Thermal Performance Enhancement Of Sodium-Magnesium-Calcium Chloride Molten Salt And Stability And Corrosion Of Doped Fluid

Improving energy efficiency and vigorously developing renewable energy are of great strategic significance for accelerating energy transformation and deepening energy reform.Heat transfer storage medium is an important part of efficient energy storage technology,which determines the efficiency and energy storage density of the system.Due to its high melting enthalpy,low viscosity,good thermal stability,wide operating temperature range and large heat storage capacity,chloride salts with abundant resources have good application prospects in high-temperature heat transfer and storage fields such as solar thermal power generation and industrial waste heat.However,the shortcomings of chloride salts with low specific heat capacity and thermal conductivity,diverse corrosion behavior in metal pipes or storage tanks,and complex mechanisms have been the main reasons limiting their industrial applications.Therefore,the research on the performance enhancement,high-temperature stability,and corrosion of metal materials such as pipelines and storage tanks of chloride molten salts in heat transfer and energy storage processes has very important reference significance for their future applications.By adding nanoparticles with lower density and excellent thermal conductivity to the chloride molten salt,its heat transfer and heat storage performance is enhanced.Nano-Mg O composite chloride salts of different sizes and concentrations were prepared by the method of mechanical stirring and high-temperature static melting.The specific heat capacity,thermal diffusion coefficient,thermal conductivity,densit y,viscosity and other properties were carefully measured.The scanning electron microscope?SEM?analyzed the effect mechanism of nano-Mg O particles on the thermal properties of chloride salt.The results show that the specific heat capacity enhancement rate of the nanofluid with 2.5 wt.%-20 nm Mg O is 37.91%,and the maximum thermal conductivity is 0.660 W/?m·K?at 470?,which is 53.49%higher than that of pure chloride salt.The nanofluid with 4.5 wt.%-20 nm Mg O has a density of 2.375 g/cm^-3 at 550?,and the maximum reduction in viscosity at the same temperature is 18.5%.The color of nanofluids deepens with the increase of the concentration of the nano-particles.The nano-Mg O in the system is well suspended without significant settling.There are a large number of cluster-like special structures in its microstructure diagram.A large number of semi-solid layers formed between the high-temperature liquid molten salt and the solid-phase nano-Mg O,resulting in a significant increase in the specific heat capacity of nanofluids.The more severe the microconvection between the chloride salt and the nano-Mg O particles is,the larger the apparent thermal diffusion coefficient and thermal conductivity of nanofluids will be.By measuring the performance changes of nanofluids after a period of service,the high-temperature thermal stability of the nanofluid is investigated.The nanofluid?SYSU-C4-20nm-2.5?with excellent thermophysical properties was selected for performance monitoring for up to 1000 h in a closed system.From the mass loss curve,it was observed that the mass remained basically unchanged after 700 h.It was found that partial evaporation of molten chloride salts and hydrolysis of Mg Cl₂ were the main reasons for the decline in the quality of the nanofluid.With the extension of working time,the melting point of the nanofluid increased slightly,but the melting enthalpy remained almost unchanged.Its specific heat capacity first decreased and then increased,and the maximum enhancement rate was 11.1%.The samples in different working times showed a uniform and stable state.The microstructure image shows that the nano-Mg O are more uniformly dispersed in the system without agglomeration,and the long-term high-temperature thermal stability is well.The hot corrosion behaviors of Inconel 625?In625?,Hastelloy X?H X?and stainless steel 316L?316L?in the nanofluids consisting of Mg Cl₂-Na Cl-Ca Cl₂ and nano-Mg O in atmosphere were investigated at 600°C for 44 days.The effects of the addition of nano-Mg O on the corrosivity of ternary chloride molten salt were investigated.The corrosion rates were determined by measuring the mass loss of alloys with times.The morphology of corroded samples was observed through scanning electron microscopy?SEM?and the mapping images of element distribution in corrosion layers were analyzed by energy dispersive spectroscopy?EDS?.The corrosion products were identified by X-ray diffraction?XRD?.The results show that the corrosion rates of In625,H X and 316L in the nanofluids were 40?m/y,137?m/y and 188?m/y.The XRD manifested that(Mg_0.984Cr_0.016)(Cr_1.984Mg_0.016)O₄,Mg Fe_0.2Cr_1.8O₄,Mg Fe₂O₄ were formed in corrosion outer layer of In625,H X and316L respectively.The SEM revealed that In625 suffered from pitting corrosion,while H X and 316L suffered from grain boundary corrosion,and the thickness of In625,H X and 316L corrosion layer was 19.4?m,35.7?m and 158.0?m consisting of oxide outer and Cr/Fe loss metal inner corrosion inner layer.So,In625 alloy is more corrosion resistant than H X and 316L in the nanofluids,which is similar to their corrosion behavior in ternary chloride salts.