Numerical and experimental study of the performance effects of varying vaneless space in high-speed micro turbine stators

•A numerical and experimental study has been made for a small size turbo-expander.•The optimal gap is close to the value calculated by Tunakov's empirical formula.•The efficiency of 0.2mm gap has a significant decline at the off-design conditions. In order to make direct performance comparisons of varying the vaneless space in high-speed micro turbomachinery, an extensive experimental and numerical program have been carried out on a 22mm leading-edge diameter mixed and inward flow radial turbine using a variety of stator designs. The stator was designed using commercial design software, having 9 vanes and the radial clearance (vaneless space) between stator and rotor was 0.5mm single side, that means the ratio of the stator trailing edge radius to the rotor leading edge radius Rte/rle is 1.0455. Two additional stators (Rte/rle is 1.0182 and 1.0727) were also designed and manufactured. According to the design condition, just like pressure at stator inlet or mass flow rate for all passages, a series of CFD simulations were carried out in order to guide design iterations towards achieving a matched flow capacity for each stator. In this way the variations in the measured stage efficiency could be attributed to the stator passages only, thus allowing direct comparisons to be made. The losses for different radial clearances have been quantified and the variations in the measured and computed efficiency were used to recommend optimum values of Rte/rle.