Study On The Crystallization Mechanism Of A High-Temperature Molten Salt During Melting And Solidification Processes

Molten salt is a potential high-temperature solid-liquid phase change thermal storage material with wide application prospects in the fields of solar thermal power generation,building heating and grid energy storage.Most of the current research on phase change thermal storage of molten salts has focused on experimental and numerical simulation studies at the macroscopic level,and lacks a microscopic perspective to study the crystallization evolution process in its phase change thermal storage process.Therefore,this paper takes a typical binary nitrate(60wt%NaNO340wt%KNO3,also known as solar salt)as the research object and explores the mechanism of crystal evolution during its solidification and melting process by means of experiments and numerical simulations,so as to provide a theoretical basis for the practical application of phase change heat storage in molten salts.Firstly,a microscopic two-dimensional monolayer crystallisation visualisation experiment is set up to observe the growth process of solar salt crystals,to study the evolution of the topology of solar salt crystals during melting and solidification,and then to analyse the morphological characteristics of crystal growth and the variation of crystal tip rate.Secondly,the variation of crystallisation rate with temperature during complete melting and solidification of solar salts in visualisation experiments is investigated and compared with the DSC test results.Again,the variation law of crystallization rate with temperature during incomplete melting and incomplete solidification is explored.Finally,the phase field method is used to simulate the crystal growth process of solar salt in two dimensions,and a validation comparison is made with the visualisation experiments,and the influence mechanism of the key parameters of the model is analysed.The paper finds that the morphology of the crystal topology during the phase transition of solar salts is generally isometric or elongated crystals with anisotropy modulo four or six,with competing main branches and branches in the growth process.During solidification the tip rate reaches a steady growth rate at the start of solidification until growth is completed or suppressed,and the increase in tip rate is not simply linear in relation to the increase in cooling rate.During the solidification/melting cycle,the evolution of the solid-liquid interface during melting is dependent on the crystal structure produced by solidification,and the 2D monolayer crystallisation visualisation experiment during solidification has a smaller subcooling rate than the DSC test.It is speculated that this may be due to the large difference in mass magnitude between the DSC test and the visualisation experiment.In the incomplete melting experiments,the crystallisation rate-temperature curve for the solidification process closely follows the melting process curve,while the incomplete solidification experiments do not show a similar phenomenon.The simulation results of the phase field method show that the crystal morphology obtained from the simulation is in good agreement with the visualisation experiment.The dimensionless latent heat parameters of the model mainly affect the growth of the main branches and the generation of the three arms,the anisotropic modulus mainly affects the number of main branches,and the anisotropic strength mainly affects the growth of the main branches and the sharpness of the tips of the main branches.