Natural convection in the melting of PCM in a cylindrical thermal energy storage system: effects of flow arrangements of heat transfer fluid and associated thermal boundary conditions

Latent thermal energy storage systems (LTESS) have received widespread attention due to their high energy density to store a significant amount of thermal energy in a form of latent heat into phase change materials (PCM) at a nearly constant melting temperature. The thermal efficiency of LTESS is usually limited by poor heat conduction in PCM but enhanced by the natural convection of molten PCM. The natural convection increases the uniformity of temperature by mixing in a PCM enclosure, and therefore increases the heat transfer rates and accelerates the melting. While there is negligible natural convection, periodic reciprocation of heat transfer fluid (HTF) through the PCM enclosure has been demonstrated to increase the heat transfer rates to PCM by increasing the melt interface area and reducing temperature gradients across PCM compared to fixed-directional flow arrangements. The current study examines the effect of HTF flow direction on the strength and duration of natural convection in Gallium as the PCM in a vertical cylindrical shell-and-tube container. The irregular melting front in the PCM is caused by both natural convection in molten PCM and thermal boundary conditions for different HTF flow arrangements. The temperature and melting front profiles of PCM with the reciprocating flow arrangement are compared to unidirectional flows in upward and downward directions. The influence of HTF operating parameters such as temperature, velocity, and reciprocation period on PCM melting are studied. Scale analysis is also applied to characterize the different melting regimes of PCM under different flow arrangements.