Influence of Jet-Induced Transition on Separating Low-Pressure Turbine Boundary Layers

Abstract

Flow measurements were made on two highly loaded low-pressure turbine blade configurations in a linear cascade facility with and without the application of flow control. The L1M blade has a design Zweifel coefficient of 1.34 with a peak c p near 47% c x (midloaded) and the Pack B blade has a design Zweifel coefficient of 1.15 with a peak Cp at 63% c x (aft-loaded). Flow and surface pressure data were taken for Re c = 20,000 with 3 % inlet freestream turbulence. For these operating conditions, a large separation bubble forms on the blade suction surface, beginning at 59% c x and reattaching at 86% c x on the L1M blade, with a nonreattaching bubble beginning at 68% c x on the Pack B blade. Data were taken using a single-element hot-film anemometer. Higher-order turbulence statistics were used to identify transition and separation zones. Similar measurements were also made in the presence of unsteady forcing using pulsed vortex generator jets located 9% c x upstream of the separation location. For the uncontrolled case, it was found that the separated laminar shear layer on the L1M blade started turbulent transition earlier than the Pack B but took longer to fully transition. This earlier transition appears to be a significant contributor to boundary-layer reattachment for the L1M blade. With the application of pulsed vortex-generating jets, the separation bubble was convected entirely off the blade for both blade configurations, but the Pack B bubble responded more slowly to the jet pulse due to a lower convection speed for the jet disturbance. Once the bubble was swept off of the L1M blade, a new bubble began to grow immediately. However, on the Pack B blade, there was a significant phase lag before bubble regrowth occurred.