Preparation And Properties Of Magnesium Based Inorganic Salt Phase Change Materials At Low Temperature

Phase-change cold storage technology is an energy-saving technology that can coordinate the imbalance of energy supply and demand in time and space.It is not only used in industrial cold storage,air-conditioning and power"peak shifting and valley filling",but also widely used in cold chain logistics.With the rapid development of economy and living standard,the demand of cold chain logistics for refrigerated transportation equipment of low-temperature products is increasing,but the phase change materials?PCMs?whose phase change temperature is in the freezing area?-10°C?-40°C?still need to be developed.The PCMs of inorganic salt solution,which are widely used,usually have the phenomenon of supercooling and phase separation and need to be modified.At the same time,it is laborious to carry out cold insulation experiments under different conditions for PCMs to be applied in the incubator.Therefore,it is necessary to establish a general applicable numerical simulation model for such problems,so as to promote the application of phase-change cold storage technology in the field of cold chain logistics.According to the literature review and selection criteria of PCMs,Mg Cl₂ solution which is easy to obtain and suitable for phase change temperature is selected as the research object.In view of the supercooling and phase separation,the best nucleating agent formula was selected:1 wt%Ca Cl₂+0.25 wt%Ca?OH?₂;xanthan gum,which integrated stabilizer and suspending agent,was selected as thickener.In order to improve the thermal conductivity,multi walled carbon nanotubes?MWCNTs?were selected as the nano particle additive to obtain magnesium based inorganic salt PCMs.Based on the physical properties,thermodynamic properties and cycle stability of magnesium based inorganic salt PCMs,the following research have been carried out:the composition analysis of FTIR shows that there is no chemical reaction between MWCNTs and inorganic salt solution.The effect of additives on supercooling degree of Mg Cl₂ salt solution was studied by step cooling experiment.It was found that adding MWCNTs and nucleating agent at the same time could play a synergistic role,and the effect of reducing supercooling degree was better than adding them alone.The supercooling degree of magnesium based inorganic salt PCMs with 1 wt%MWCNTs was reduced to 2.2?.By measuring the latent heat of the sample,it is found that with the increase of additives,the latent heat slightly decreases,but it can meet the application requirements.The thermal conductivity was measured,and the results showed that the thermal conductivity increased with the increase of the mass fraction of MWCNTs.When the amount of MWCNTs was 3 wt%,the thermal conductivity reached 0.5628W·?m·K?^-1 with an increase of 10.7%.The results of 400 thermal cycles of PCMs show that its chemical composition and thermodynamic properties are stable,and it can meet the requirements of practical application.Using the magnesium based inorganic salt PCMs as the cool storage agent,and a common heat preservation box is selected to design the cold storage experiment.The physical and mathematical models are simplified and established,and the convergence and accuracy of the internal contact surface of the box are solved by using the hexahedral grid mixed polyhedral grid.In the Fluent software,the simulation calculation of the cold storage experiment is carried out,and the conjugate heat transfer and equivalent heat capacity method are used to simulate the heat transfer and phase change process.By comparing the simulation results with the actual experimental results,the phase change time and average temperature,the effective cold holding time in the box and the average temperature are obtained.The above errors are all within 6%.It shows that the experimental model and calculation flow can accurately simulate and analyze these problems,which has a good reference value for the application of low temperature PCMs.