## 1 Introduction
[16] used iron oxide-nanofluids as working fluids to study the effects of different parameters on the thermal performance of two-phase closed thermosyphons, wherein they focused on the heat transfer rate, evaporative condensation heat transfer coefficient, and thermal resistance. They experimentally studied different working fluid nanoparticle concentrations, working temperatures, and thermosyphon inclination angles. In addition, they compared the performance provided by the nanofluid to that provided by pure water. [PERSON] et al. [17] studied the effects of different operating parameters such as heat flux, carbon nanotube (CNT) mass concentration, and working fluid filling ratio on the thermal performance and thermal resistance of the flat heat pipe. The results show that, with increasing heat flux, the heat transfer coefficient of the heat pipe evaporator section increases, and the heat transfer coefficient of CNT/water is 40% greater than that of deionized water. CNTs change the contact angle between the liquid and surface by forming a fouling layer on the screen structure and enhancing the capillary force and thermal performance of the structure. [PERSON] et al. [18] systematically studied the effect of the liquid filling rate on the thermal performance of a thin novel flat heat pipe (FHP) heat exchanger under natural convection conditions. Their results showed that the composite material had an optimum filling range, and the temperature difference was very small. Therefore, the FHP heat exchanger exhibited good thermal diffusivity. [PERSON] et al. [19] used computational fluid dynamics (CFD) to study the transient thermal performance of phase change and heat and mass transfer in a two-phase closed thermosyphon. A CFD model based on fluid volume technology was established. The working fluid was deionized water. The CFD model reproduces the evaporation and condensation processes of the thermosyphon under different heating inputs. By increasing heating input, the thermal resistance decreases to the minimum (0.552 K/W) and the effective thermal conductivity increases to the maximum (2.07 \(\times\) 106 W/m\(\cdot\)K). [PERSON] et al. [20] used Fluent to study the influence of the filling rate and thermosyphon inclination angle on the thermal performance of a thermosyphon and then compared the computational fluid dynamics (CFD) simulation results with experimental data. Their results showed that the maximum deviations of the temperature distribution and thermal resistance were 4.2% and 8.1%, respectively, and the evaporator temperature increased significantly at low filling rates and low inclination angles. [PERSON] et al. [21] performed a comprehensive study of the boiling characteristics of geysers in two-phase closed thermosyphons with water as the working fluid at different inclinations and filling rates. The results showed that the inclination angle and liquid filling rate had significant effects on the occurrence and cycle of geyser boiling under high heat input conditions. The inclination angle is the angle between the horizontal plane and the thermosyphon axis, and this angle has an important influence on the heat transfer characteristics of a thermosyphon [10; 15; 21]. In the literature [15], the evaporator heat transfer coefficient of the thermosyphon can be increased by 33% by choosing the appropriate inclination angle. Previous studies have explained the effects of various factors on thermosyphons through experimental phenomena or simulation data, but no good theories that explain the response of the effects and predict the thermal resistance value have been presented. In this paper, by analyzing the main influencing factors of the inclination angle, a formula for calculating the actual heating area is deduced, and a theory for the areal thermal resistance is proposed, providing a research method for studying the heat transfer characteristics of thermosyphons.