Effect of Nano-additives on Solidification of Phase Change Material in a Sphere Enclosure

Solidification of phase change materials (PCMs) is important for latent heat storage in buildings, thermal control of electric modules, and cooling of power plants. Solidification of PCMs in a spherical shell is of particular interest due to its high surface-to-volume ratio that can potentially improve the heat transfer rates. Nanoadditives, such as graphite nanofibers, exfoliated graphene nanoplatelets (xGPNs), and graphene-based foams have been utilized to enhance the heat transfer coefficient. However, there is still a fundamental lack of understanding regarding how nanoadditives affect solidification process and the stability of the nanoadditive-enhanced PCMs. In the present work, the solidification of eicosane with and without xGNPs in spherical shells of different diameters (38mm and 50mm) is investigated under two cooling conditions, i.e. freezing in fixed temperature water bath, and in controlled air flow. The thermal conductivities of pure PCM and PCM with xGNPs of different concentrations (1.5%, 3%, and 4.5%) are measured with the hot wire method. It is found that the thermal conductivity of the base PCM is enhanced by 54% with 3wt% of xGNP. When the solidification of PCM is conducted in water, the reduction in solidification time is about 51% for small sphere and 53% for large sphere at nanoadditive concentration 4.5% compared with pure eicosane. Additionally, when the solidification of PCM in performed in air, the reduction in solidification time is about 36% for small sphere and 39% for large sphere at 4.5% xGNP concentration. As a result, solidification in air takes longer time than solidification in water. Moreover, it is found that the solidification time decreases by decreasing the water temperature and by increasing air velocity respectively for both the small and large spheres.%%%%M.S., Mechanical Engineering  – Drexel University, 2015