Modified Biot Number Models For Startup And Continuum Limits Of High Temperature Thermosyphons

Abstract

Two-phase thermosyphons are very efficient devices with excellent temperature uniformity due to their high thermal conductance characteristics. Large temperature gradients observed in sodium two-phase thermosyphons under steady state operation is a clear sign of design flaws. Bad designs may lead to the decrease of the device thermal performance, as well as to significant thermal stresses in the thermosyphon walls, affecting the operation life and promoting leakages. This study presents steady state theoretical models concerning the startup limit, the continuum limit and the temperature distribution along the condenser. The models proposed in this study are based on the analogy between a two-phase thermosyphon and a thermal fin, where a modified Biot number is proposed to predict the vapor flow behavior inside two-phase thermosyphons. The results obtained are in good agreement with experimental data, showing that the proposed models can be used for the design of reliable and safe high temperature two-phase thermosyphons .