Numerical Study on the Effect of Surface Wettability on Heat Transfer Characteristics of High-Temperature Two-Phase Closed Thermosiphon

Abstract To better understand the heat transfer mechanisms and influencing factors of a high-temperature two-phase closed thermosiphon, this study conducted a volume of fluid-based numerical simulation using sodium as the working fluid. The transient phase change process inside the heat pipe was examined, along with the movement and condensation behavior of wall bubbles under varying inner wall wettabilities. Their impact on the overall heat transfer characteristics was also analyzed. The results indicate that the vaporization core first forms on the evaporator wall, with vapor gradually growing and merging until stable nucleate boiling occurs. In the evaporator, hydrophilic walls enhance the bubble heat transfer, while hydrophobic walls tend to cause bubbles to adhere to the surface. In the condenser section, hydrophilic walls primarily experience film condensation, while hydrophobic walls exhibit dropwise condensation. The axial velocity of the evaporator section increases with fluctuations, reaching a peak of 0.92 m/s. Upon encountering the cooler condenser section, the velocity and temperature decrease. As the contact angle decreases from 157 deg to 30 deg, the average axial velocity in the evaporator section increases from 0.11 m/s to 0.2 m/s, but the average axial temperature in the condensation section increases from 1078.68 K to 1094.72 K. Additionally, when the heat pipe's inner wall exhibits superhydrophilic and superhydrophobic properties, the evaporator and condenser heat transfer coefficients improve by 14% and 0.68%, respectively. Compared to a uniformly wetted surface, modifying the inner wall so the evaporator section is superhydrophilic and the condenser and adiabatic sections are superhydrophobic reduces thermal resistance by 0.22%.