Experimental Characterization of Inward Freezing and Melting of Additive-Enhanced Phase-Change Materials Within Millimeter-Scale Cylindrical Enclosures

The inward melting and solidification of phase-change materials (PCM) within millimeter-scale cylindrical enclosures have been experimentally characterized in this work. The effects of cylinder size, thermal loading, and concentration of high-conductivity additives were investigated under constant temperature boundary conditions. Using a custom-built apparatus with fast response, freezing and melting have been measured for time periods as short as 15 s and 33 s, respectively. The enhancement of PCM thermal conductivity using exfoliated graphene nanoplatelets (xGnPs) has also been measured, showing a greater than 3× increase for a concentration of 6 wt.%. Reductions in the total melting and freezing times of up to 66% and 55%, respectively, have been achieved using xGnP concentrations of only 4.5 wt.%. It is shown that the phase-change dynamics of pure and enhanced PCM are well predicted using a simple conduction-only model, demonstrating the validity of approximating enhanced PCM with low additive loadings as homogenous materials with isotropic properties. While general consistency between the measurements and model is seen, the effect of additives on heat transfer rate during the initial stages of freezing and melting is lower than expected, particularly for the smaller cylinder sizes of 6 mm. These results suggest that the thermal resistance of the PCM is not the limiting factor dictating the speed of the solid–liquid interface during these initial stages.