Numerical investigation on flow characteristics and thermal performance of phase change material in spherical capsules with smooth or dimpled surfaces

The utilization of phase change spherical capsules with dimpled surfaces in a packed-bed latent thermal energy storage (PLTES) system provides a novel approach to addressing the intermittency and fluctuation in solar thermal utilization processes. In the context of accounting for wall effects, a comparative analysis of the flow characteristics and thermal performance of smooth and dimpled spherical phase-change capsules is conducted using the Large Eddy Simulation (LES) and an enthalpy-porosity method. Results show that the dimpled spherical capsule exhibits a substantial improvement in thermal performance compared to the smooth spherical capsule, with a maximum increase of 28.3 % in time-averaged Nusselt number when the difference in time-averaged drag coefficient is within 2 %. Compared to the smooth spherical capsules, the transient characteristics of dimpled spherical capsules exhibit significant changes, with the fluctuation amplitude of the drag coefficient and the fluctuation frequency of the lift coefficient both increasing by approximately twice. The local heat transfer performance also exhibits significant differences. The increase in surface temperature enhances the melting rate of PCM inside spherical capsules, but with the temperature rise, the enhancement efficiency decreases. The dimpled structure has a promoting effect on the melting process of PCM inside spherical capsules, and with the increase in surface temperature, this promoting effect becomes more pronounced. The time for PCM to achieve complete melting can be reduced by up to 19.07 %. The work contributes a novel idea for the application of nonsmooth structures in PLTES systems.