Heat transfer performance analysis of a large-scale separate heat pipe with a built-in tube

Large-scale separate heat pipes (SHP) are preferable to establish passive cooling systems for spent fuel pool (SFP). But the SHP performs not well in long heat transport applications. In this paper, a built-in tube separate heat pipe (BTSHP), which is a modification of SHP by inserting a straight smooth tube in the heat pipe evaporator is proposed to improve the heat pipe performance. Based on the SHP numerical model, which is validated by experimental results, the BTSHP heat transfer model is established. And the heat transfer performances of both SHP and BTSHP under different filling ratios, heat transport distances, and thermal loads are studied. It is found that BTSHP generally shows 8-28% extra heat transfer capacity than SHP. And the maximum thermal resistance is 5.2 degrees C/kW, which is 1.6 degrees C/kW lower than that of SHP. Compared with SHP, the BTSHP evidently decreases the overall circulation resistance and considerably improves the heat transfer capacity, especially in long heat transport distance, moderate and high filling ratio applications. Besides, when the cooling air velocity increases, the BTSHP can further release its potential by reducing fraction of liquid that flows into the built-in tube and get a better performance than SHP.