A two phase closed thermosiphon is an efficient heat transfer device utilizing phase change phenomenon in closed volume. Since the boiling and condensation of pure fluid occurs at fixed temperature in given pressure, the heat transfer in thermosiphon is possible with negligible temperature difference. This superior heat transfer characteristic of thermosiphon is a great advantage in cryogenic applications where an object is cooled by cryocooler. The cryocooler-cooled thermosiphon is utilized in diverse fields. Conduction-cooled superconducting magnets use cryocoolers for their operation without using any cryogenic fluid. In such an application, the operation temperature range of thermosiphon is generally from the critical point to the triple point of the working fluid. Therefore, its working fluid experiences very large temperature and pressure variations with its physical properties during the cool-down process.
In this study, cryogenic thermosiphons are designed, fabricated and tested to investigate the operation characteristics. A Stirling cryocooler of which the cooling capacity is measured is used to cool down thermal load made of copper block. The temperature and pressure of the thermosiphon is mearured during the cool-down process. From the experimental results, the operation regime of thermosiphon is divided into 4 periods according to thermal condition of the thermosiphon. The thermodynamic condition of the working fluid in the thermosiphon in this transient period is different from that of the steady operational thermosiphon. The temperature difference between each end of the thermosiphon is large since there is an initial cooling period when natural convection only occurs at the early step of the cool-down process. Later, thermosiphon operates also with large temperature difference because the evaporator is still hot. When the evaporator temperature reaches close to the saturation temperature of the working fluid, the thermosiphon operates stably. The heat transfer mechanism in the thermosiphon undergoes various thermal conditions with its thermodynamic condition. Modeling of the thermosiphon operation is also carried out to predict the performance of thermosiphon. The appropriate modeling is presented and verified by the numerical simulation.
