| Promoting the application of organic phase change materials in thermal energy storage systems,which is of great significance for improving energy utilization efficiency and practicing low-carbon development.Organic phase change materials limit their application in thermal energy storage systems due to their lower thermophysical properties.With the development of nanomaterial technology,high thermal conductivity nanomaterials are widely used in phase change materials to improve their thermophysical properties.In this paper,graphene and carbon nanotubes were selected as additives for phase change materials to improve the thermophysical properties of fatty acid-based organic phase change materials.By using the molecular dynamics simulation method,the thermophysical properties of graphene/fatty acid and carbon nanotube/fatty acid composite phase change materials were obtained.And the microscopic behavior of fatty acid molecular chains and their interactions with graphene,carbon nanotubes were analyzed during the phase change.The mechanism of phase change temperature change and thermal conductivity enhancement of the system is revealed.It provides a theoretical basis for the future application of fatty acid-based phase change materials in thermal energy storage systems.The main contents and conclusions are as follows:(1)Lauric acid system and lauric-palmitic acid binary system of were established.The changes in the microstructure of fatty acid molecular chains during the phase change were investigated,and the phase change temperature and thermal conductivity of the two systems were compared.The results show that the phase change temperature of the binary system is significantly lower than that of the lauric acid system,and the thermal conductivity is higher than that of the lauric acid system by about 6.0 %.When the phase change occurs,the thermal conductivity all appears to decrease significantly;The structure of the fatty acid molecular chain changes from an extended conformation to a bent conformation during the phase change,with no change in the bond lengths between the atoms within the system.The phase change process is a process of decreasing orderliness.(2)Three graphene/fatty acid composite phase change systems with different length-width ratios were established.The effect of length-width ratio of graphene on the microstructure and thermophysical properties of graphene/fatty acid composite system was investigated and compared with the binary system.The mechanism of the influence of length-width ratio of graphene on the thermophysical properties of the composite system was revealed by analyzing the interaction energy and phonon vibration power spectrum between graphene and fatty acids.The results show that the addition of graphene limits the thermal diffusion movement of fatty acid molecules,thus improving the overall orderliness of the composite system;As the length-width ratio increases,both the phase change temperature and the thermal conductivity of the system increase.This is due to the increasing binding energy between graphene and fatty acids,which requires more energy for the phase change to occur,the increasing overlap area of phonon vibrations between graphene and fatty acids,thus contributing to heat transfer;In addition,the effects of orientation and mass fraction of graphene on the thermal conductivity of the composite system were investigated.When graphene is parallel to the direction of heat flux,more heat is transported in the direction of the graphene plane.The heat flux increases and the temperature gradient decreases,thus improving the thermal conductivity of the composite system.The results show that the larger the mass fraction,the better the dispersion of graphene in fatty acids,which helps to improve the thermal conductivity of the composite systems.(3)Three carbon nanotube/fatty acid composite phase change systems with different length-perimeter ratios were established.The effect of length-perimeter ratio of carbon nanotube on the microstructure and thermophysical properties of the composite system was investigated.The results show that the phase change temperature and thermal conductivity of the composite system gradually increase with the increase of the lengthperimeter ratio.However,all peaks of the vibrational power spectrum of carbon nanotubes are significantly lower than those of graphene,and the smaller overlap area of phonon vibrations between carbon nanotubes and fatty acids results in lower heat transfer efficiency,making its thermal conductivity lower than that of the graphene/fatty acid composite system.In addition,the microstructure and thermal conductivity of the system of fatty acid molecules filled inside carbon nanotubes were also investigated.The results show that the extension of fatty acid molecules inside carbon nanotubes is much higher than that of the binary system,and the overall orderliness of the composite system is higher.The average thermal conductivity of the composite system is about 62.0% higher than that of the binary system. |
