Capillary characteristics and capillary limit of stainless steel screen mesh wicks for sodium-based high-temperature heat pipes

Sodium-based high-temperature heat pipes (HTHPs) exhibit great potential for aerospace thermal management and nuclear reactor cooling due to their high thermal conductivity and long service life. Their heat transfer performance is primarily constrained by the capillary characteristics of internal wick structures. This work systematically investigates the capillary characteristics and capillary limit of stainless steel screen mesh wicks for sodium-based HTHPs. A visual experimental system in a vacuum glove box is established to conduct sodium capillary suction tests under different structural and thermal conditions. The results show that capillary absorption initiates at 500 °C, and the liquid rising height increases with temperature. Capillary pressure is enhanced by higher mesh numbers but is nearly independent of mesh layers. High-temperature interfacial reactions improve wick permeability with increasing temperature, while permeability decreases with both rising mesh number and layer quantity. The capillary performance factor slightly increases with temperature but declines with more mesh layers, presenting temperature-dependent mesh number sensitivity: it increases monotonically with mesh number at 500 °C, while increasing first and then decreasing above 500 °C. The 1-layer 400-mesh wick achieves the optimal comprehensive capillary performance with a maximum factor of 6.984 × 10⁻⁷ m at 600 °C. Compared with the inaccurate self-developed theoretical model, the BP neural network realizes precise height prediction of capillary height with a correlation coefficient (R) of 0.9954. Furthermore, an optimized capillary limit prediction model is proposed, whose relative errors are reduced by 10.42% and 8.00% compared with existing models with improved prediction accuracy. This study provides fundamental data and reliable models for the design and optimization of sodium-based HTHPs.