Solidification of additive-enhanced phase change materials in spherical enclosures with convective cooling

Solidification of eicosane with and without nanoadditives is experimentally investigated in spherical enclosures subject to convective cooling in water and air. The effects of additive volume fraction and external convective cooling conditions (i.e., the heat transfer medium, subcooling, and flow velocity) on the solidification process are examined. The results are compared with a conduction-controlled thermal network model accounting for the enclosure and PCM resistances, as well as the convective subcooling. The experimentally determined solidification time is found to be consistently lower than the model prediction, likely due to asymmetric and dendritic solidification, as well as natural convection inside the enclosure and possible thermocouple position errors. A simple correlation is proposed to predict the solidification time of a phase change material (PCM) in a spherical enclosure subject to convective cooling based on the same enclosure subject to a constant temperature boundary. Results show that the solidification time decreases with the volume fraction of nanoadditives due to the improved PCM conductivity. In addition, the nanoadditives are found to be more effective for solidification in water than in air, due to the large air-side convective resistance that does not benefit from improving PCM conductivity.