Counter-rotating low-pressure turbine vanes with non-axisymmetric endwall contouring: Numerical investigation of aerodynamic performance considering seal leakage flow

The integrated design philosophy incorporating the intermediate turbine duct (ITD) with guide vanes between counter-rotating high and low-pressure turbines is a relatively new approach that can effectively shorten the axial length of aero-engines and improve the thrust-to-weight ratio. However, the highly diffusive flow within the ITD tends to induce flow separation on the blade. Meanwhile, the incorporation of sealing flow markedly promotes the development of secondary flow structures within the hub region. These effects lead to aerodynamic losses in the vane passage. Therefore, this study investigates the effect of high-degree-of-freedom non-axisymmetric endwall contouring on the aerodynamic performance of counter-rotating low-pressure turbine (CR-LPT) vanes in the ITD. Initially, based on the Bézier surface method, 17 variable control points are defined between the sealing flow inlet and the blade mid-chord. Non-axisymmetric contouring of the lower endwall in this region is realized by adjusting the radial distance between these control points and the rotational axis. Subsequently, a surrogate model is constructed with the control parameters as inputs and the total pressure loss coefficient as the objective function, aiming to determine the optimal endwall contour configurations. Numerical results show that, compared to the prototype flat endwall, the optimal endwall contouring configuration (OPT3) considerably improves the aerodynamic performance of the CR-LPT vane, reducing the passage total pressure loss coefficient by up to 2.3%. The contour effectively weakens the legs of the horseshoe vortex, suppresses the shear-induced vortex arising from the sealing flow-mainstream interaction, and improves flow behavior on the blade pressure surface. These results confirm that high-degree-of-freedom non-axisymmetric endwall contouring offers a promising method to manage losses within the hub region of CR-LPT vanes in the presence of sealing flow.