Structural design and heat transfer performance study of a diverging-converging sodium heat pipe via numerical simulation

This paper proposes a novel heat pipe with a diverging evaporator section and a converging condenser section to overcome the sonic limit of heat pipes and improve their heat transfer efficiency. The heat transfer performance and structural advantages of this design are systematically studied through a combination of theoretical modeling and numerical simulation. A three-dimensional transient heat transfer numerical model is established using ANSYS Fluent to analyze the thermal performance under different diverging-converging angles of 0° (traditional cylindrical), 0.2°, 0.4°, 0.6°, 0.8°, and 1.0° at heating powers of 300 W, 500 W, and 700 W. The research results indicate that the designed diverging-converging sodium heat pipe meets the structural design requirements. The diverging structure in the evaporator section effectively reduces the vapor flow velocity, avoids the sonic-limit effect during startup, and promotes full evaporation. The converging structure in the condenser section provides an accelerated channel for condensed liquid. The temperature difference of the heat pipe exhibits a nonlinear relationship with heating power, decreasing first and then increasing. At 500 W, the temperature difference is the smallest, indicating optimal heat transfer performance. Compared to traditional cylindrical heat pipes, the diverging-converging structure reduces the total thermal resistance by approximately 28.56 %, improves condensation heat-transfer coefficient by up to 47.23 %, and enhances the overall heat transfer coefficient by 39.98 %. This design offers significant application advantages in fields such as heat recovery, electronic cooling, and nuclear reactor core heat transfer.