Research on alternative energy resources has intensified during the past few decades as a consequence of the alarming increase in energy cost. Effective thermal energy storage systems have become a true necessity, especially in solar energy applications. Using the phase change of some materials in thermal energy storage systems is advantageous in various ways. For example, the heat capacity of the storage reservoir is tremendously increased (on a unit volume basis), since latent heat of fusion is involved. In addition, there are advantages as far as heat transfer from and to the reservoir is concerned. Until recently, most papers on phase change dealt exclusively with conduction heat transfer, although it has been known for some time that natural convection may play a key role during melting and freezing. Lately, however, several studies (Viskanta, 1983) considering buoyancy effects on phase-change heat transfer have been reported. The present paper reports on experimental measurements undertaken to investigate outward melting around a vertical cylinder embedded in a solid initially at its fusion temperature. The cylinder is maintained at a uniform temperature that exceeds the fusion temperature. The top and bottom of the phase-change material are adiabatic, and a small air gap provides a slipmore » boundary condition at the top. The main objective of the research reported here was to obtain experimental data for the above mentioned configuration. It is remarkable that, although the isothermal boundary condition at the heat source has been extensively studied experimentally for melting to a vertical plate (e.g., Ho and Viskanta, 1984; Okada, 1983), there are very few experimental data available for melting to an isothermal cylinder positioned vertically.« less