Molecular Dynamics Simulation On Thermophysical Properties Of Thermal Physical Enhancement Of Molten Carbonates

In the field of solar thermal power generation,molten salt as a common high temperature thermal medium is widely used due to its excellent characteristics such as wide working temperature range,low price,high heat of dissolution and low corrosiveness.The thermal parameters of molten salts（such as specific heat capacity and thermal conductivity）have an important influence on the energy conversion efficiency.Improving the thermal conductivity and specific heat capacity of molten salts and thus the efficiency of solar thermal power transfer and storage systems has become the main research content.Therefore,in this paper,the specific heat and thermal conductivity of carbonate nanofluids are simulated and calculated using molecular dynamics methods,and the mechanisms of enhancing the thermal storage and thermal conductivity of nanofluids are investigated at the molecular level.In this paper,a binary sodium carbonate and potassium carbonate mixture model is first developed,and then a composite material based on amorphous Si O₂nanoparticles and binary carbonate is proposed and designed in order to improve the thermal performance of carbonate molten salts.Molecular dynamics methods were used to simulate and calculate the specific heat of the binary mixed carbonate and the change in specific heat capacity of the composite fluid after doping the carbonate with nanoparticles of different mass fractions and radii.The addition of Si O₂nanoparticles can effectively enhance the specific heat of molten salt,and the enhancement of specific heat capacity decreases with the increase of Si O₂particle size,and there is an optimal amount of nanoparticles added with the same Si O₂particle size,and the maximum increase of specific heat of nanofluid can reach 24.38%when the addition mass fraction is 1.5%.The results of the study illustrate that there is a compressed interfacial layer around the Si O₂nanoparticles,and the movement of the base fluid ions in the interfacial layer is constrained by the nanoparticles,which requires more energy input from outside,so the thermal storage of the carbonate base fluid can be enhanced,while the dispersion of the nanoparticles can provide more specific surface area for the molten salt ions to gather near its surface This provides additional energy storage.The enhanced electrostatic interaction in the nanomolten salt composite liquid is another reason for the increased specific heat capacity.The thermal conductivity of the binary mixed carbonates and the thermal conductivity of the molten nanosalt fluids under different working conditions were simulated and calculated.The results demonstrate that the addition of Si O₂nanoparticles can effectively enhance the thermal conductivity of carbonates.The enhancement of the thermal conductivity of the nanofluid does not correlate significantly with the particle size of Si O₂nanoparticles,but increases with the addition of Si O₂nanoparticles,and the maximum increase of the thermal conductivity is up to55.1%.A mechanistic study of the enhanced thermal conductivity of molten salts with Si O₂nanoparticles revealed that the thermal properties of the interfacial layer in the nanofluid itself are not responsible for the enhanced thermal conductivity,but the interfacial layer also generates interfacial thermal resistance which weakens the thermal conductivity.Microstructural analysis of the system demonstrates that the mean Brownian motion of the molten salt nanofluid is also not responsible for the enhanced thermal conductivity,and that the addition of Si O₂nanoparticles reduces the distance between the anions and cations in the molten salt.Finally,the reduction of the potential energy of the cations in the carbonate demonstrates that the cations in the base salt are attracted to the Si O₂nanoparticles and cause reflection-collisions within the interfacial layer,leading to the enhancement of the thermal conductivity of the nanofluid.