Molecular Dynamics Simulation Of Molten Salt Phase Change Materials

In order to achieve the carbon peaking and carbon neutrality goals,large-scale development and utilization of clean energy are becoming more and more important.Solar energy has the advantages of low cost,a large amount of irradiation,and no pollution.Solar thermal power generation has high application value and development prospects.Its key technology is the transmission and storage of heat energy.Nitrate is the most commonly used heat transfer medium and heat storage material at present.However,nitrate has strong water absorption,easy decomposition at high temperature,and strong corrosivity,which has great damage to the test instruments and hinders the conventional physical property test methods.The theoretical calculation is also an effective method to obtain the thermophysical properties of materials other than experimental testing,in which the molecular dynamics method is widely used.In this paper,LAMMPS open-source software package is used to calculate several nitrate melts and nitrate melts doped with nanoparticles.It not only obtains the thermophysical properties of phase change materials by non-experimental means but also provides a theoretical basis for the design and performance improvement of materials,which has important guiding significance for the research of phase change materials and their thermophysical properties.With Buckingham potential function,three kinds of mono KNO₃,NaNO₃,and Li NO3 molten salts were calculated in LAMMPS.Radial distribution function and mean square displacement were analyzed,several thermophysical properties were calculated and compared with the experimental data.The effect of interatomic force on the thermophysical properties of KNO₃ phase change materials was studied by modifying the potential function after calculation.It is found that there are two types of the five RDF curves.It shows that the change of interatomic force leads to the transformation of crystal structure.The density of KNO₃ increases with the increase of interatomic force,because the interatomic force reduces the atomic gap,which reduces the total volume and increases the density.The thermal conductivity of KNO₃ increases with the increase of interatomic force.Heat transfer is realized through the vibration between adjacent atoms.This is because the force between atoms enhances the vibration between atoms and improves the heat exchange efficiency,which is reflected in the improvement of material thermal conductivity.With the increase of interatomic force,the viscosity increases.Because when the fluid flows,the moving atoms are attracted by other atoms.When the interatomic force increases,the greater the attraction will hinder the fluid flow.Therefore,the increase of interatomic force will lead to an increase in viscosity.The effects of different simulation control parameters on the density and specific heat capacity of KNO₃ were studied.When the number of atoms is lower than 480,the deviation of the calculation result of density is large,and the relative error is higher than 7%.When the number of atoms is higher than 1200,the deviation of the result is small,the relative error is lower than 3%,and the fluctuation between the calculation results is small.For the specific heat capacity,when the number of atoms is small,the calculated value fluctuates greatly.When the number of atoms is greater than 2400,the calculated value tends to be stable and the relative error is small.When the selected timestep is less than 0.1fs,the fluctuation of density calculation value is small.For the specific heat capacity,the calculated value will tend to be stable only when the time step is less than 0.05 fs.Several thermophysical properties of KNO₃,NaNO₃,and Li NO3 nitrate are calculated.All the results are close to the experimental values in the literature except the viscosity of KNO₃,and the relative errors are less than 10%.Using the random number algorithm,the initial model was established by substituting between Na and K atoms and taking NaNO₃ and KNO₃ as the basic structures respectively.After the system reached equilibrium,the thermophysical properties of Solar Salt（KNO₃/NaNO₃ Binary Molten Salt）are calculated.Compared with the NaNO₃ structure,the calculation deviation of density,specific heat capacity,and viscosity of the KNO₃ structure is more obvious except for the thermal conductivity.It shows that the model of NaNO₃ structure is more suitable for the calculation of KNO₃/NaNO₃ binary molten salt.5 groups of random numbers are selected to model NaNO₃ structures,and the binary molten salt of Solar Salt was calculated after initialization.The difference between the five groups of calculation results is very small.It shows that when the number of model atoms is enough,the random number has no influence on the calculation results,and the random model can meet the calculation requirements.The first RDF peak of KNO₃/NaNO₃ binary molten salt is 3.438 ?,the density is2.1g/cm3,the specific heat capacity is 1.496 J/（g·K）,the thermal conductivity is 0.495w/（m·K）,and the viscosity is 1.45 c P.KNO₃ molten salt phase change materials doped with Si O2 nanoparticles were studied.The thermophysical properties of KNO₃-Si O2 nanofluids were calculated.The specific heat capacity will first increase with the increase of the mass ratio of nanoparticles.After the specific heat capacity reaches the maximum value,the specific heat capacity will decrease with the increase of the content of nano-Si O2 particles.Meanwhile,the thermal conductivity of nanofluids will increase with the mass ratio of nanoparticles,which is because the thermal conductivity of KNO₃ is much lower than that of Si O2 nanoparticles,Si O2 becomes a heat conduction enhancement phase,which improves the overall thermal conductivity of nanofluids.With the increase of silica content,the viscosity of nanofluids will also increase,because solid particles hinder the flow of KNO₃ liquid,thus improving the overall viscosity of the mixture.The influence of size effect on thermophysical properties is discussed.With the increase of the radius of nanoparticles,the specific heat capacity increases slightly.The specific heat capacity of nanofluids increases by 6.6% with the increase of the radius of nanoparticles from 7 ? to 13 ?.However,the thermal conductivity of the molten salt remains relatively stable（within 3%）when the radius of nanoparticles changes between 9 and 13 ?.This shows that the size of nanoparticles has little effect on the thermal conductivity of the mixture in this range,while the results of using nanoparticles with a radius of 9 ? can summarize the real nanofluids in the nanometer range,at least for the specific heat capacity and thermal conductivity.In terms of viscosity,first,it will increase with the increase of nanoparticle radius.This is due to the increase of particle radius and the surface area,which hinders the movement of fluid.When the radius of nanoparticles continues to increase,it will decrease slightly,because the Brownian motion in the system is weakened,which leads to the decrease of viscosity.In the experiment,five kinds of quaternary molten salts were prepared by the mixed melting method.The melting point of the quaternary molten salt was tested by DSC.The results show that the melting point of sample B（mole fraction of KOH is6%）is the lowest,347.29 K.The specific heat capacity of sample B was measured.It was found that the specific heat capacity of sample B was 2.03J/（g·K）at 773.15 K.The curve of specific heat capacity in the solid temperature range was by the quadratic equation of one variable.In the temperature range of liquid,the specific heat capacity showed a linear growth trend according to the temperature.TG/DSC was used to determine the weight loss of the molten salt.The thermal stability experiment shows that mixed salt can be used normally under 857.36 k.Compared with the two traditional nitrates,it has a higher specific heat capacity and a significant advantage in melting point.