Experimental study on the geyser boiling phenomena and periodic correlation in high temperature sodium heat pipes

Temperature fluctuations pose a critical challenge in the operation of high-temperature heat pipes, potentially compromising the stability and integrity of heat transfer systems. This research bridges a significant gap by examining the influence of filling ratio on the occurrence of temperature fluctuations, and presents an innovative semi-theoretical correlation. We have meticulously designed and constructed sodium heat pipes with varying filling ratios ranging from 33.3 % to 100.1 %, and conducted extensive experimental campaign to investigate their behavior. This finding indicates that the operational states of sodium heat pipes under different design parameters and operating conditions are categorized into four distinct regimes, which are categorized as 'Fin regime, Geyser Boiling (GB) regime, GB-Ideal transitional regime, and Ideal regime' with specific ranges identified. The experimental results demonstrate that the filling ratio is a pivotal factor influencing the operational spectrum where GBP manifest. Furthermore, this research has developed a novel semi-theoretical correlation that describes the numerical relationship between the heat pipe's fixed angle, filling ratio, heat transfer rate, and the thermophysical properties of the working fluid, with respect to the temperature fluctuation period. The results show that the fluctuation period decreases with an increase in the fixed angle, increases with an increase in the filling ratio, and decreases with an increase in heat transfer rate. This study provides valuable insights for the prediction and optimization of high-temperature sodium heat pipe performance, which is crucial for enhancing heat transfer efficiency, ensuring stable operation of heat pipes, and conducting safety analyses for the design of heat pipe reactors.