Flow characteristics and thermal performance of walnut-shaped phase change material capsules for latent heat thermal energy storage

Latent heat thermal energy storage (LHTES) technology can effectively address the intermittency and instability in solar energy utilization. However, conventional spherical capsules generally suffer from high flow resistance and low heat transfer efficiency. Inspired by the surface morphology of walnut shells, this study proposes a walnut-shaped phase change material (PCM) capsule with continuous surface grooves. The irregular geometric and surface grooves of the walnut-shaped capsule disturb the near-wall boundary layer, reduce the wake recirculation region, and provide a larger heat transfer area under the same volume, thereby enhancing the overall heat transfer performance. Paraffin was adopted as the PCM, and various capsules with same volume were fabricated using 3D printing technology. A visualization experimental system was established to investigate the phase change behaviors of different capsules under unconstrained conditions. The results demonstrate that the unconstrained melting time of the walnut-shaped capsule is 26.4% shorter than that the spherical capsules, while its flow resistance is reduced by 48.1%. Furthermore, the effects of the length, depth, and number of grooves on the capsule surface on the performance were further investigated. Furthermore, the effects of the groove depth, number, and length (G, N, K) on capsule performance are systematically investigated. The results indicate that the walnut-shaped capsule with the parameter combination of (3/8, 6, 6/7) achieves the best balance between low flow resistance and enhanced convective heat transfer performance. These findings are expected to provide new guidance for the geometric design of biomimetic PCM capsules in latent heat thermal energy storage systems.