Unconstrained melting heat transfer in a spherical container revisited in the presence of nano-enhanced phase change materials (NePCM)

The classical problem of unconstrained melting in a spherical container was revisited experimentally in the presence of nano-enhanced phase change materials (NePCM). The NePCM samples were prepared with graphite nanosheets at various loadings up to 1% by weight. A number of important thermophysical properties of the NePCM samples were in-house measured in terms of the loading and temperature. A variety of boundary temperatures were adopted in the melting experiments. It was shown that the total melting time is shortened by 10% for the 0.5 wt.% sample at the lowest boundary temperature (only 8 degrees C above the melting point), as a result of the enhanced thermal conductivity in comparison to that of pure PCM. However, all of the other cases, corresponding to either increasing the loading or raising the boundary temperature, were demonstrated to slow the melting down because the contribution by natural convection is significantly suppressed, or even vanished, by the dramatic growth in viscosity, which was more than 60-fold for the 1.0 wt.% sample. The rise in viscosity was also understood to thicken the thin molten layer within the close-contact melting region, thus impeding heat conduction through this layer. In addition, both the melt fraction and surface-averaged heat flux were correlated to a grouping of dimensionless variables that govern this problem. Universal correlations were proposed for all of the NePCM samples together with an overall uncertainty below 15%. (C) 2016 Elsevier Ltd. All rights reserved.