Numerical Analysis of the Flow Field through a Turbine Stage with Bucket Tip Clearance

A numerical technique for the computation of three-dimensional turbulent flows through a turbine stage with bucket tip clearance is presented. To calculate nozzle and bucket flow regions simultaneously, steady interaction is assumed to occur at the connecting boundaries. To analyze the exact flow-region geometry of the bucket tip clearance, an H-type computational grid system, which is also used in the nozzle and bucket flow regions, is used in the tip clearance region. A finite volume method is used to obtain the spatially discretized governing equations, while the damping surface technique is employed for the time integration. In the present analysis, a two-equation model of turbulence is introduced to estimate the turbulence effect. In order to verify the validity of the present method, computations are carried out for the flow through turbine stages of different tip clearance heights. The results show clear simulation of secondary flow phenomena such as the tip leakage vortex and passage vortex motions in the bucket flow region. The effects of the clearance height on the turbine stage flow fields are also well predicted qualitatively.