Numerical Study on Flow and Heat Transfer of Alkali Metal High-temperature Heat Pipe during Frozen Startup Based on FVM

Abstract

In recent years, heat pipe has been widely used as an efficient heat transfer equipment in fuel chemistry, electronic communication and so on. The main structure of a heat pipe mainly includes the vacuum tube containing alkali metal working fluid and the composite mesh wick on the inner wall of the tube. In the axial direction, heat pipes are generally divided into three parts:evaporation section, adiabatic section and condensation section. Heat is input into the heat pipe through the evaporation section and output through the condensation section. The adiabatic section only plays a role in connecting and mass transfer. The length of each area can be flexibly arranged. In the radial direction, heat pipes are generally divided into outer wall surface, wick and vapor area. Alkali metal high-temperature heat pipes show broad application prospects in the nuclear reactor cooling and other aspects because of their strong heat transfer capacity and inherent safety at high temperature. Because the working fluid of these heat pipes is solid at room temperature, and whose saturated vapor pressure relatively low, their frozen start-up has a complex phase change process of the working fluid, so it brings some difficulties to the study of the frozen start-up of the high-temperature heat pipes. For different application scenarios, it is necessary to study the transient and steady-state operating characteristics of high-temperature heat pipes, to provide support for the application of high-temperature heat pipes in a variety of applicable scenarios. To establish a method for predicting the transient start-up and steady-state operation characteristics of alkali metal high-temperature heat pipes, this study utilized the finite volume method (FVM) to establish the pipe wall heat conduction model, the wick flow heat transfer model, and the vapor zone model. A frozen start-up transient analysis program for alkali metal high-temperature heat pipes was developed and verified using the C programming language, with a maximum relative deviation of 9.8%. Transient start-up of a single horizontal sodium heat pipe was simulated, and sensitivity analysis was conducted. The steam zone of the heat pipe enters the continuous flow state completely 700 seconds after start-up, and after a total of 3 000 seconds, the heat pipe reaches steady-state operation. Under steady-state operation, the heat pipe exhibits good isothermal properties, with a stable axial temperature difference of 22.5 K on the outer wall and an internal pressure drop of approximately 47 Pa in the wick. Furthermore, the ambient temperature primarily impacts the time required for the heat pipe to reach steady-state, as well as the vapor pressure and velocity distribution under steady-state conditions. Meanwhile, the length of the adiabatic section of the heat pipe influences the time required for it to reach steady-state and has a significant effect on the pressure and velocity distribution of the wick.

Keywords

• high-temperature heat pipe
• numerical simulation
• frozen start-up
• finite volume method