Experimental investigation of heat transfer limitations in concentric annular sodium heat pipes and thermosyphon

Abstract

This paper presents experimental investigations on the heat transfer performance of concentric annular heat pipe (CAHP) and thermosyphon (CATP) using sodium as a working fluid for microreactor applications. CAHP characterized by wicks on the inner and outer walls offers the potential to enhance capillary force and enable more compact microreactor designs. The research aims to understand the implications of structural differences between CAHP, CATP and conventional heat pipe (CHP) and identify favorable conditions for their use in microreactors. The thermal behavior and heat transfer limits of CAHP and CATP were examined under various boundary conditions. The experimental results revealed that CAHP exhibited capillary limits approximately 71.2% higher than cylindrical heat pipes under horizontal conditions. However, the Chi model used for predicting capillary limits showed a difference about 19.7% in horizontal operation and exceeded 44.6% in vertical operation, highlighting the need for considering the effect of non-uniform capillary forces in CAHP. The presence of additional inner wick in CAHP provided large evaporation areas, which facilitated rapid vapor generation and enabled the development of a continuum flow faster than in CHP, by approximately 70 degrees C. However, in vertical operation, CATP necessitates higher heat fluxes and exhibits notable temperature disparities between the evaporator and condenser regions compared to CAHP. This difference can be explained depend on the vapor generation mechanisms: heat pipes utilize additional wick area to facilitate evaporation, while thermosyphon depend on boiling that the advantages related to the annular flow path in CATP are not clear. Furthermore, the presence of an adiabatic section yields contrasting effects: while it promotes the formation of larger menisci in heat pipes, its presence is unfavorable in thermosyphons as it can elevate boiling temperatures that delayed operations. The findings underscore the advantages of CAHP over CHP, particularly in microreactor applications, provide insights into design optimizations for compact microreactors, and showed gaps in operating prediction models and experimental results.