Experimental Characterization of the Melting and Freezing of Encapsulated Phase-Change Materials under Convective Boundary Conditions

Abstract

This work was conducted as part of a multidisciplinary project on the development of alternative dry-cooling technologies for power plants using phase change materials (PCM). The research focuses on the dynamics of the melting and freezing of phase change materials in millimeter scale cylindrical polymer tubes under convective boundary conditions. The effect of air temperature, air velocity, and encapsulant material and wall thickness were studied using a custom built experimental apparatus. It is shown that as air velocity increases the time to complete melting and freezing is reduced, and the overall heat transfer coefficient between the air and the PCM is increased. As expected, this work showed that the rate of phase change was increased by reducing the thermal resistance of the encapsulant using thin walled tubes with larger thermal conductivities. Additionally, a larger temperature difference driving heat transfer produced larger melting and freezing rates. During the initial stages of phase change in thin-walled tubes, the experimental results matched well with a simple quasi-steady 1D heat transfer model neglecting the effects of sensible heating and natural convection. Substantial deviations from the model were seen for tubes with large wall thickness and higher thermal resistances, due to the effects of sensible heating of the polymer encapsulant. At the later stages of phase change, deviations from the predictions were seen for both freezing and melting for all wall thicknesses. Freezing was under-predicted using the simplified modeling approach, while melting was over-predicted. This is attributed to the observed deviations of the experimental results from the 1D radially inward melting and freezing assumed in the analysis. This work demonstrated the viability of creating low-thermal resistance encapsulated PCM (EPCM) structures for the proposed dry-cooling system. Specifically, the long-term performance of EPCM was shown for over 100 cycles (> 42 hours) meeting the target thermal performance and cost requirements of the project.%%%%M.S., Mechanical Engineering and Mechanics – Drexel University, 2016