Heat pipe (HP) (Fig. 1) is one of the most remarkable achievements in the field of thermal physics and heat transfer engineering in recent century because of its unique ability to transfer heat over considerable distances with minimal or without considerable heat losses. It is a highly passive device that can quickly transfer heat from one point to another. They are often referred to as the ‘‘superconductors’’ of heat as they possess an extraordinary heat transfer capacity and rate with almost no heat loss [1, 2]
Heat pipe was first coined by Gaugler  in 1994 of the General Motors Corporation in the U.S. Patent No. 2350348. In 1962, Trefethen  revived the idea of a heat pipe in connection with the space program. However, serious development started in 1964 when the heat pipe was independently reinvented and a patent application was filed by Grover at Los Alamos National Laboratory in New Mexico. After the work of Grover, the heat transfer community started usage of heat pipe in different applications [4, 5]D.A. Reay and P. Dunn  described different applications of heat pipes in the areas of space craft, chemical reactors etc. The main applications of HPs are to protect the environment and as well to save energy. The utilisation of heat pipes in a nuclear application improves the heat transfer coefficient, which is higher than in conventional heat exchanger systems.
The operation of a heat pipe  is easily understood by using a cylindrical geometry. Though, it can be of any size or shape. Working fluids such as water, methane, etc can be used with respect to its operating temperature. HP length is divided into three sections: The evaporator, adiabatic and condenser sections. Heat applied externally to the evaporator section is conducted through the pipe wall and wick structure, where it vaporizes the working fluid. The resulting vapor pressure drives the vapor through the adiabatic section to the condenser, where the vapor condenses, releasing its latent heat of vaporization to the provided heat sink. The capillary pressure created by the menisci in the wick pumps (or drives) the condensed fluid back to the evaporator section. Therefore, the heat pipe can continuously transport the latent heat of vaporization from the evaporator to the condenser section. This process will continue as long as there is a sufficient capillary pressure to drive the condensate back to the evaporator. During the condensation process, the menisci in the condenser section are nearly flat. A capillary pressure exists at the liquid-vapor interface due to the surface tension of the working fluid and the curved structure of the interface. The difference in the curvature of the menisci along the liquid-vapor interface causes the capillary pressure to change along the pipe. This capillary pressure gradient circulates the fluid against the liquid and vapor pressure losses, and adverse body forces such as gravity or acceleration.
1.Zohuri, B., Heat Pipe Design and Technology Modern Applications for Practical (z-lib.org).pdf. second ed. 2016: Springer International Publishing.
2.Zohuri, B., Heat Pipe Design and Technology. 2016.
3.Wang, P., et al., A laboratory scale heat pipe condenser with sweating boosted air cooling. Applied Thermal Engineering, 2020. 170.
4.Mochizuki, M., et al., Heat pipe based passive emergency core cooling system for safe shutdown of nuclear power reactor. Applied Thermal Engineering, 2014. 73(1): p. 699-706.
5.Kuang, Y., C. Yi, and W. Wang, Heat transfer performance analysis of a large-scale separate heat pipe with a built-in tube. Applied Thermal Engineering, 2020. 167.
6.Faghri, A., Heat Pipes: Review, Opportunities and Challenges. Frontiers in Heat Pipes, 2014. 5(1).
7.Faghri, A., Heat Pipes: Review, Opportunities and Challenges. Frontiers in Heat Pipes, 2014: p. 48.
Writer: Emmanuel Osei Tutu (Harbin Engineering University)
Editor: Priscilla Oforiwaa
Designers: Zhang Jing & Zhang Chao
Translation : Zhang Chao