Sodium heat pipe startup and non-condensable gas interactions

This study investigates the startup behavior of sodium heat pipes, focusing on how different startup methods, the presence of non-condensable gases (NCGs), and flow instabilities near the mixing layer affect thermal performance and operational stability. Four startup methods were evaluated, ranging from slow, incremental power increases to rapid, one-step power applications. Thermal instabilities were observed to emerge at a critical power of 50.43 W, corresponding to an operating temperature of 340 °C. Slow startups initiated below this threshold enabled a gradual displacement of NCGs toward the condenser, resulting in a uniform temperature distribution and extended effective heat transfer lengths. At 200 W, the effective length exceeded 800 mm in slow startups, whereas rapid startups showed shorter lengths due to mixing at the vapor-NCG interface. At 1000 W, rapid startups exhibit significant portions of the heat pipe remaining below the sodium melting point of 97.8 °C, particularly near the condenser. This occurs due to incomplete displacement of NCGs during the initial phase of startup, leading to uneven temperature distributions and inactive regions. The abrupt vaporization of sodium causes unstable flow patterns that prevent the vapor from fully engaging the condenser region. Slow startups, by contrast, gradually transition the entire pipe into operation, minimizing inactive regions and maintaining a more uniform temperature profile. These results underscore the need to manage startup rates carefully, especially at higher power levels, to ensure complete activation of the heat pipe. The results validate a theoretical model treating the flow near the mixing layer as compressible in time and incompressible in space. This approach successfully modeled the interface dynamics and instability mechanisms caused by rapid interactions between sodium vapor and NCGs. The findings demonstrate that gradual power increases are shown to maximize thermal performance and operational stability. Future research should refine startup methodologies, develop strategies to mitigate instabilities, and improve the interaction between sodium vapor and NCGs for high-temperature applications. This study provides critical insights into optimizing sodium heat pipe performance in high-temperature applications, particularly for advanced nuclear reactors and other demanding thermal management systems.