Complex Secondary Flow and Associated Loss Generation in Ultra-Highly Loaded Turbine Cascade

An increase of turbine blade loading decreases the numbers of blades and stages, and results in the improvement of the performance characteristics of gas turbines. However, in such highly loaded turbine cascade with high turning angle, the secondary flow becomes much strong due to the steep pressure gradient across the blade-to-blade passage and deteriorates the performance of turbine enormously. In this study, the computations were performed for the flow in the ultra-highly loaded turbine cascade in order to clarify the effects of the inlet boundary layer thickness and the incidence angle which strongly influence the secondary flow structure in a turbine cascade. Moreover, the experimental oil flow visualization was conducted on the blade surface and the endwall, and the measurements of blade surface static pressure were performed at the midspan. The computed results agreed well with the oil flow visualization and the measured blade surface static pressure. The effects of the incidence angle and the inlet boundary layer thickness on the secondary flow structure, the total pressure loss, the secondary flow kinetic energy and the blade loading distributions were examined in detail. The positive incidence angle induced the characteristic vortex released from the endwall. Moreover, it was revealed that the interactions among the horseshoe vortex, the passage vortex and the characteristic vortex strongly increase the secondary loss in the cascade passage.