Robust Heat Transfer Enhancement During Melting and Solidification of a Phase Change Material Using a Combined Heat Pipe-Metal Foam or Foil Configuration

Experiments are performed to analyze melting and solidification of a phase change material (PCM) enclosed in a vertical cylinder by a concentrically located heat pipe (HP) surrounded by either aluminum foam or radial aluminum foils. The PCM liquid fraction, temperature distribution, melting (solidification) rates, and effectiveness are reported to quantify the improvement in thermal performance relative to a base case, a Rod-PCM configuration. Parameters of interest include the porosity of the PCM-metal composite, the foil thickness, the number of foils, and the foam pore density. The main contributor to enhanced performance is shown to be the porosity for both the HP-Foil-PCM and HP-Foam- PCM configurations. Both of these configurations improve heat transfer rates relative to either the HP-PCM or the Rod-PCM configuration. However, the HP-Foil-PCM configuration with one-third of the metal (foil) mass is shown to have approximately the same performance as the HP-Foam-PCM configuration, for the range of porosities studied here (0.870-0.987). This may be attributed to the metal morphology and resulting contact area between the metal enhancer and the HP. The HP-Foil-PCM configuration, with a porosity of 0.957 using 162 foils of thickness 0.024 mm, attained an overall rate of phase change that is about 15 times greater than that of the Rod-PCM configuration and about 10 times greater than that of the HP-PCM configuration. The greatest degree of enhancement was achieved with the HP-Foil-PCM configuration (with porosity 0.957) yielding an average effectiveness during melting (solidification) of 14.7 (8.4), which is an extraordinary improvement over the base case.