Dry-out limit enhancement and optimal filling ratio determination in sodium heat pipes based on the overfilling effect

Abstract

This study deals with the critical aspects of heat pipe technology, focusing on the characteristics of operating limits, especially the dry-out limit of sodium heat pipes under various filling conditions. Understanding, predicting, and enhancing the dry-out limit of a sodium heat pipe is a key consideration for the safe design and operation of microreactors. Therefore, this research encompasses a comprehensive analysis of overfilled, optimally filled, and underfilled heat pipes, highlighting the implications of the filling ratio on thermal behavior and dry-out limit. Key areas of investigation include the analysis of thermal behavior in overfilled sodium heat pipes. The characteristics of overfilled sodium heat pipes appear as distinctive temperature patterns and operational limitations influenced by the movement of excess liquid and the formation of liquid plugs under high heat flux conditions. Continuum vapor flow leads excess liquid to the condenser end, forming a liquid pool in the condenser end region. This process smooths the temperature rise at the evaporator, rather than causing a rapid temperature increasing in dry-out conditions. Additionally, a continuous temperature decrease at the condenser end occurs due to the impediment of vapor flow to the condenser end by the liquid pool. Based on the liquid pool effect at the condenser end caused by overfilling, this research develops a modified capillary Chi model by integrating the effect of the filling ratio, addressing a gap in existing models. This modified model accounts for the liquid pool formed by initial excess liquid, which impedes the vapor's reach and can shorten the capillary transportation length. This improvement leads to more accurate predictions of the dry-out limit points, especially in overfilled conditions, suggesting that overfilling can delay the dry-out limit but may reduce the effective condenser length. A significant part of the study involves the development and validation of a semi-empirical method to determine the optimal filling ratio for sodium heat pipes. The method, based on comparing theoretical and actual liquid plug lengths, predicts a 161% charge as optimal. Experimental validation with a 172% filled heat pipe confirms this, showing superior isothermal performance and alignment with the Chi model's predictions compared to overfilled and underfilled heat pipes. The research extends to examining the heat pipes' performance in microreactor conditions, specifically under gas- cooled conditions, where different filling ratios distinctly influence the heat transfer rate and operational limits. Overall, the findings of this study enhance the understanding of sodium heat pipe behavior across a range of operational scenarios. The results not only validate the proposed semi- empirical method for optimal filling but also underscore the importance of considering filling ratios in the design and operation of microreactors, ultimately contributing to the advancement of heat pipe technology.