Effect of Voids on Solidification of Phase Change Materials Infiltrated in Graphite Foams

As a fundamental process during production of composite thermal energy storage systems, infiltration of phase change materials (PCM) leads to formation of voids (air pockets) inside the pores of graphite foams. The presence of voids inside graphite cells (i.e. the presence of air pockets next to the conductive walls of the porous structure) markedly affects the thermal and phase change behavior of the composite. Therefore, it is vitally important to investigate the effect of voids on phase change behavior of latent heat energy storage composites. In complementing recent work devoted to modeling of the infiltration of PCM into graphite foams, a numerical approach was employed to study the solidification of PCM infiltrated into a graphite pore in the presence of a void. For this purpose, a two-dimensional model of the porous structure was developed based on the typical geometrical features of the pores. Grid independence study was performed on different unstructured grid systems. Since more than one fluid phase is present in this problem (PCM being the liquid phase and air pocket or void as the gas phase), the volume-of-fluid (VOF) method was utilized for investigation of solidification problem and tracking the interface. Considering various forces operating at the scale of the pore (i.e. 500 microns in diameter), this problem is under the influence of surface tension, gravity, and pressure gradient. The simulation was transient and continued until the entire liquid PCM inside the pore freezes. The volume of final void space will represent a combination of infiltration and shrinkage voids. Results of the simulation indicate the presence of 9.8% void (from the infiltration process) that can greatly alter the solidification rate of the PCM inside the pore. It is concluded that formation of shrinkage void during solidification can be predicted using this multi-phase model. For verification purposes, the volume of the predicted infiltration void was compared to reported experimental measurements and the volume of shrinkage void was compared to theoretical volume change. Good agreements were found in both cases.