A Computational Study of Tip Desensitization in Axial Flow Turbines: Part 2 — Turbine Rotor Simulations With Modified Tip Shapes

This study used Computational Fluid Dynamics (CFD) to investigate modified turbine blade tip shapes as a means of reducing the leakage flow and vortex. The subject of this study was the single-stage experimental turbine facility at Penn State University, with scaled three-dimensional geometry representative of a modern high-pressure stage. To validate the numerical procedure, the rotor flowfield was first computed with no modification to the tip, and the results compared with measurements of the flowfield. The flow was then predicted for a variety of different tip shapes: first with coarse grids for screening purposes and then with more refined grids for final verification of preferred tip geometries. Part 2 of this two-part paper focuses on flow-field predictions with modified blade tip geometries, and the corresponding comparisons with the baseline, flat-tip solutions presented in Part 1. Fifteen different tip shapes were computed using the ADPAC CFD Solver and moderately sized grids (720,000 nodes). These modified tip shapes incorporated different combinations of blade tip edge rounding and squealer cavities, both square and rounded, as means of reducing the leakage flow and vortex. Rounding of the suction side edge of the blade tip resulted in a considerable reduction in the size and strength of the leakage vortex, while rounding of the pressure side edge of the blade tip significantly increased the mass flow rate through the gap. Rounded squealer cavities acted to reduce the mass flow through the gap and proved advantageous over traditional, square squealer cavities. The presence of a square squealer cavity without edge rounding showed no aerodynamic advantage over a flat tip. Final computations of two preferred tip shapes were then carried out using more refined grids (7.2 million nodes). The final, refined grid computations reconfirmed a reduction in the leakage flow and vortex, as well as their associated losses.