Study On Heat Transfer Characteristics Of High Temperature Molten Salt Doped With Nano-particles

China’s "dual carbon target" strategy puts forward higher demand for new energy represented by solar energy and wind energy.Solar energy as the cleanest energy has attracted much attention because of its wide distribution and infinite reserves.The establishment of tower solar thermal power station can concentrate the scattered solar radiation for utilization and greatly improve the utilization rate of solar energy.As the core component of the photothermal conversion of the solar thermal power station,the utilization and conversion efficiency of the solar radiation of the receiver determines the overall efficiency of the solar thermal power station.The receiver is composed of a parallel heat transfer pipeline.The heat transfer working fluid in the equipment converts the solar radiation into heat energy for storage.At present,the main heat transfer working fluids are water and molten salt.Adding nanoparticles to prepare nanofluids can significantly improve the heat transfer performance of the fluid.Therefore,in this paper,aiming at the two most widely used molten salts solar salt and Hitec salt,the former is doped with SiO₂ nanoparticles and Al₂O₃ nanoparticles,and the latter is doped with TiO₂ nanoparticles.The heat transfer and flow of molten salt in the pipeline are numerically simulated.The effects of nanoparticles doping on the heat transfer characteristics and heat transfer effect of molten salt were analyzed,and the effects of different types of nanoparticles and different particle sizes of nanoparticles were compared and analyzed.The heat transfer enhancement mechanism of molten salt-based nanofluids was analyzed by numerical simulation.In order to reduce the calculation conditions,nine working conditions were designed by orthogonal experiment.The effects of specific heat capacity,viscosity and thermal conductivity on the heat transfer enhancement of composite molten salt were analyzed.The results show that the increase of specific heat capacity,viscosity and thermal conductivity of molten salt after doping nanoparticles has different effects on Nusselt number.Nusselt number increases with the increase of specific heat capacity and viscosity.For thermal conductivity,when the enhancement degree of thermal conductivity is between 5%and 10%,the Nusselt number in the tube decreases,and when the enhancement degree of thermal conductivity increases to 15%,the Nusselt number increases again.In addition,the effects of heat flux and inlet temperature are compared.It is found that heat flux has little effect on Nusselt number,and the increase of inlet temperature will reduce the pressure drop and Nusselt number of molten salt flow in the tube.In the simulation of solar salt doped SiO₂ and Al₂O₃ nanoparticles,it is found that within the simulation range（0.5%-2%）,from the perspective of heat transfer,the optimal doping concentration of the two nanoparticles is different.The optimal concentration of SiO₂ nanoparticles is 1.5%,and the optimal concentration of Al₂O₃nanoparticles is 2%.From the perspective of heat storage,SiO₂ nanoparticle is the highest at 1%,and the optimal concentration of Al₂O₃ is the same as heat transfer.From the simulation of ternary molten salt Hitec salt,it is found that TiO₂ particles with a particle size of 25 nm have the best effect,and 40 nm and 100 nm have little influence on the molten salt.Combined with the simulation of binary and ternary molten salt,it can be seen that the addition of nanoparticles will reduce the temperature of molten salt,decrease the temperature difference,decrease the temperature gradient,and greatly increase the heat transfer effect.With the increase of the concentration of nanoparticles,the viscosity of molten salt in the tube increases,and the pressure drop increases.