•Combined effect of fin length, fin position, heated wall orientation and nanoparticles•For side-heating, assembling fin near the bottom wall (l/W = 0.25) is more effective•Position of fin may promote melting rate from 13 to 40% for bottom-heating•Shorter fins could be as effective as longer ones by introducing nanoparticles•Combined utilization of fin and nanoparticles augments melting rate up at least 54% A comprehensive numerical study is presented to investigate the melting process in a square enclosure whose two facing walls are maintained at different constant temperatures while the other facing walls are thermally insulated. Thermophysical properties of the utilized PCM depend on both temperature and phase. The influence of two different orientations of heated and cooled walls, various lengths and positions of the fin that is attached to the hot wall on phase change process is studied. Besides, analyses are performed considering the contribution of CuO nanoparticles and the contribution of each investigated parameter on the enhancement of the melting process is identified. The obtained results show that all the studied parameters have a remarkable influence on melting rate and energy storage, besides, natural convection has an incontrovertible role in melting process. Assembling the fin in the enclosure shortens the melting time between by 27 to 52% depending on its length and 13 to 68% depending on its position compared to the case without fin. It was also found that assembling fin promotes the melting rate up to 52% depending on the heated wall orientation. A further enhancement is attained with the addition of nanoparticles particularly for shorter fin lengths and bottom-heating case. The computed results also reveal that a combined utilization of a shorter fin together with nanoparticles can provide a thermal performance as high as that of a long fin, thus the utilization of nanoparticles with shorter fins may be an alternative to assembling of long fins without nanoparticles. Display omitted