Phase change materials for thermal management and thermal energy storage applications

Abstract

Present dissertation reports the theoretical and experimental investigations pertaining to the heat transfer characteristics of phase change materials (PCMs) during melting/solidification. The objective of the present study is to analyze the thermal performance of PCM based systems for various applications including thermal management of electronic components and thermal energy storage systems. Initially, analytical models have been developed to analyze melting and solidification process of PCM in various energy systems involving cylindrical annulus and rectangular enclosure for different thermal boundary conditions. Variational formulation technique has been employed to solve the conduction equation with associated boundary conditions in cylindrical annulus. The model yields closed form solution for temperature distribution of PCM during melting and solidification process. The theoretical prediction is found to exhibit good agreement with available experimental results. Later on, the analytical models have been proposed to analyze melting and solidification of PCM inside rectangular energy storage system with internal plate fins. Heat balance integral method is employed to solve the conduction equation with associated boundary conditions. Closed form expressions have been obtained for various parameters such as temperature distribution, solid-liquid interface location and melt/solid fraction. The results obtained from the analytical models are compared with the available experimental, analytical and numerical results. Based on the theoretical analysis, correlations have been proposed for melt/solid fraction for different boundary conditions. Subsequently, computational fluid dynamics (CFD) simulations have been performed to analyze the unconstrained and constrained melting of PCM inside the spherical capsule by using Ansys Fluent. Numerous models involving symmetric/axisymmetric formulations with varying density, constant density and Boussinesq approximation have been simulated to analyze the melting behavior. Volume of fluid model and solidification and melting model are used to simulate the melting of PCM. The results obtained from numerical simulation such as melting pattern and melt fraction of PCM are found to be in good agreement with the available test data. Furthermore, a test facility has been developed to analyze the heat transfer performance of PCM based heat sinks for thermal management of electronic devices. Tests have been carried out to investigate the thermal performance of PCM based heat sinks involving fin number, inclination angle and heat flux values. The evolution and propagation of melt front inside the heat sink is studied through photographic observation. For two different cases such as unfinned heat sink without PCM and PCM based three finned heat sink configurations, the operating time is found to relatitaively indpendent of inclination angle. In addition, metallic foams are employed in different unfinned and finned heat sinks. Thermal performance of different configurations of unfinned and finned heat sinks involving pure PCM and metallic foam (MF) PCM composite have been compared. The effect of various parameters such as volume fraction of PCM, effect of heat flux and type of heat sink on the stretching of operating time to achieve a set point temperature has been studied. Enhancement ratios are obtained for various heat sink configurations. It has been observed that four finned heat sink assembly with MF integrated with PCM exhibits the highest enhancement ratio. In addition to this, nanoparticles are embedded in PCM based unfinned and finned heat sinks to study the thermal performance of nano enahnced PCM based heat sinks through experimental investigation. The thermal performance of nano-enhanced PCM based heats sinks are compared with pure PCM based heat sink configuration. Lower nanoparticle concentration in PCM is found to provide better thermal performance and beneficial for thermal management applications. Keywords: thermal management, thermal energy storage, phase change materials, melting/solidification, melt/solid fraction, unconstrained, constrained, heat sink, unfinned/finned, enhancement ratio.