Experimental and numerical characterization of natural convection in a vertical shell-and-tube latent thermal energy storage system

In this paper, physics of heat transfer mechanism in vertical cylindrical shell-and-tube latent heat thermal energy storage (LHTES) systems is investigated. Visualized experiments are carried out to investigate the evolution of the liquid/solid interface of a phase change material (PCM). The temporal variation of the experimental temperature is then used to investigate the effect of natural convection on heat transfer in the PCM. A combined conduction/convection model is applied to investigate the melted PCM's convective circulation to better understand the heat transfer within the liquid PCM. The results show that during the charging process liquid PCM ascends to the upper part of the system and the melting front moves downward. Thermal energy is transferred from the heat transfer fluid (HTF) to the liquid PCM through convection in a vertical liquid PCM layer formed around the HTF pipe. Later on, the high temperature liquid PCM induces horizontal convective circulation at the upper part of the system. During the discharging process, the results show that the solidification front moves along both radial and axial directions. The outcomes help to understand the effect of natural convection in order to be effectively utilized for the design and optimization of shell-and-tube LHTES systems.