Infiltration, Void Formation and Thermal Conductivity Improvement in Thermal Energy Storage Composites of Graphite Foam/Phase Change Materials

Abstract

Thermal energy storage (TES) composites open new opportunities in utilization of renewable energies and recycling waste heat. Development, utilization and thermal conductivity improvement of TES composites were investigated in this dissertation. Development of such composites mainly depends on the characteristics of the infiltration process that is linked to the formation of voids and was investigated numerically and experimentally. The numerical investigation was conducted with the purpose of extracting the details of liquid interface evolution and behavior during the infiltration. The penetration of wetting and non-wetting liquids into a porous structure (graphite foam) was studied using the volume-of-fluid (VOF) method. The effects of different driving forces and interface behavior were investigated as well as the observed phenomena during the infiltration of wetting (interface pinning and wicking flow) and non-wetting liquids (void formation). The numerical results were verified against those obtained from the coupled VOF-Level Set method, known to have higher accuracy in capturing the interface. Furthermore, the numerical results of horizontal wicking flow through a network of pores in series were validated against the experimental results with good agreement. Regarding the utilization of TES composites, the numerical simulation of the phase change processes was performed considering presence of voids and corresponding effects. The proposed combined VOF and enthalpy-porosity method takes into account the variation of density with temperature, making it capable of predicting the shrinkage void. Numerical simulations were conducted at the pore level and the evolution of the freezing and melting fronts were extracted along with the volume of shrinkage void. With regard to verification of results, it was found that the volume of the shrinkage void is in good agreement with the theoretical volume change due to density variation and its distribution was found in accordance with the observed convection patterns within the pore. During the phase change processes, a temperature gradient was observed along the interface between phase change material (PCM) and void. Therefore, thermocapillary effect was included by considering the variation of the surface tension with temperature. The final status of phase change processes, position and shape of infiltration and shrinkage voids, convection patterns within the pore and phase change duration were extracted and compared between cases with and without thermocapillary convection. It was found that thermocapillary forces influence the convection pattern within the pore and cause a reduction of about 8% in duration of phase change. As a novel method for thermal characterization of graphite foam/PCM composite, the effective thermal conductivity was investigated numerically and experimentally. A three-dimensional body-centered cube arrangement of uniform spherical pores saturated with PCM was considered as the numerical model. Unidirectional thermal analysis of the model was conducted and the total heat flux was integrated over hot/cold surfaces. Knowing the applied heat flux and temperature difference, the effective thermal conductivity was evaluated based on the Fourier’s law. Experimental investigations were conducted on samples of graphite foam and graphite foam/PCM composite using the direct (absolute) method of thermal conductivity measurement. Applying a unidirectional heat flux on the sample, the temperature distribution was measured within the sample and the effective thermal conductivity was evaluated using the direct method, based on the Fourier’s law. The numerical and experimental results were found to be in good agreement. It was concluded that highly-conductive and highly-porous structures such as graphite foam are excellent candidates for thermal conductivity improvement of PCM.