| Molten salt has become a common thermal energy storage material because of its low cost,high energy storage and high heat capacity.Nanoparticles were added to molten salt for better thermophysical properties.It is found that the specific heat,heat capacity and other thermophysical properties can be improved at low concentration,which is considered to be an effective means to strengthen the heat transport capacity.For the sake of economy,there are many reports on the improvement of thermophysical properties such as specific heat,heat capacity and thermal conductivity,but few on the supercooling characteristics of base salt.A large number of supercooling literatures focus on low-temperature nanofluids,and it is concluded that nanoparticles can significantly reduce the degree of supercooling.In this paper,the supercooling characteristics of molten salt based nanofluids are discussed based on the study of thermophysical properties.The eutectic nitrate was synthesized by two-step method and its composition was verified by X-ray diffraction.The influence of different cooling rates on the supercooling characteristics was studied.For lower cooling rate,the solidification process is close to equilibrium solidification.Atoms or ions have enough time to diffuse and collide with each other.The critical nucleation number formed by energy and structure fluctuation will increase gradually with the decrease of temperature,and the critical nucleation point will grow up by diffusion.For higher cooling rate,the formation of early nucleation sites is inevitably inhibited.The greater the cooling rate is,the greater the degree of supercooling is.SiO2 nano-powders with different particle sizes were used to fabricate nitrate-based nanofluids.Depending upon DSC,SEM,etc.techniques,investigations on both the solidification and supercooling behavior of the molten salt based nanofluid was carried out in this paper.A modified Johnson-Mehl-Avrami equation model was recommended to analyze the crystallization process of the as-prepared composite and the influence of nanoparticles on the cooling kinetics was further analyzed based on experimental data.The research results show that JMA equation can be only used to describe the crystallization process of pure eutectic salt,but not suit for nanofluids since the addition of nanoparticle can alter the crystallization kinetics of the composites.Depending on the particle size and concentration,nanoparticles addition was found to be able to improve the degree of supercooling and reduce the crystallization completion time by varying degrees.Further verification by a modified JMAK equation shows that with the nanoparticle addition,the component 'n' would fluctuate within a certain range,which indicates that the nanoparticles can improve the crystallization process by altering the nucleation mechanism and adjusting the cooling rate resulted from the increased thermal conductivities of the nano-fluids.Nanofluids doped with different dopants were prepared using alumina,silica and titanium dioxide particles.The influence of different particles on the supercooling characteristics was studied in the same method.The results indicated that silica particles seem to bring about the minimum half crystallization time,which is 42%lower than that of pure salt.In addition,the increase of concentration will lead to the decrease of semi crystallization time.The maximum shortening of crystallization time caused by concentration change is 11.3%(alumina),19.9%(silica)and 7.5%(titanium dioxide),respectively.It can be seen that the sensitivity of SiO2 to concentration is the best,while that of TiO2 is the least.This is consistent with the rule of undercooling:the supercooling degree of silica doped samples is always smaller than that of samples doped with titanium dioxide at high concentration.Moreover,thermal cycling experiments were carried out,and it was found that thermal cycling would lead to the deterioration of heat capacity and specific heat performance. |
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