Experimental investigation on internal interface dynamics and heat transfer enhancement in a fluidized PCM capsule

Latent heat thermal energy storage using phase change materials (PCMs) is constrained by the low intrinsic thermal conductivity of the solid phase. Fluidized beds of encapsulated PCM spheres offer a dual pathway for performance improvement: enhanced external convection and mechanically agitated internal phase change. However, the microscopic mechanisms by which macroscopic particle motion accelerates internal solid-liquid phase transition remain poorly quantified. Here, we investigate the internal phase-change dynamics of 25 mm paraffin-filled spheres undergoing downward liquid fluidization using high-resolution optical diagnostics. Through advanced image processing, we extract the temporal evolution of the solid PCM fraction and, during melting, when the solid–liquid interface is optically accessible, track its instantaneous displacement. These melting-phase measurements reveal quasi-periodic interface oscillations with an amplitude of ∼0.4 mm and a dominant frequency of 2.8 Hz, which is closely aligned with the bed pressure fluctuation frequency, confirming a fluid-structure resonance mechanism. The root-mean-square velocity of the internal interface correlates linearly with the external sphere kinetic energy (R2 = 0.96). During solidification, although direct interface visualization is precluded, the same resonance mechanism, evidenced by the matching pressure fluctuation dynamics, drives internal advective mixing. Decomposition analysis demonstrates that this internal advection accounts for 60–70% of the total solidification time reduction, with the internal Nusselt number peaking at 3.5–4.0 during intermediate solidification stages. A dimensionless fit for the effective thermal conductivity enhancement factor is proposed within the tested range. These findings elucidate the governing physics and identify the key dimensionless parameters for fluidized bed thermal storage employing large PCM capsules, while the generalizability of the proposed scaling framework to other capsule sizes and PCM properties remains to be established through future multi-parameter studies.