The Onset of Low Reynolds Number Separation in High-Lift High-Work LPT Passages

Low Reynolds number conditions occur over portions of the operating envelope in the low-pressure turbine section of aircraft engines, leading to a reduction in efficiency due to laminar boundary layer separation. The resulting “Reynolds-lapse” can greatly influence the performance of high-lift high-work low-pressure turbine blades. While the mid-span Reynolds-lapse phenomena has attracted much attention in the literature, the focus of the current paper is on the evolution of separation across the blade span. Extensive experimental measurements were obtained in a low-speed linear cascade arrangement of low-pressure turbine research profiles. Two different high-lift (Zw = 1.78 and Zw = 1.88) high-work profiles were considered. Multiple planes of high-speed particle image velocimetry were acquired to study the development of separation across the span as flow conditions were varied through the Reynolds-lapse curve. In the case of the Zw = 1.78 airfoil, the suction surface boundary layer separation bubble size increases steadily and monotonically as Re is decreased to the lowest Reynolds number measured, Re = 15k. However, the Zw = 1.88 profile experiences a rapid onset of un-reattaching separation at the knee in the curve. A unique four camera composite velocimetry measurement of the suction surface flow was used to capture the progression in separation across the span. A difference in the onset of separation between the two blade profiles was observed. In one case, the separated flow region progressed from the endwall toward the mid-span; and in the other blade set, it progressed from the mid-span toward the endwall. The findings provide a new and deeper understanding of the separation development across the passage of high-lift high-work turbine blades at low Reynolds number.