Wetting Characteristics of Alkali Metal Sodium on Wire Mesh Wicks in High-temperature Heat Pipes

With their compact structure, high reliability, and inherent safety, heat pipe reactors demonstrate significant potential in energy supply for deep-space, deep-sea, and remote island applications. As a critical heat transfer component in heat pipe reactors, the high-temperature sodium heat pipe relies on the wetting characteristics of sodium on the wire mesh wick. These characteristics govern the capillary-driven flow of the working fluid from the condenser to the evaporator section, thereby directly influencing the thermal performance of the system. The wetting behavior of alkali metal sodium on wire mesh wicks was systematically investigated in this study using a self-developed visualization experimental platform, which includes a sodium purification system, a temperature- and pressure-controlled argon gas system, and a visualization measurement system. Experimental results reveal that ambient pressure has minimal impact on the contact angle. Overall, at the same temperature, higher pressure leads to slightly smaller contact angles. This is attributed to the reduced surface tension between the gas and liquid phases under lower pressure due to thinner gas molecules. A consistent wetting transition temperature T_trans (T_trans=500 ℃ for single-layer 600-mesh wire mesh) was observed across different pressures. Below T_trans, the contact angle remains nearly constant (average values: 128.58°, 125.46°, and 113.57° under 600, 800 and 1 000 Pa, respectively, with standard deviations of 2.28°, 4.19°, and 7.11°). Upon reaching T_trans, the contact angle decreases abruptly, and further temperature increases cause to drop suddenly to 0° (manifested as rapid droplet spreading). This phenomenon arises from reduced sodium surface tension at elevated temperatures and the disruption of the hydrophobic chromium oxide film on stainless steel surfaces by sodium at high temperatures, enhancing wettability. Additionally, increasing the number of wire mesh layers enlarges the contact angle, while higher mesh numbers reduce it. According to Cassie’s wetting theory, the apparent contact angle is negatively correlated with the solid-liquid contact area in a material’s microstructure. Higher mesh numbers increase the wire density per unit area, expanding the contact area and thus lowering the apparent contact angle. Both increasing mesh layers and mesh numbers elevate T_trans (T_trans for 1-layer 400-mesh, 1-layer 600-mesh, and 3-layer 600-mesh wire meshes are 450, 500, and 525 ℃, respectively). This is likely due to greater pore volume in multi-layered or high-mesh structures, which retains more residual oxygen, thereby requiring higher temperatures for sodium to reduce and disrupt the chromium oxide film. This study provides experimental insights for optimizing the design and high-precision performance analysis of high-temperature heat pipes, advancing the engineering application of heat pipe reactors.