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In this work,an experimental study of melting heat transfer of nano-enhanced phase change materials (NePCM) in a differentially-heated rectangular cavity was performed.Two height-to-width aspect ratios of the cavity,i.e.,0.9 and 1.5,were investigated.The model NePCM samples were prepared by dispersing graphene nanoplatelets (GNP) into 1-tetradecanol,having a nominal melting point of 37℃,at loadings up to 3 wt.％.The viscosity was found to have a more than 10-fold increase at the highest loading of GNP.During the melting experiments,the wall superheat at the heating boundary was set to be 10℃ or 30℃.It was shown that with increasing the loading of GNP,both the heat storage and heat transfer rates during melting decelerate to some extent,at all geometrical and thermal configurations.This suggested that the use of NePCM in such cavity may not be able to enhance the heat storage rate due to the dramatic growth in viscosity,which deteriorates significantly natural convective heat transfer during melting to overweigh the enhanced heat conduction by only a decent increase in thermal conductivity.This also suggested that the numerically predicted melting accelerations and heat transfer enhancements,as a result of the increased thermal conductivity,in the literature are likely overestimated because the negative effects due to viscosity growth are underestimated.