Simulation of melting and solidification of graphene nanoparticles-PCM inside a dual tube heat exchanger with extended surface

In this research, the charging and discharging processes of nano-phase change material (nano-PCM) CaCl2 center dot 6H(2)O containing Graphene nanoparticles in a two-dimensional circular enclosure are simulated. The enclosure consists of two concentric circles so that the inner circle has a high temperature (charging mode) and low temperature (discharging mode), and the outer one is insulated. Twelve rectangular blades are placed on the walls of the inner circle with the same temperature as the wall temperature. By changing the length from 0 to 1.5 mm in the process of charging and discharging of nano-PCM, the values of Nusselt number, percentage of changed phase, melting and freezing contours, etc., are studied. COMSOL Multiphysics 5.5 software and finite element method were employed for the simulations and the melting front was simulated using the enthalpy method. The results demonstrate that using a blade with a longer length causes the enclosure temperature to be higher in the charging mode and lower in the freezing state than the case without blades mode at the same time. Adding a blade with a length of 1.5 to the enclosure causes the Nusselt number to enhance by 252% in 100 s and to decrease by 87% in 1000 s in charging mode, while the Nusselt number is reduced by 94% and is enhanced by 115% at the same times, respectively, in the discharging process. An increase in the blade length causes the amount of molten nano-PCM to enhance in the charging mode and the amount of frozen nano-PCM to increase in the discharging state. Also, using an enclosure with a longer blade length makes the charging and discharging processes take place faster.