A visualization study on the unconstrained melting of paraffin in spherical container

A visualization experiment is carried out to investigate the void-affected unconstrained melting process in a spherical container. Factors including the diameter of spherical container (20.05-70.30 mm), filling ratio of PCM (0.8-0.98), heating temperature (32-47 degrees C) and initial temperature (7-22 degrees C) are studied. Melting behaviors concerning the evolution of solid-liquid interface and the motion of solid PCM are revealed, from which a new unconstrained melting mode (i.e., floating melting which occurs before the close-contact melting) is observed due to the existence of void in solid PCM. In addition, the quantitative results of floating melting time, total melting time and liquid fraction are obtained. The results show that as the melting mode changes from floating melting to close-contact melting, the melting rate is slightly enhanced. With the increase of spherical container diameter, the floating melting time increases first and then decreases, while the total melting time has a continually increasing trend. The higher heating temperature and higher initial temperature will decrease the floating melting time and total melting time. As the filling ratio of PCM increases, the floating melting time and the total melting time increase at first and then decrease. Finally, the correlations of dimensionless total melting time and liquid fraction are proposed.