Effect of radiation on the effective thermal conductivity of encapsulated capsules containing high-temperature phase change materials

The use of packed beds containing encapsulated capsules can markedly improve the efficiency of latent heat thermal energy storage systems. The capsule effective thermal conductivity is a crucial parameter for modelling the melting process within the capsule and to investigate the thermal performance of the packed bed. This study simulated the effect of radiation on the melting process of encapsulated high-temperature molten salt using a numerical model. Results showed that radiative processes inside the molten salt could accelerate the melting process. When comparing simulations that incorporated radiation compared to those that did not, melting time differed by as much as 18% because the radiation changed the temperature distribution of the liquid molten salt and enhanced heat transfer. The effect of radiation on the molten salt melting process was then qualitatively analyzed using dimensionless numbers. When the ratios of conductive and radiative heat transfer N were 0.91, 1.62, and 2.24, the effect of radiation became less significant and the time required to complete the melting process was reduced by 25%, 19%, and 7%, respectively. An equation for effective thermal conductivity considering radiative heat transfer in encapsulated capsules was derived, which was valid within a limited range.