Pore-scale study on the effects of randomly distributed void cavities on the thermal performance of composite phase change materials

Abstract

Composite phase change materials (PCMs) can increase the overall effective thermal conductivity of latent heat thermal energy storage (LHTES) systems and improve their heat transfer performance. However, owing to density differences caused by solid–liquid phase change, void cavities with low thermal conductivity are generated in small spaces where the volume shrinks, thereby increasing the heat transfer resistance. To analyze the pore-scale effects of void cavities on the thermal performance of composite PCMs, a model without void cavities and six models with randomly distributed void cavities were established in this study. A graphics processing unit (GPU) accelerated multiple-relaxation time lattice Boltzmann method was adopted to implement the phase change process and obtain the evolutions of temperature distribution, average liquid fraction, and energy storage performance. The results indicated that, as the volume fraction of the void cavities increased, the average liquid fraction of the composite PCMs slightly increased, whereas the heat storage significantly decreased. At a Fourier number of 0.1, the heat storage per unit width of the composite PCMs was reduced by 3.81 %, 8.02 %, and 11.22 % when the volume fractions were 5 %, 10 %, and 15 %, respectively, compared to the case without void cavities. Additionally, the composite PCMs with large pore-size void cavities had slightly better heat storage efficiency than those with small pore sizes at the same volume fraction of void cavities. It was also found that the LHTES cavity with right-half void cavities had the minimal influence on the heat storage performance.