables an independent variation of the inlet boundary layer. An experimental validation confirms the achieved design goals of more distinctive secondary flow and a more consistent effect of the periodically incoming wakes. The classic investigation approach centered around probe-based measurements up- and downstream of the blade passage is extended and enhanced by an advanced multi-methodology approach. Optical measurement setups are implemented inside of the blade passage utilizing Particle Image Velocimetry (PIV) and unsteady Pressure-Sensitive Paint (PSP). Furthermore, the potential of ultra-fast Ru(phen)-based Temperature-Sensitive Paint (TSP) is demonstrated in a flat plate test case under relevant operating conditions. A continuous combination of the experimental results and supplementation by CFD clearly showcases the added value in their synergy. This multimethodology approach enables the quantification of the attenuating effects of periodically incoming wakes on the secondary flow and the analysis of the underlying mechanisms. In the time-averaged flow field downstream of the passage, the secondary flow attenuation is apparent by a reduction of over-/underturning, secondary losses, and the passage vortex liftoff. In order to put the relevance of wake effects into perspective, the associated losses up-, inside, and downstream of the blade passage are compared to the effects of reduced blade loading and reduced endwall boundary layer height. While all three factors lead to lower downstream secondary losses, the mechanisms inside the passage are quite different. Also, the loss reduction due to lower blade loading is about one order of magnitude higher than the relatively small periodic wake effect in this regard. By combining phase-locked and synchronized PIV and PSP measurements, the movement of the bar wakes can be traced throughout the blade passage. On the blade suction surface, the ’negative-jet-effect’ of the wakes induces a negative pressure gradient in space i.e. positive gradient in time. This effect is intensified by around 33% near the endwall and alters the interaction of the blade flow with the secondary flow. The passage vortex exhibits a periodic reduction in turbulent kinetic energy and reduced spanwise distance from the endwall (’penetration depth’). However, its pitchwise extension is increased due to augmented vortex dissipation by the wakes which leads to less pronounced i.e. more diffuse downstream secondary flow. Based on a phase lag to the downstream wake passing and the measured blade-to-blade velocity field, the origin of these effects can be retraced to a large endwall separation area inside the passage. This separation appears to be triggered by the migration of the horseshoe vortex pressure side leg. The interaction with turbulent wakes significantly increases the robustness of the endwall boundary layer against this separation. The presented findings on the mechanisms of secondary flow development demonstrate that periodically unsteady inflow conditions should be taken into account during secondary flow evaluations in turbine design.     «

A comprehensive investigation of the secondary flow in a low-pressure turbine cascade under periodic inflow conditions is conducted. Initial analysis reveals some uncertainties regarding the inlet endwall boundary layer due to leakage flow through the gap of a bar wake generator. Therefore, an improved cascade design is presented based on CFD studies. The main design feature is an adjustable two-part flat plate, integrated at part-span and acting as a cascade endwall. This enables an independent...     »