Study On The Modulation Of Nanomaterials On The Thermophysical Properties And Thermal Storage Performance Of Mixed Nitrate

Molten salt is widely used in heat storage systems as an efficient heat transfer and storage mass.However,single-component and multi-component molten salts also have disadvantages such as high melting point and poor thermal stability,which hinder melting.The development and application of salt in heat transfer and storage.The thermophysical properties of molten salt can be effectively improved by adding nanomaterials to molten salt.In this paper,nano-materials are dispersed into binary nitrate（60wt.%Na NO₃-40wt.%KNO₃）,research on its related thermal performance through experimental and theoretical methods.The density and surface tension of the molten salt nanofluids are measured and analyzed by Archimedes method and pulling escape method,and the microscopic morphology of the solid molten salt nanofluids was characterized by Scanning Electron Microscope（SEM）.At the same time,ANSYS software was used to simulate the melting and heat transfer process of molten salt in the thermal storage tank.The study found that after adding SiO₂nanoparticles of different particle sizes to binary nitrate（60wt.%Na NO₃-40wt.%KNO₃）using the high-temperature melting method,both the density of the molten salt nanofluids and the density of the based salt decrease with the increase of temperature,and have little effect on the density of the based salt;when the MgO nanoparticles are added,the density of molten salt nanofluid decreases with increasing temperature,but when the temperature is greater than 450°C,the density value of the MgO nanoparticles is added.It should be greater than the density value of the based salt.The density value of 15 kinds of molten salt nanofluids was calculated using the mixture density calculation model.After comparing with the experimental measurement value,it was found that the calculated value of 11#～15#molten salt nanofluids was quite different from the measured value.Calculate the volume expansion coefficient of based salt and 15 kinds of molten salt nanofluids.The order of volume expansion coefficient of based salt and 1#～5#molten salt nanofluids is:based salt>1#>3#>2#>4#>5#;The volume expansion coefficient of based salt and 6#～10#molten salt nanofluids is in order:8#>6#>9#>based salt>10#>7#;based salt and 11#～15#molten salt nanofluids;The order of the volume expansion coefficient of salt is:based salt>13#>12#>11#>14#>15#.Adding 20nm SiO₂ nanoparticles,30nm SiO₂ nanoparticles and MgO nanoparticles to the based salt will increase the surface tension of the based salt;the surface tension of 15 kinds of molten salt nanofluids will decrease with the increase of temperature.In the 1#～5#molten salt nanofluids,relative to the based salt,the surface tension value of 1#molten salt nanofluid increased the least,and the surface tension value of 5#molten salt nanofluid increased the most.Among the 6#～10#molten salt nanofluids,relative to the based salt,the surface tension value of the 6#molten salt nanofluid increased the least,and the surface tension of the 10#molten salt nanofluid increased the most.The particle size of the SiO₂ nanoparticles has no effect on the surface tension of the molten salt nanofluids.In the 11#～15#molten salt nanofluids,the surface tension of the molten salt nanofluids increases with the increase of the concentration of MgO nanoparticles.By analyzing the SEM image of the molten salt nanofluids in the solid state,it is observed that the surface tension of the molten salt nanofluids is related to the coating layer formed by the molten salt molecules wrapped on the surface of the nanoparticles,and it is proposed that the presence of highly active atoms on the surface of the nanoparticles makes the molten salt molecules in a chemical adsorption layer is formed on the surface of the nanoparticles,and a physical adsorption layer is formed due to the stirring action,and finally a nano cloud nuclei is formed in the molten salt system.The effect of nanoparticles on surface tension of based salt was explained.Modeling the thermal storage tank in the thermal storage system using ANSYS software and simulating the melting thermal storage characteristics of the based salt and molten salt nanofluids in the thermal storage tank using Fluent software.It is found that the molten salt nanofluids in the heat storage tank is mainly natural convection conducts heat transfer,and when the heat storage tank is surrounded by insulation,the molten salt in the tank can store all the heat;in based salt and 3#,4#,5#molten salt nanofluid,the heat storage capacity ranges from the small order is 3#molten salt nanofluid>based salt>5#molten salt nanofluid>4#molten salt nanofluid.