Effect of Tip Clearance on the Performance of Helium Turbine Expander

High power helium turbine expander is a key equipment in the Comprehensive Research Facility for Fusion Technology (CRAFT) cryogenic system, which has a direct impact on the CRAFT cryogenic performance. As a core part of helium turbine expander, the impeller determines the refrigeration efficiency of the turbine. This article adopts the numerical simulation method to investigate the flow characteristics of cryogenic helium gas in the impeller passage. In addition, the effect of tip clearance on turbine performance under design and off-design conditions are investigated. The results show that with the decrease of the tip clearance, the leakage of the tip clearance decreases, while the efficiency and power of the turbine expander increase obviously. When the relative rotational speed reaches the interval of 0.8–1.2, the tip clearance has a great influence on the power, and the power increases by about 90–150 W for every 0.1 mm reduction in the clearance. Turbine efficiency increases first and then remains unchanged with increasing mass flow. When the relative mass flow is 0.8, the efficiency increases by 0.6% as the clearance decreases by 1 mm.