Separation Control on a Low-Pressure Turbine Blade using Microjets

Abstract

Flow separation that occurs over low-pressure turbine blades at low Reynolds numbers has been a cause of concern due to its detrimental effect on engine performance. In the present study, the effect of microjet actuation, a low-mass, high-momentum device, is demonstrated for the elimination of separation on a low-pressure turbine blade over a wide range of Reynolds numbers using a range of complementary diagnostics, which include the surface pressure, velocity field, and wake-loss measurements. The U.S. Air Force Research Laboratory L1A low-pressure turbine blade used in this study is a highly aftloaded profile that experiences a nonreattaching separation at approximately 60% axial chord at low Reynolds numbers. Baseline blade pressure distributions as well as wake-loss coefficient measurements show that the blade experiences nonreattaching separation for Reynolds numbers based on axial chord less than 50,000 for a freestream turbulence intensity of 1% with steady inlet conditions. Microjet-based control was activated at various blowing ratios for each Reynolds number of interest. The integrated wake-loss coefficient was reduced by 85% with microjet control at low Reynolds numbers. Ensemble-averaged particle image velocimetry velocity fields show an elimination of reverse flow on the blade’s suction surface when steady microjets were activated at the optimum blowing ratio during low Reynolds number conditions. Turbulence statistics found through particle image velocimetry showed a concomitant dramatic reduction in unsteady velocities with control. Boundary-layer reattachment is attributed to the increase of local streamwise vorticity caused by the microjets (jets in crossflow), which lead to enhanced freestream entrainment to the near wall. The boundary layer is further energized due to enhanced turbulent mixing far downstream of the microjets. It was also observed that excessive control at high blowing ratios actually leads to a much larger separation. Prior studies applying active control on low-pressure turbines have shown the effectiveness of pitched and skewed vortex-generating jets for eliminating flow separation. The current study demonstrates that simple, normally issuing microjets with zero pitch angles and very low mass flow can be as effective as vortex-generating jet actuators for low-pressure turbine flow control.