Investigation of improved VOF method in CFD simulation of sodium heat pipes using a multi-zone modeling method

Abstract

Alkali metal heat pipes are the vital components within the core of heat pipe-cooled reactors. The mechanism characteristics of alkali metal heat pipes need to be further analyzed. CFD simulation with the traditional Volume of Fluid method provide an essential means to analyze the flow and heat transfer mechanism in alkali metal heat pipes. For Lee heat and mass transfer equation within the VOF method, the evaporation and condensation coefficients have significant effects on the simulation results, and their values are typically determined empirically, which results in inaccurate or even unreasonable simulation results. To establish a reasonable numerical relationship for the evaporation and condensation coefficients of the working fluid, this paper employs a multi-zone modeling approach for heat pipes and proposes an improved VOF method. During the iterative process, the temperature and pressure values in the corresponding regions are updated based on the iteration results. The mass changes caused by evaporation and condensation processes in each wick and vapor chamber region are calculated and compared with the theoretical value. To validate the proposed method, a high-temperature experimental test platform was constructed, and an 820 mm sodium heat pipe was fabricated. Furthermore, experimental research was carried out at different heat pipe inclination angles and under various heat transfer powers, with the experimental results being compared to those obtained from the model simulations. The simulated temperature values at different points of the model agree well with the experimental values at different heat transfer levels. This research provides insights into the multiphase distribution and pressure change within the heat pipe, offering important references for the optimization design of alkali metal heat pipes.