Investigation On Solidification Behavior Of Phase Change Materials In Confined Geometry And Nucleation Enhancement Technology

Phase change energy storage technology is an effective means to improve energy utilization structure and enhance energy utilization efficiency.As a new type of phase change energy storage material,microencapsulated paraffin can solve the problem of mismatch between energy supply and demand in space and time.However,paraffin wax in the micron scale space has high supercooling degree during the solidification process,which is not conducive to its application and development.Therefore,it is an important problem to explore the factors influencing nucleation of phase change materials in confined space and the strengthening means and action mechanism of crystal nucleation.In this study,the microencapsulated phase change materials with paraffin as the core material were selected as the research object,and the problems of solidification and strengthening nucleation means of phase change materials in the confined space were studied in depth.The specific work is as follows:?1?The factors affecting the solidification of microencapsulated phase change materials in micron scale were studied.microencapsulated phase change materials with uniform particle size distribution were prepared based on coaxial microfluidic chip.The solidification process of microencapsulated phase change materials in monodisperse and polydisperse systems was visualized.The influence of polydispersity of particle size on the supercooling degree of microencapsulated phase change materials was determined by comparing the monodisperse supercooling degree with the same average particle size.The results show that the supercooling degree of microencapsulated phase change materials decreases with the increase of particle size.Compared with the monodisperse system,as the PDI of microencapsulated phase change materials particles in the polydisperse system increases,the supercooling deviation between the polydisperse system and the monodisperse system will also increase.Therefore,particle size and particle diameter polydispersity are important factors to determine the supercooling degree of microencapsulated phase change materials.?2?Nanosheets/nanospheres particles were selected for physical mixing in different proportions to enhance the dispersion stability of TiO₂ nanoparticle suspension.The LUMiSizer stability analyzer was used to test the dispersion stability of nanoparticle suspensions with different ratios and mass concentrations,so as to establish the relationship between the dispersion stability of TiO₂ nanoparticles and the supercooling degree of microencapsulated phase change materials.The results show that the main reason for the supercooling-degree change of microencapsulated phase change materials is the different steric resistance force between the nanoparticles when the ratio of different shaped TiO₂ nanoparticles changes.When the total mass concentration of TiO₂nanoparticles changes,the main reason for the supercooling-degree change of microencapsulated phase change materials is the effect of span-80 on the dispersion of nanoparticles.Therefore,the control of the ratio between TiO₂ nanoparticles and the surfactant concentration has a very important effect for reducing the nucleation supercooling degree of microencapsulated phase change materials.?3?The effect of ultrasonic on heterogeneous nucleation of nanoparticle suspension was studied.Dispersing different hydrophilic nanoparticles?Al₂O₃,SiO₂?stably in water,and ultrasonic is introduced in the solidification process of three kinds of aqueous nanometer suspension of.Nanoparticles concentration and the ultrasonic intensity on the supercooling degree of water was studied.The relationships between the concentration of nanoparticles and the intensity of ultrasound,which are significant for reducing the subcooling degree of water,are summarized.The mechanism of ultrasonic synergistic nanoparticles to strengthen the nucleation was revealed.The results show that both nanoparticles and cavitation bubbles can be used as nucleating agents to re duce the supercooling degree of deionized water.In the positive region of ultrasound and nanoparticles,the lost part of cavitation bubbles can be compensated by nanoparticles.But in the negative region of the ultrasound and the nanoparticles,the surface of the nanoparticles,which can provide effective nucleation area,decreases due to adsorption on the air bubbles,so unable to compensate lost part of cavitation bubbles,resulting in higher undercooling degree of ionized water compared to that induced by ultrasound alone.