THE AERODYNAMIC OPTIMIZATION DESIGN OF TURBINE CASCADE WITH NONAXISYMMETRIC ENDWALL AND EXPERIMENTAL VALIDATIONS

摘要

The nonaxisymmetric endwall profiling has been proven to be an effective tool to reduce the secondary flow loss in turbomachinery. In the present work, an endwall optimization design procedure for reducing secondary flow losses has been developed which allowed complete 3-dimensional parameterization design of the turbine endwall. A so-called shape function and a decay function were used for the definition of the nonaxisymmetric endwall. The shape function was used to control the curvature in the circumferential direction and the decay function was used to control the curvature in the axial direction. The design of the endwall was generated by the product of these two functions. The sinusoidal function was used for the shape function and the B-spline was used for the decay function. This parametrization allowed influencing the contouring of the specific endwall region. The profile of the endwall has been optimized using automatic numerical optimization by means of an improved efficient global optimization algorithm based on kriging surrogate model. The niching micro genetic algorithm was used to get the correlation vector of Kriging model, which eliminated the dependence of correlation vector starting search points. This method reduced the difficulty of finding appropriate penalty parameters and increased the robustness of the optimization method. The 3D-Reynolds-averaged Navier-Stokes flow solver based on CFX, with a k-ω model for turbulence model, was used for all numerical calculations. An in-house optimization design system was developed to close the loop of the geometry definition, flow solving and the optimization algorithm which allowed the solution of non-linear problems. A large-scale linear cascade with a low-speed wind tunnel has been chosen for the experimental validation of the optimization results. The experimental measurements and numerical simulations both demonstrated that the total pressure loss and secondary flow intensity were reduced with the nonaxisymmetric endwall used in the cascade passage. The detailed flow pattern comparisons between the passage with based flat endwall and the optimization nonaxisymmetric endwall were given by the numerical simulations method and entropy generation rates analysis were used for the investigation of the secondary flow loss reduction mechanism in the nonaxisymmetric endwall profile cascade.