Investigation of Endwall Secondary Flows in a Low Aspect Ratio Transonic Turbine, Part I: Passage Normal Shock

Abstract A significant source of loss in turbines includes the generation of endwall secondary flows and subsequent mixing losses downstream of a blade row. In the case of reduced aspect ratios, these secondary flows assume a greater spanwise penetration attributing to increased loss generation. In the first part of this two part series, the endwall secondary flow of a low aspect ratio turbine with AR = 0.66 (airfoil span to axial chord length) and Zweifel loading coefficient (Zw = 1.0) is characterized at an exit isentropic Mach number of 0.95. Both experimental and numerical findings are compared. Numerical predictions are validated by airfoil static pressures and downstream survey measurements in the SLU linear turbine cascade polysonic wind tunnel facility. Numerical analysis is completed using Cadence Fine/Turbo employing RANS with the SST k-omega turbulence closure model. This paper discusses the development of the endwall secondary flows throughout the airfoil passage including the development of the main secondary flow vortices (passage, counter and corner vortices) in conjunction with trailing edge shock impingement. Further, the subsequent mixing losses of the endwall secondary flows are shown with contours of pressure loss coefficient for five downstream survey planes ranging from 0.2Cx to 2.0Cx downstream of the trailing edge. Results show the effect of shock impingement and resulting shock boundary layer interaction on the suction surface resulting in an impact on the endwall secondary flow vortex development unique to a low aspect ratio turbine. The combination of endwall secondary flows and suction surface shock result in shock vortex and shock boundary layer interactions which affect the downstream mixing losses. Finally, the resulting work presents a new schematic detailing the endwall secondary flow development for low aspect ratio turbines in the transonic regime.