Low-Reynolds-Number Performance of Micro Radial-Flow Turbines at High Altitudes

The low-pressure and low-density conditions encountered at high altitudes significantly reduce the operating Reynolds number of micro radial-flow turbines, frequently bringing it below the self-similarity critical threshold of 3.5 × 10⁴. This departure undermines the applicability of conventional similarity-based design approaches. In this study, micro radial-flow turbines with rotor diameters below 50 mm are investigated through a combined approach integrating high-fidelity numerical simulations with experimental validation, aiming to elucidate the mechanisms by which low Reynolds numbers influence aerodynamic and thermodynamic performance. The results demonstrate that decreasing Reynolds number leads to boundary-layer thickening on blade surfaces, enhanced flow separation on the suction side, and increased secondary-flow losses within the blade passages. These effects jointly produce a pronounced and non-linear deterioration of turbine efficiency. Geometric scaling analysis further indicates that efficiency losses intensify with decreasing turbine size, and become particularly severe at low rotational speeds and high expansion ratios. Detailed flow-field analyses reveal a direct link between the degradation of blade loading distribution and the amplification of transverse pressure gradients under low-Reynolds-number conditions, providing physical insight into the observed performance decline.