Conjugate heat transfer and melting mechanisms of a metallic phase change capsule with an internal cavity under external forced convection

Conjugate heat transfer during charging of encapsulated metallic phase-change materials is controlled by external convection, shell conduction and transport within the melt. The previous research commonly prescribe uniform shell temperatures or neglect the influence of internal structural features. This study numerically investigates the conjugate heat-transfer characteristics of an aluminium PCM capsule with an internal cavity encapsulated by an Al₂O₃ shell and heated by external molten-salt flow. A fully coupled computational fluid dynamics model is employed to resolve the interaction between turbulent external flow, shell heat conduction, and internal melting. The influences of external flow orientation relative to gravity and Reynolds number on shell heat transfer, wake development, and melting evolution are examined. The results show that external flow orientation primarily affects the conduction-dominated initial melting stage by modifying the shell temperature distribution. Axial flow produces a nearly symmetric shell temperature field and more uniform melting, whereas cross-flow generates asymmetric surface heating caused by non-uniform external convection, leading to different melting rates during the early stage of phase change. At Re = 14000, the predicted complete-melting times are 25.76 and 24.97 s, respectively, with a difference of 3.1%. Reducing Reynolds number increases the external thermal resistance and extends the melting time to 334.97 s at Re = 35. The local Nusselt-number distribution is shown to be governed by boundary-layer development and wake behaviour, with rear-surface heat-transfer recovery progressively weakening as the Reynolds number decreases. These external hydrodynamic characteristics directly influence the heat flux entering the capsule and subsequently regulate the evolution of internal natural convection and melting. An otherwise identical gravity-off control shows that buoyancy-driven motion increases the PCM heat-absorption rate by 11.7% at a liquid fraction of 0.50 and reduces the complete-melting time by 10.2%. By resolving the continuous pathway from external boundary-layer and wake behaviour, through shell conduction and cavity-induced resistance, to buoyancy-driven melting inside the capsule, this study explains how external forced convection governs the transient charging response of an encapsulated metallic PCM.