Study On The Solidification Characteristics Of Cool Storage Media With Magnetic Field Directed CNTs

Cool storage technology is an important means of shifting peaks and valleys,relieving power supply tensions and improving the overall efficiency of power investment.However,there are problems of large subcooling and low thermal conductivity,which leads to high evaporation temperatures and long storage/release times in the storage system.Nano additives have been extensively investigated in recent years.Not only does it with excellent thermal conductivity shorten the storage/release time,but it provide a nucleation surface to reduce subcooling.Carbon nanotubes（CNTs）are coaxial circular tubes with several to tens of layers,composed of carbon atoms arranged in a hexagonal pattern.With their large specific surface area and ultra-high axial thermal conductivity,they have become a research hotspot in the field of heat transfer enhancement,but they tend to agglomerate and settle in polar base fluids and are randomly oriented.Therefore,the CNTs are magnetized so that they are directed under magnetic field induction.Their high axial thermal conductivity and large specific surface area are fully exploited,thus enhancing their cold storage properties.In this work,CNTs with ultra-high thermal conductivity were selected as nanomaterials,and Fe₃O₄nanoparticles were wrapped around CNTs to form magnetic CNTs nanomaterials.In order to give full play to the advantages of high axial thermal conductivity and the large specific surface area,a magnetic field was used to induce direction and contribute to the unagglomeration of magnetic CNTs nanomaterials.The main research content and results are as follows:（1）In the research on the preparation of magnetic CNTs cool storage working media,functionalized CNTs were firstly formed by acid treatment and dispersant modification,and then magnetic CNTs nanomaterials were prepared by chemical precipitation method.The effects of reaction conditions on the physical properties of materials in the process of chemical precipitation were studied.At the same time,the materials were characterized by transmission electron microscope,infrared spectrometer,X-ray diffractiometer,and vibrating sample magnetometer.Finally,the nano materials were dispersed using ultrasonic vibration and the addition of surfactants to prepare magnetic CNTs cool storage media.The results showed that the optimum preparation is achieved at a reaction temperature of 70°C and a reaction time of 30 min.The CNTs nanomaterials are evenly wrapped with Fe₃O₄particles,whose saturation magnetization is 31.26 emu/g,and the Fe₃O₄particle size is controlled at about 10 nm.In a word,the prepared nanocomposites have excellent dispersivity and stability.（2）In the study of the solidification characteristics of cool storage media strengthened by magnetic fields,based on the theory of the preparation and phase transition crystallization of magnetic CNTs storage media,the nucleation,solidification and interfacial properties of the magnetic CNTs storage media were investigated by a magnetic field induced nucleation test device.In addition,the directional regulation mechanism of CNTs and the improvement of thermal conductivity in nano cool storage media were analyzed.The results indicated that the directional arrangement of magnetic CNTs nanomaterials under magnetic field induction leads to a significant improvement in thermal conductivity.Differential scanning calorimetry measurement found that the latent heat of magnetic CNTs cool storage media was slightly reduced and the freezing point was increased.Under the induction of magnetic field,magnetic CNTs cool storage media has a 68.2%reduction in subcooling compared with the traditional medium of water,an increase in total cold storage time of 36.2%,a significant increase in solidification rate and heat flux density,and excellent thermophysical properties and solidification characteristics.（3）In the research of the influence of magnetic induction intensity on the solidification characteristics of working media,magnetic CNTs cool storage media with different mass ratios of Fe₃O₄and MWCNTs（Fe:C）were prepared and characterized.The influence of magnetic induction intensity on the solidification characteristics of magnetic CNTs cool storage media was obtained by placing them under different magnetic field intensities.At the same time,the force on magnetic CNTs nanomaterials during solidification and the phagocytosis behavior of nanomaterials between solid and liquid interfaces were studied.The results showed that the Fe:C is proportional to the magnetization of the material.Due to the increase of magnetic Gibbs free energy on the driving force of nucleation,the solidification characteristics of magnetic CNTs cool storage media under magnetic field are improved.Under the induction of a magnetic field,magnetic CNTs have the optimal magnetic field strength,which means that their solidification characteristics are optimal under the optimal magnetic field strength.Through force analysis and calculation of nanomaterials,it was found that the range of magnetic induction intensity is 219-283 m T,which can cause magnetic CNTs to be engulfed by the growing solid-liquid interface and uniformly distributed after complete solidification,fully utilizing their strong nucleating agent and thermal conductivity.（4）In the numerical simulation of the solidification process of the cool storage media,the finite element method was used to simulate the whole solidification process,including the subcooling stage and the phase change stage.The effects of magnetic CNTs nanomaterials and magnetic fields on the fluid flow development characteristics and temperature distribution were discussed.The results showed that by comparing the step cold curves,it is found that the average error between the numerical simulation and the experiment is 5.6%,which is basically consistent with the experimental results.The temperature field results showed that magnetic CNTs cool storage media have better heat transfer characteristics under magnetic field.The flow field revealed the development characteristics of magnetic CNTs cool storage media during the subcooling stage.Compared with deionized water,the stable flow field and eddy current occurrence time are shortened by 37%and 69.4%,respectively.