An investigation of the size effect and molecular orientation on the values of thermal conductivity of long-chain n-alkane by molecular dynamics simulation

Phase change materials (PCM) such as n-alkanes are expected to be used in thermal energy storage (TES) application for the merits of high latent heat, chemical stability and negligible supercooling. However, the low thermal conductivity of PCM constrains the thermal performance in TES system. Therefore, understanding the thermal transport phenomena of n-alkane such as n- eicosane (C20H42) is crucial for enhancing the heat transfer efficiency of TES. In this study, the thermal conductivity of long chain n-alkanes (C20H42) is studied by utilizing equilibrium molecular dynamics (EMD) and non-equilibrium molecular dynamics (NEMD) simulations. In order to remove the possible size effect from the results for solid n-alkanes, a new method is used to determine the thermal conductivity of the solid system. Traditionally, to remove the size effect, the base structure is replicated in one direction and the macroscopic thermal conductivity is determined by making the length of the system long enough. However, it might not be a suitable method for solid systems such as n-alkanes. To address this issue, a new approach is conducted, where the length of the system increases by increasing the number of molecules in the base structure and macroscopic thermal conductivity can be determined by fitting the results from the simulations. Besides, a parameter based on the distribution of the molecular orientation is determined to show the connection between molecular orientation and thermal conductivity. All simulations were performed with the large-scale atomic/molecular massively parallel simulator (LAMMPS) molecular dynamics package.