Numerical Investigation of Effects of Non-Uniform Tip Clearance on Flow Field Inside a Turbine Cascade

Abstract

In the unshrouded axial turbine, the tip clearances can result in the loss of turbine efficiency and the penalty of turbine performance. Therefore, investigating the blade tip geometry of improving the turbine performance has a great significance. This paper is to study the effects of non-uniform tip clearance on the flow field in a turbine cascade. The numerical works are performed at the incidence angle of 0 degree and the exit Reynolds number of 1.7 × 105 based on the blade chord. In the investigations, the flat tip (Basic) geometry was employed as a benchmark, and three different tip geometries, including the pressure side squealer (PSQ), suction side squealer (SSQ) and grooved tip (Grooved), were studied. The tip clearances are all specified as 1.18% of the chord. The squealer height is set to 2.94% of the chord. The endwall static pressure, tip leakage loss, flow capacity and the development of tip leakage vortex are discussed. And the numerical results show that the grooved tip which can obtain the least total pressure loss, is helpful to smooth the pressure change from pressure side to suction side and suppress the intensity of tip leakage vortex. The tip clearance flow in the pre semi-passage is mainly involved in the passage vortex, and in the post semi-passage it is added to the tip leakage vortex. Compared with the Basic, PSQ and SSQ tips, the Grooved tip contributes to reducing the tip leakage flow and the tip leakage loss. And the leakage flow can be strengthened in the middle passage for the PSQ. The difference between the area averaged streamwise coefficient and mass averaged loss is almost opposite for the SSQ and Grooved tip, which is uncertain the performance of the turbine cascade with the SSQ and Grooved tip is better than the Basic tip.