Free convective melting-solidification heat transfer of nano-encapsulated phase change particles suspensions inside a coaxial pipe

The nano encapsulated phase change materials are of the great energy storage potential in various engineering applications. Since they are new nanomaterials, new models for understanding their thermal behavior and capability are essential. This work aims to investigate the unsteady thermal behavior of Nano-Encapsulated Phase Change Material (NEPCM) suspensions in a cylindrical cavity. The particles contain a Phase Change Material (PCM) core, which can absorb/release a substantial amount of thermal energy upon phase change. The phase change particles are well dispersed in a liquid fluid and freely move along with the fluid. The flow, heat transfer, and the particle phase change were modeled using partial differential equations. A non-dimensional approach was employed to generalize the study. The unsteady charging and discharging behavior of the NEPCM suspension are investigated for the volume fraction of the NEPCM particles, fusion temperature of nanoparticles, Stefan number, and the Rayleigh number. Numerical results show that an increment in the Stefan number, i.e., Ste, can significantly reduce the Nusselt number, i.e., Nua, at the charging mode of the system. However, the dependency of the Nua at the discharging mode on the Ste is negligible. Also, it was found that the effect of the fusion temperature of the particle's core (hf) on heat transfer depends on the working mode of the system. In the charging mode, using a higher value of hf decreases the heat transfer rate. Reversibly, a higher value of hf inhibits the Nua during discharging state. Furthermore, the results show that for Ra = 106, Ste = 0.2, and hf = 0.1, a rise of / from 0 to 0.05 leads in about 1.73 and 1.55 times of improvement in the value of Nua for the cases of the melting and solidification of the core of NEPCM particles. (C) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.