Numerical simulation on the impact of dimpled structure on flow and thermal performance in phase-change spherical capsules within confined spaces

Developing a phase change capsule structure that can improve heat transfer performance while maintaining non-directionality and ease of aggregation is of great significance for enhancing the performance of packed-bed latent heat thermal energy storage systems. Inspired by a “golf ball,” introducing a dimple structure to the surface of a smooth phase-change spherical capsule is a potential method to promote thermal performance. However, the mechanism by which the dimpled structure affects the flow and heat transfer characteristics inside the packed bed channels is still unclear. Therefore, this study numerically investigates the flow and thermal characteristics of phase-change spherical capsules with smooth and golf ball-inspired surface structures. The results reveal two critical values: first, the wall effect becomes significant and cannot be ignored when the channel-to-sphere diameter ratio (W/D) is <4; second, when the number of dimples (N_d) exceeds 184, further increasing the number of dimples has minimal impact on the external flow and heat transfer characteristics in confined spaces. Secondly, the incorporation of the dimpled structure leads to a maximum reduction in C_d by 7.75 % due to the delay of flow separation. Besides, the dimpled structure destroyed the heat transfer deterioration zone and formed the semilunar heat transfer enhancement region, which increased Nu by 10.6 %. Finally, the presence of the dimpled structure enhances the natural convection intensity, which accelerates the melting rate of PCM, resulting in a reduction of 82 s in the time required for complete melting inside the dimpled spherical capsule.