High-temperature heat pipes, as heat transfer components with high efficiency, safety, and the advantage of not requiring additional power, have broad applications in space nuclear power and small, mobile nuclear power sources. Due to the complexity of the internal mechanisms of high-temperature heat pipes, steady state performance analysis is important for design and operation of lithium heat pipe.

This study aims to develop an improved lumped parameter numerical heat pipe model with a more complete physical model and a simpler solution for steady state performance analysis of lithium heat pipe.

First of all, the physical operation of the high-temperature heat pipe was considered to be composed of heat transfer cycles and fluid flow cycles, and the influence of different flow forms, compressibility, and Mach numbers on steam flow, as well as the variation of the liquid core working fluid, were taken into account into the fluid flow cycle. Then, the high-temperature heat pipe was divided into the evaporation section, adiabatic section, and condensation section, each consisting of solid, liquid, and vapor regions, and each part was treated as a node, with physical parameters concentrated on the nodes. Subsequently, thermal conduction differential equations, fluid flow differential equations, and thermodynamic differential equations were established for each node as needed, combining all the differential equations to form a system of differential equations based on the lumped parameter heat pipe with a combined annular and mesh wick. Thereafter, the finite difference method was employed to discretize the system of differential equations, and a Python program was used for solving these equations. Finally, the above-mentioned model was employed to analyze the flow and heat transfer characteristics of the ultra-long lithium heat pipe in the HP-STMC space reactor, and simulate the variations in operating parameters of the lithium heat pipe under fixed heat sink and working temperature conditions.

The research results indicate that: 1) The program demonstrates good predictive accuracy when compared with literature data. 2) Under fixed heat sink condition, with increasing heating power, thermal resistance, steam velocity, and the dryness of the liquid core decrease. 3) Under fixed working temperature of 1 600 K, the steam does not reach turbulent flow when the heat transfer power is below 8.5 kW, resulting in minimal changes in both total thermal resistance and steam thermal resistance. However, when the heat transfer power exceeds 8.5 kW, steam enters turbulent flow, causing a rapid increase in both total thermal resistance and steam thermal resistance. Simultaneously, steam velocity and the dryness of the liquid core also increase. In contrast, the liquid working fluid does not enter turbulent flow and maintains an extremely low velocity, with a maximum Reynolds number and velocity of approximately 260 m·s^-1 and 0.12 m·s^-1, respectively. 4) For ultra-long lithium heat pipes operating at 1 800 K and below, the steam thermal resistance accounts for about 3.9% of the total thermal resistance.

The findings of this study enhance our understanding of the complex dynamics within high-temperature heat pipes, providing a theoretical foundation and guidance for the design and engineering application of alkali metal heat pipes, represented by lithium heat pipes. This study may also serves as a technological basis for the design and operation of heat transfer systems in space nuclear power and portable nuclear power sources.