Axisymmetric Endwall Contouring in a Four-Stage Turbine: Comparison of Experimental and Numerical Results

Secondary flows and leakage flows lead to complex vortex structures in the 3-D flow field of a turbine blading. Aerodynamic losses are the consequence. Reducing these aerodynamic losses by axisymmetric endwall contouring is the subject of a current experimental and numerical investigation of the flow field in a 4-stage test turbine with repeating stages. Numerical 4-stage simulations for the reconstructed turbine with an axisymmetric off-set arc endwall contour at the casing have been performed and compared to corresponding numerical investigations of the original machine without endwall modifications. The 3-D flow fields have been calculated by application of a steady 3-D Navier-Stokes code. Based on these results the experimental setup is modified to the off-set arc endwall design. The characteristics of the reconstructed machine are measured and compared to the original test rig. Special emphasis is put on the determination of the aerodynamic efficiencies over the four stages. For a detailed assessment of the radial and spanwise flow field properties inside the blading, 5-hole pressure probes are used for steady flow measurements in the narrow axial gaps before and after the 3rd stage. Finally, the measured radial distributions of the flow field properties and the machine characteristics are compared to the corresponding numerical predictions. All results show a significant positive influence of the endwall contouring on the radial distribution of the flow angle, the pressure field and the aerodynamic efficiency.