Phase-change materials (PCM) are particularly promising for thermal storage in various energy plants as solar plants, district heating, heat pumps, etc. mainly because of the possibility to reduce the volume of storage tanks, but also because the problems related with thermal stratification are considerably reduced. On the other hand, research is necessary in order to address technical problems, mainly related to the heat transfer in the medium, which needs to be enhanced in order to achieve reasonable charging and discharging processes. The present paper describes the application of computational fluid-dynamics (CFD) for the analysis of PCM thermal storage systems. The numerical analysis is directed at understanding the role of buoyancy-driven convection during constrained solidification and melting inside a shell-and-tube geometry. The 2D model is based on a finite-volume numerical procedure that adopts the enthalpy method to take in account the phase change phenomenon. The time-dependent simulations show the melting phase front and melting fraction of the PCM and incorporate the fluid flow in the liquid phase. The obtained temperature profiles are compared to a set of experimental data available in the literature. The results show that during the melting process natural convection within the PCM has non negligible effects on the behavior of the system. The numerical simulations of the solidification process show that the increasing solid fraction of the PCM inhibits the buoyancy in the remaining liquid portion of the phase-change-material. Furthermore, the paper discusses the effects on the phase-change processes of the main operating conditions, including inlet temperature and mass flow rate of the heat transfer fluid.

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