Experimental investigation of thermal performance in a shell-and-tube phase change thermal energy storage unit with an inner square tube

Experimental investigations of phase change processes in a shell-and-tube latent heat thermal energy storage unit with an inner square tube were carried out. Paraffin OP44E was selected as a phase change material, and the water heated or cooled by constant temperature water tanks flowed into the inner square tube as the heat transfer fluid. Visualization experiments allow for the observation and analysis of the solid-liquid interface during the phase change process. Due to natural convection, the melting interface progresses from top to bottom, while the solidification interface advances from the center outward, as heat transfer is primarily governed by conduction. The experimental results demonstrated that the initial temperature of the PCM has a negligible impact on the thermal performance of the energy storage unit, particularly during the solidification process. The dimensionless analysis reveals the relationships of Nusselt numbers, Stefan numbers, and Reynolds numbers. Furthermore, four stages of conduction, mixed conduction-convection, convective enhancement, and convective attenuation were divided according to the effects of natural convection. Finally, empirical formulas for effectiveness were established to provide guidance for the heat exchanger design.