Heat transfer performance of high-temperature heat pipe startup in small heat pipe reactors

Abstract

High-temperature heat pipe is an important heat transfer component in small heat pipe reactors (SHPR). Research on the transient heat transfer characteristics during heat pipe startup is crucial for safety analysis of SHPR. This manuscript investigates the startup performance of a lithium heat pipe from the frozen state with various influencing factors, including heating power, inclination angle, and convection heat transfer coefficient. The objective of this work is to speed up the heat pipe startup process in a safe and economic manner based on its heat transfer characteristics. In order to illustrate the startup characteristics and further to reduce the startup time, this manuscript constructs a CFD heat pipe model via the multi-physics simulation software, COMSOL. The phase change process of heat pipe startup is described by the Phase Change Material method, with the fluid velocity and pressure fields of wick established using the Brinkman equation. Results show that the heat pipe thermal resistance decreases with increasing heating power, positive inclination angle, or convection heat transfer coefficient. On the other hand, the heat pipe startup time reduces with increasing heating power, increasing positive inclination angle, or decreasing convection heat transfer coefficient. Moreover, numerical analysis indicates that heat pipe temperature rises faster in the evaporation section than in the condensation section at the beginning of startup. Temperature rise process in the condensation section occupies the majority of the entire heat pipe startup process. Therefore, to speed up the startup process, this work proposes a novel extra heating method applying to the condensation section at early startup. Compared with the conventional startup, the proposed method is able to shorten the startup time by 10.5 % and 15.8 % with an extra heating power of 10 % Full Power (FP) and 20 %FP respectively.