Aerodynamic Performance Evaluation of Leading Edge Tubercles on Low Pressure Turbine Blades at Very Low Reynolds Numbers and Higher Turbulence

At higher altitude, an aero-engine low pressure turbine is susceptible to a substantial decrease in performance commonly called the Reynolds lapse. Alleviating the negative impact of decreasing Reynolds number on performance could lead to enhanced high-altitude operations. This paper presents results and recommendations from an experimental application of leading edge tubercles as passive flow control devices for a purpose-designed, aft-loaded LPT profile. These bio-inspired geometric features have been shown to delay stall and improve post-stall operability of airfoils and wings. In the present study, tubercle geometry was based on an optimized tubercle geometry that was designed previously for another application with guidance provided by a neural network technique called Self-Organizing Maps, trained by available published data. In addition to low turbulence tests, the performance of the baseline and tubercled blade were also investigated at higher turbulence that is more representative of engine operation. The experimental LPT performance evaluation took place in a cascade rig and included measurement of mid-span surface pressure, signal variance of blade surface hot film data, and cascade-exit mixed-out losses. Conclusions were drawn after consideration of wake traverses, exit losses, surface pressure measurements, and hot film signal variance at various axial-chord Reynolds numbers between 15 000 and 90 000. The suction-surface pressure provided information on blade loading and flow separation while hot-film provided information on the state of the boundary layer. Some reduction in exit losses and increased performance at higher Reynolds numbers was noted. Valuable insight has been gained on the challenges associated with very low Reynolds number testing and recommendations have been made for future work.