A computational study of tip desensitization in axial flow turbines

by 1974- Tallman, James A.

iii This study investigates the use of modified blade tip geometries as a means of reducing the leakage flow and vortex in axial flow turbine rotors. Computational Fluid Dynamics (CFD) was used as a tool to compute the flowfield of a low-speed, singlestage, experimental turbine. The results from three separate baseline turbine rotor computations all showed good agreement with experimental measurements, validating the numerical procedure’s ability to predict complex turbine rotor flowfields. This agreement was, in part, due to an advanced, multi-block method of discretizing the turbine rotor into a computational mesh, which was developed as part of the study. After validating the numerical procedure, three different classifications of tip geometry modification were investigated through CFD simulation: chamfering of the suction side of the blade tip, rounding of the blade tip edge, and squealer-type cavities. Chamfering of the blade tip was shown to cause the leakage flow inside the gap to turn toward the camber direction of the blade. This turning led to reduced mass flow through the gap and a smaller leakage 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 greatly 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. Final, detailed computations using a very refined mesh reconfirmed the findings of more rapid, preliminary computations. Detailed, three-dimensional analysis of the computed flowfields revealed the physics behind the modified tip geometries’ reduction of the leakage flow and vortex.