Unsteady boundary layer studies on ultra-high-lift low-pressure turbine blades

The unsteady boundary layer development on the suction surface of two ultra-high-lift low-pressure (LP) turbine blades, known as T106C and U2, was investigated to further understand the loss reduction mechanism, which is integral to optimize the blade design in unsteady flow. The T106C profile is a mid-loaded ultra-high-lift LP turbine blade. Owing to the strong local adverse pressure gradient, the laminar boundary layer on the suction surface separates shortly after the suction side peak velocity. The turbulence within the incoming wakes cannot induce transition around the separation point because of the low receptivity of the laminar boundary layer. This allows the wake's negative jet to induce roll-up vortex, which reduces the benefits of the wake-induced transition. The wake-turbulence induced transition, which occurs downstream of the separation point, helps the separated boundary layer to reattach earlier. Owing to its mid-loaded nature, a large portion of the suction surface is covered by the reattached turbulent flow, which also contributes to the high profile loss. Therefore, LP turbine blades designed to make best use of unsteady flow effects should be aft-loaded. The ultra-high-lift blade U2 is an aft-loaded profile. Furthermore, the mild local adverse pressure gradient after the suction side peak velocity allows the laminar boundary layer to further develop before separation and results in a local Re θ of about 250 at separation. Therefore, the turbulence in the passing wake is able to induce transition close to the separation point. The earlier wake-induced transition and calmed region significantly reduce the size of the separation bubble. Furthermore, the earlier occurrence of wake-turbulence induced transition prevents the wake's negative jet from generating roll-up vortex, which also leads to lower losses.