Numerical and Experimental Study of the Melting Process of a Phase Change Material in a Partically Filled Spherical Shell

A three dimensional axisymmetric model of the heat transfer during the melting process of a phase change material (PCM) inside a spherical container was analyzed both numerically and experimentally. A study of PCM phase change is important to understand the behavior of these materials in thermal storage applications. A void space was provided within the container to consider the volumetric expansion of PCM during the melting process. The PCM's properties used in the simulations, include the melting temperature, latent and sensible specific heat, thermal conductivity, and density in the solid and liquid states, and were based on a commercially available salt hydrate, calcium chloride hexahydrate. This salt has been used mainly in latent heat-based heat storage systems. The mathematical model was solved using the "Volume of Fluid Method," and the "Enthalpy-Porosity Formulation" was employed to solve the energy equations in both the liquid and solid regions of the PCM, using the Fluent software. A detailed sensitivity investigation was performed for melting in spherical shells of 40, 60, and 80 mm in diameter, while the outer surface temperature of the container was set to 5, 10, and 15 °C above the mean melting temperature of the PCM. The simulations showed the melting process from the start of phase change to the end, and incorporated some phenomena such as convection in the liquid phase, volumetric expansion due to melting, sinking of the solid phase, and close contact melting. It was found that at a constant value of Stefan Number, increasing the Grashof Number will enhance the heat transfer rate. Additionally, the combined effect of the Grashof and Stefan Numbers at an increase of the outer surface temperature of the enclosure could also enhance the melting rate of the PCM. Finally, appropriate dimensionless variables, based on a combination of the Fourier, Grashof and Stefan Numbers, were introduced in order to obtain a generalized correlation for the liquid mass fraction and the Nusselt Number during melting of the phase change material.