Abstract: To elucidate the micro-scale heat transfer mechanisms of the liquid-vapor phase change process in the wick of the high-temperature alkali metal heat pipe, we investigate the micro-scale heat transfer characteristics at the evaporating meniscus region for different alkali metals including potassium, sodium, and lithium by the contact line heat transfer model in this work. Distributions of the liquid film thickness, contact angle, interface temperature, and heat flux at the evaporating meniscus region for different alkali metals are obtained under the same saturation vapor pressure and wall superheat. Our results show that due to the high thermal conductivity of alkali metals, contact line heat transfer characteristics of potassium, sodium, and lithium are quite different from those of water. For alkali metals, the heat transfer in the micro region near the three-phase contact line is dominated by the thermal resistance at the vapor-liquid interface. Among potassium, sodium and lithium, lithium has the highest micro-scale heat transfer performances. We demonstrate that the thickness of the non-evaporating liquid film, the apparent contact angle and the pressure gradient of the liquid film are self-tuned according to the wall superheat, and a higher superheat results in a thinner non-evaporating liquid film, larger apparent contact angle and larger pressure gradient. The adsorbed film region, where a non-evaporating liquid film is adsorbed on the wall, is dominated by the disjoining pressure. In the thin-film region, both disjoining pressure and the capillary pressure contribute to the total pressure difference that pumps the liquid from the intrinsic meniscus region. The curvature of the vapor-liquid interface remains constant, and the capillary pressure dominates in the intrinsic meniscus region.

Key words: alkali metal, contact line heat transfer model, evaporating meniscus