Melting experiments in a vertical square enclosure are undertaken with a solid-liquid phase-change material (n-octadecane) dispersed with nanoparticles (Al2O3) as the nano-phase-change-material (nano-PCM). The enclosure has a cross-section dimension of 25 by 25 mm and it was 60 mm long. The vertical side walls were differentially heated isothermally while the remaining side walls were thermally insulated. The effect of dispersing nanoparticles in the base phase change material (PCM) on the melting heat transfer characteristics in the enclosure is examined for the relevant parameters in the following ranges: the mass particle fraction of nanoparticles, omega(p) = 0, 5, 10 wt.%; the Rayleigh number, Ra-f = 1.71 x 10(6)-5.67 x 10(7); the Stefan number, Ste(f) = 0.037-0.108; and the subcooling parameter, Sb = 0.042-0.537. The surface-averaged heat transfer results obtained during the melting experiments indicate that natural convection heat transfer into the melted region of the enclosure tends to degrade markedly with increasing mass fraction of nanoparticles dispersed in the nano-PCM, when compared with that of the base PCM. (C) 2013 Elsevier Ltd. All rights reserved.