Aerodynamics of a High-Speed Low-Pressure Turbine Cascade with Unsteady Wakes and Purge Flow

An experimental investigation on the aerodynamics of a high-speed low-pressure turbine
was performed at engine-relevant Mach and Reynolds numbers. Measurements were performed in a transonic low-Reynolds linear cascade. Rotating bars were used to recreate the incoming wakes characterized by engine-representative wake velocity triangles. A secondary air system connected to a cavity slot injects purge flows upstream of the cascade.
Time-averaged measurements of the midspan inlet and outlet flowfield and blade loading
were performed to characterize the inlet boundary conditions and cascade performance. The blade aerodynamics were compared against numerical simulations to highlight the challenges of the latter in capturing the correct physics of high-speed low-pressure turbines. Surface-mounted hot-films and fast-response pressure sensors were used to comprehensively characterize the blade boundary layer and transition mechanisms. Classical transition features were observed for all cases up to Mach numbers at which passage shocks occur. For the highest Mach number investigated, the separation was impacted by the shock impingement on the SS for the steady inlet flow cases. In the time-resolved frame of reference, the separated wake-induced transition mechanism was characterized by Kelvin-Helmholtz instabilities. An exception occurred for the case described by a shock in the passage.
The cascade inlet boundary layer was also characterized with time-averaged and time-resolved instrumentation. The secondary flows were found to be weakened with increasing Mach number. Upon the introduction of unsteady wakes in the flow domain, the secondary flows were found to be modulated. The periodic thinning of the inlet boundary layer partially suppressed the passage vortex during one wake cycle. Similar findings were found for cases characterized by purge flow injection. The secondary flows were strengthened and displaced away from the endwall with an increasing purge massflow ratio. The classical secondary flow structures were still observed with the purge: passage vortex, trailing shed vorticity, and corner vortex. The structures were found to be modulated by the wake passing. Opposite from the case without purge, the passage vortex is always identifiable during each wake-passing cycle. The passage vortex suffered the most considerable degree of fluctuations due to the wake. The endwall flow is largely uncorrelated to the unsteady wake.
The experimental data was used to challenge state-of-the-art loss models. Classical empirical loss models failed to predict the profile loss correctly. A physics-based loss correlation that models the boundary layer development along the blade surfaces revealed that further calibration is required to widen its application to the current blade geometry. Classical secondary loss correlations also struggled to capture the loss. Physics-based loss correlation satisfactorily predicted the secondary loss for the investigated flow conditions without unsteady wakes and purge flow. The latter demonstrated to be sensitive to the inlet boundary layer, and therefore, improving the experimental characterization of the inlet boundary layer must be sought to assess the capability of correlations to predict secondary loss correctly.