Investigation On Tip Clearance Flow In A High-speed Radial Turbine Rotor

The tip clearance between impeller and endwall is an important cause of turbine efficiency drop. Relative researches show that larger tip clearance will lead to more leakage flow and stronger influence on aerodynamic performance of turbine rotor. As a hot and difficult subject in turbomachinery field, tip clearance flow has not been completely comprehended yet. Nowadays the most of research attention is paid to low-speed axial impeller, with little to high-speed radial impeller. As to tip clearance geometry parameters, researchers always focus on conventional tip clearance, conducting insufficient investigations on tight one. Recently, more and more turbochargers have been applied to vehicle engines. Based on the actual situation above, the main purpose of this paper is to investigate the effects of tip clearance, especially tight tip clearance, on flow field of impeller, and the sensitivity of calculation results to turbulence models and grid resolution of tip clearance.In this paper,firstly NACA 65-1810 cascade was studied to assess the numerical calculation adopted by combining experiment research and numerical calculation, with the existing experiment data. And then based on a high-speed radial turbine, Reynolds-averaged Navier-Stokes equations were calculated to investigate the influence of tip clearance on flow field. The research results indicate that so far none of these turbulence models can successfully predict all of flow characteristics. Fine enough spanwise grid of tip clearance is necessary to accurately predict the characteristics of tip clearance flow, which is not sensitive to axial and tangential grid. In a rotor with tight clearance, fluid with low-momentum and low-energy at the tip clearance entrance causes reversed flow. Different from conventional flow, reversed flow significantly alters the thermal field, with high temperature region on the pressure side and low temperature region on the suction side, with stronger viscosity effects than pressure effects. What’s more, tight clearance can weaken leakage vortex intensity. As tip clearance increases, the total-static efficiency of turbine linearly drops, and the amount of leakage mass flow broadly grows.