Flow and heat transfer characteristics of fractal tree-shaped heat pipe in enhancing the melting process of phase change material

Heat pipe, as a type of device with extremely high heat transfer rate, has been widely used in latent heat storage (LHS) to alleviate the problem of poor thermal conductivity in phase change material (PCM). Inspired by the application of tree-shaped fins in enhancing the thermal performance of LHS, this study innovatively proposes a tree-shaped heat pipe. This study aims to investigate the flow and heat transfer characteristics of tree-shaped heat pipe and further explore its availability in improving the heat transfer performance in the field of LHS. The flow characteristics and surface heat transfer coefficient distribution between tree-shaped heat pipe and traditional cylindrical heat pipe are compared under different heating power. The thermal advantages of Case 3 (tree-shaped heat pipe charge) compared to Case 2 (cylindrical heat pipe charge) and Case 1 (boundary charge) for PCM melting process are investigated. The results show that the tree-shaped branches increase the flow resistance and change the flow pattern of the vapor steam, which results in the vapor working fluid not being able to carry enough heat to reach the ends of the branches. The temperature and surface heat transfer coefficient in the condensation section decrease with increasing height, and the decreasing gradient of tree-shaped heat pipe is greater than that of cylindrical heat pipe. At the end of the condensation section, the surface heat transfer coefficient of cylindrical heat pipe is 2116.09 W·m⁻²·K⁻¹, while the heat transfer coefficient of tree-shaped heat pipe is only 1340.51 W·m⁻²·K⁻¹. Despite this, compared to Case 1 and Case 2, tree-shaped heat pipe in Case 3 shortens the total melting time of PCM by 30.15 % and 18.80 %, and reduces the average equivalent thermal resistance by 79.07 % and 51.4 %, respectively. Although the temperature and surface heat transfer coefficient of TSHP in the condensation section are lower than those of CHP, its more uniform mass distribution reduces the heat transfer path and equivalent thermal resistance between PCM and the heat pipe, resulting in a significant reduction in the melting time of PCM.