Experimental Study on Cooling Down Process of a Nitrogen-Charged Cryogenic Loop Heat Pipe

Cryogenic loop heat pipes are highly efficient heat transfer devices at cryogenic temperature range, which have promising application prospects in satellites, spacecrafts, electronics, and so on. Cooling down process is a most critical process for a CLHP before startup. At present, secondary loop is a major way for a CLHP to fulfil cooling down and most studies are concentrated on heat transfer characteristics during normal operation. However, few investigations have been carried out on the cooling down process. In this paper, the cooling down process of a nitrogen-charged CLHP assisted with a secondary loop was experimentally investigated. A simple qualitative approach to estimate the cooling down time was proposed according to the law of conservation of energy. The two flow paths of the working fluid in the CLHP during the cooling down process were described. Experimental studies on the cooling down process with various secondary heat loads and working fluid inventory were presented in detail. With the increase of secondary heat load, the elapsed time of Stage III decreased significantly due to the larger mass flow rate in Path I. In addition, the effect of the working fluid inventory on the cooling down time was generally small in the range from 2.99 MPa to 3.80 MPa. However, with 2.80 MPa working fluid inventory, it required much longer cooling down time, which was because of the lack of liquid in the CLHP with low working fluid inventory. Moreover, the influence of gravity on the temperature variation of the components during the experiments was analyzed. This work is beneficial for better understanding of the cooling down process and optimizing of CLHPs.