Abstract
We numerically investigate heat storage characteristics of a small heat storage vessel packed with phase-change material (PCM) encapsulated into a spherical hollow. The heat storage vessel is dealt with as a porous medium, and the flow of the working fluid is analyzed two-dimensionally using the modified Darcian momentum equation which takes into account both the buoyancy effect and the channeling effect. The heat transfer in the encapsulated PCM is analyzed using a one-dimensional heat conduction model. It is clarified that the completion of the heat storage process is delayed by natural convection, since the incoming hot working fluid flows mainly near the upper part of the vessel. It is also found that the nonhomogeneity near the boundary walls prolongs the heat storage process since it decreases the flow resistance near the walls and promotes a bypass flow near the upper wall due to the natural convection. A nondimensional equation is derived for the time taken to complete heat storage, where the Fourier number is expressed as a function of the modified Stefan number, the modified Reynolds number and the modified Grashof number.
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