Influence of fin configuration and nanofluid-assisted heat transfer on melting dynamics in a concentric tube latent heat storage system

Fin and nanofluid-based performance enhancement techniques have been widely investigated for latent heat storage (LHS) systems. However, their relative contributions and interaction during the melting of phase change material (PCM) within a concentric-tube LHS configuration remain insufficiently clarified. This study numerically investigates the influence of fin configuration and nanoparticle incorporation strategy on the melting dynamics of PCM in a concentric-tube LHS system. Four configurations are analyzed: (i) solid fin–HTF, (ii) solid fin–nanoHTF, (iii) porous fin–HTF, and (iv) porous fin–nanoHTF. Simulations are performed using the enthalpy–porosity method to evaluate melting duration, temperature evolution, and melt-front progression. The results reveal a clear transition from conduction-dominated to natural convection-dominated heat transfer as melting progresses. Among all cases, the solid fin–nanofluid configuration exhibits the shortest melting time, indicating superior heat transfer performance. Nanofluid incorporation enhances melting primarily during the intermediate stage, modestly improving the melt fraction by 3%, and then diminishes further and becomes non-monotonic near thermal saturation. Among the porous-fin cases, increasing fin porosity generally degrades melting performance under the investigated conditions, with high porosity introducing additional thermal resistance and momentum damping. For instance, the melting performance of the 10% porosity configuration outperforms those at 50% and 90% porosity configurations. However, a porosity of 10% remains inferior to the solid-fin configuration under the conditions investigated. The enhanced convective heat transfer due to nanoparticle loading further accelerates the melting process. Increasing Reynolds number produces a substantially stronger enhancement than increasing nanoparticle concentration, indicating that flow intensity is a dominant factor in melting performance. The comparison between nanoHTF and nano-enhanced PCM further shows that modifying the HTF provides greater melting enhancement. The configuration with double fins exhibits the fastest melting rate, followed by the backward single-fin, middle single-fin, and forward single-fin configurations. Overall, double square solid fin in pure PCM with 5% nanoHTF turbulent flow is the best configuration among all investigated cases for PCM melting enhancement.