Investigation of Melting Process in a Double Tube Energy Storage Unit: Effect of the Eccentricity of the Inner Tube

As reported in the literature regarding the melting process in energy storage systems, the relatively low thermal conductivity of most phase change materials (PCMs) results in reduced heat transfer during the phase transition, which increases the charging time of the storage systems. To address this issue, several methods have been explored to reduce the melting time, such as the insertion of metallic surfaces and/or nanoparticles into the PCM. In the current study, we numerically investigated the impact of the upward and downward vertical displacement (eccentricity) of the inner tube of a shell-and-tube energy storage unit, which is oriented horizontally. The annular space is filled with adipic acid as the PCM, with a melting temperature in the range of [150 °C- 152 °C]. During the charging phase, an industrial hot heat transfer fluid (HTF) circulates through the central tube. The energy storage unit was modeled using the conservation equations of momentum and energy for both the PCM and HTF, and the resulting numerical model was validated by comparing the numerical results with experimental data. Numerical investigations were conducted using Ansys Fluent 2024. The mass flow rate and inlet temperature of the HTF were fixed at 0.017 kg/s and 180 °C, respectively. The eccentricities explor²ed in this study were 0.13, 0.26, and 0.4 for downward and upward displacements of the central tube of the storage unit. The results indicate that the downward displacement of the inner tube provides better energy performance compared to the upward displacement, resulting in a shorter melting time and a higher heat transfer rate.