Evaporation of Alkali Metal Sodium Droplet in the Circular Tube

Abstract High-temperature heat pipe is a passive and efficient heat transfer element, which is generally filled with a certain mass of alkali metal as the heat transfer medium, and utilizes vapor-liquid phase change to efficiently transfer heat from the evaporation section to the condensation section of the heat pipe. In the normal operation of high-temperature heat pipe, the alkali metal will experience evaporation in the evaporation section, vapor diffusion in the adiabatic section, condensation in the condensation section, and reflux under the action of capillary force of the wick structure and other complex heat and mass transfer behaviors. This study focuses on the evaporation behavior of sodium alkali metal inside a high-temperature heat pipe. First of all, an experimental rig is constructed to visualize the evaporation behavior of alkali metal sodium droplets, which mainly includes the alkali metal sodium purification system (to remove the kerosene and impurities inside the commercial sodium to ensure the high purity of sodium), the visual measurement system (to record the droplet morphology), and the constant-temperature and constant-pressure argon system (to provide a high-purity argon environment to avoid the chemical reaction between sodium and the water vapor and oxygen in the air during the experimental process and to provide different temperature and pressure environments for the evaporation of sodium). Then, the evaporation behavior of alkali metal sodium droplets in a circular tube with different wall temperatures and pressure is experimentally investigated by using the previously constructed experimental rig, and the morphological changes of the droplets during the evaporation process were recorded by a high-speed camera. After that, the image analysis software ImageJ is used to analyze the droplet morphology to obtain the change curves of droplet height, contact line length, and contact angle with time during the evaporation of sodium alkali droplets. And the curves are further analyzed to obtain the information of sodium droplet evaporation mode and evaporation rate. By comparing and analyzing the experimental data of sodium droplet evaporation from round tubes with different temperatures and ambient pressures, the influence of the temperature and ambient pressures on the evaporation behavior of sodium droplets is obtained. In this study, the first experimental data of sodium alkali droplet evaporation in a high-temperature heat pipe is obtained by visualization experimental means, which reveals the mechanism of sodium droplet evaporation and lays a foundation for further improvement of the existing heat pipe simulation model.