Effect of Large Stagger Angle Endwall on Secondary Losses in a High-Lift Turbine

The performance of low-pressure turbine (LPT) plays an extremely important role in high-bypass-ratio aero-engine, which significantly influences the engine performance and efficiency. Because of the requirement of weight reduction of LPT, high-lift profiles have been widely used in modern axial turbines. However, the usage of high-lift profiles usually accompanies strong secondary flow. How to reduce the secondary losses in the high-lift blade design has become an important research area. This paper proposes a novel design concept, large stagger angle endwall (LSAE). This kind of new design reconstructs the blade profile around the endwall by obviously increasing the stagger angle. The results obtained show a smooth transition between cylindric leading edge and hub endwall and obviously improved the local aerodynamic performance. A detailed numerical investigation was performed in a 7-stage high-lift LPT and the LSAE technology was implemented on the 2nd guide vane. The result revealed that, without LSAE technology, the complicated secondary flow in the upcoming inlet boundary layer brought inevitable effects on the downstream flow of blade or vane. The incoming flow has a high positive incidence angle near the hub endwall and leads to a high-pressure region and a pressure blockage. This could finally intensify the inlet boundary separation and cause high secondary losses. However, LSAE is able to adapt the high deviation of inlet flow angle. With better gradual transition on geometry from the leading edge to hub endwall, it firstly optimized the pressure distribution. The static pressure recovery between leading edge and 40% axial chord length position disappeared, and the cross-pressure gradient between pressure side and suction side at leading edge decreased. The pressure blockage was eliminated and the through-flow capability was enhanced at endwall. Then, LSAE changed flow structure. It reduced the roll-up of horseshoe vortex and made the streamlines of horseshoe vortex brunches follow the vane geometry well. As a result, the passage vortex and suction side corner vortex became smaller, resulting in the disappearance of the wall vortex. Also, the deviation of exit flow angle decreased. Finally, the loss of secondary flow decreased and the spanwise mass flux was more uniform at exit plane. Mass-averaged loss was reduced by 11.3%. The efficiency of the 2nd stage increased by 0.75%. Performance experiments of a 7-stage LPT were carried out in which LSAE technology was applied to the 2nd, 3rd, 4th and 5th vanes. The experimental result verified the feasibility and reliability of LSAE for engineering application. The final result improved the turbine efficiency by 0.29%.