Phase change materials are widely used in thermal energy storage systems because of their high latent heat capacity and near-isothermal heat exchange characteristics. This study presents a numerical investigation of the melting performance of paraffin-wax-based phase-change material (PCM) enhanced by aluminum oxide (Al₂O₃) nanoparticles in plain and finned square enclosures subjected to differential wall heating. The
primary novelty of this work lies in the systematic, combined evaluation of fin geometry and nanoparticle loading as synergistic enhancement strategies in a natural-convection-driven PCM storage system. The fin subdivides the flow into small convection cells, reducing the velocity of the molten PCM. The solid-liquid phase change process was modeled using the enthalpy-porosity method within a transient finite volume framework (ANSYS Fluent). The Rayleigh number (Ra) was varied from 10³ to 10⁶ by adjusting the temperature difference (ΔT) while keeping a fixed cavity dimension of 8 cm. Nanoparticle volume fractions of 1%, 2%, and 3% Al₂O₃ were examined. When the Rayleigh number is Ra = 10³, the fin-induced subdivision of convective cells reduces the maximum flow speed, while the melting time is reduced to 49 Sec compared to 80 Sec in the plain enclosure. In the plain enclosure at Ra = 10³, complete melting of pure paraffin occurs in 80 Sec; the addition of 3% Al₂O₃ reduces the melting time to 57 s (28.75% reduction). The introduction of an internal fin induces twin-cell convective structures that, despite reducing peak velocity size by a factor of approximately 3.5 compared to the plain enclosure, decrease total melting time to 49 sec 38.75% improvement over the plain pure-paraffin baseline. The Nusselt number increases monotonically with Ra, confirming enhancement.