Abstract In this experimental study, the unconstrained melting process of nano-enhanced phase change material (NePCM) in a spherical container is investigated. For this purpose, the green synthesized copper oxide nanosheets (CuO NSs) as nanomaterial and paraffin wax as phase change material (PCM) is used. The density, thermal conductivity, and viscosity of the samples were measured. The factors of interest were melt fraction (f), Grashof number (Gr), Stefan number (Ste), Nusselt number (Nu), Fourier number (Fo), wall temperature (T, 65, 75 and 85 °C) and nanomaterial mass fraction (wt., 0.00, 0.05, 0.10 and 0.20%). Melt fraction was calculated and validated with a maximum deviation below 16% by using volume shrinkage and image processing methods. Results show that, at the wall temperature of 65 °C, the melting rate of the samples are the same; also, the total melting time for all samples at 65 °C are approximately 40 min, which is 15 and 20 min more extended than the wall temperatures of 75 °C and 85 °C cases. Nu number curves clarify that in the first 5 min of the melting, pure PCM graph is upper than other curves and becomes smoother as the CuO NSs loadings increase. Finally, the melt fraction and Nusselt number correlations are developed with a combination of f, T, Fo, Ste, and Gr. These proposed correlations are valid for the range of 0.002 ≤ FoSte ≤ 0.147 and 12400 ≤ Gr ≤ 169000. Based on the general suggested correlation, the squared correlation coefficient (R2) and mean absolute error (M.A.E.) values for the melt fraction, are 0.912 and 16.4%. Also, these values for the Nu number correlation, are 0.920 and 21%, respectively.