Experimental investigation of the heat transfer characteristics of a helium cryogenic thermosyphon

• The heat transfer characteristics of the helium cryogenic thermosyphon are studied.
• The thermal resistance and the heat transfer rate of the dry-out limit are investigated.
• The effect of the helium filling ratio on the heat transfer performance is discussed.

The heat transfer performance of a cryogenic thermosyphon filled with helium as the working fluid is investigated experimentally with a G–M cryocooler as the heat sink in this study. The cryogenic thermosyphon acts as a thermal link between the cryocooler and the cooled target (the copper evaporator with a large mass). Helium is charged in different filling ratios, and the cooling down process and the heat transfer characteristics of the cryogenic thermosyphon are investigated. The cooling down process of the cooled target can be significantly accelerated by the presence of helium in the cryogenic thermosyphon and the cooling down period can be further shortened by the increase of filling ratio. The heat transfer mode changes from the liquid–vapor phase change to natural convection as the increase of the heating power applied on the evaporator. The heat transfer limit and thermal resistance are discussed for the liquid–vapor phase change heat transfer, and they can be estimated by empirical correlations. For the natural convection heat transfer, it can be enhanced by increasing the filling ratio, and the natural convection of supercritical helium is much stronger than that of gaseous helium.