The aerodynamic optimization design of turbine cascade nonaxisymmetric endwall and the midgap influence assessment

The nonaxisymmetric endwall profiling has been proven to be an effective tool to reduce the secondary flow loss in turbomachinery. In the present work, first, without considering the endwall midgap in the real machine, an endwall optimization design procedure for reducing secondary flow losses has been developed, which allowed complete three-dimensional parameterization turbine endwall design. The profile of the endwall has been designed using automatic numerical optimization by means of an improved efficient global optimization algorithm based on kriging surrogate model. Next, 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. Then, in order to evaluate the ability of the optimized nonaxisymmetric endwall with the midgap, the midgap was added in for both the baseline flat endwall and the optimized nonaxisymmetric endwall in the numerical simulations analysis. The entropy generation rates analysis were used for the investigation of loss distribution in the passage. For the cascade in the present work, with the midgap added in, the optimized nonaxisymmetric endwall did not perform as well as the situation without the midgap in the loss reduction. In addition, comparing to the baseline flat endwall, the optimized nonaxisymmetric endwall needed more net leakage flow to avoid the ingress of passage flow into the midgap.