Experimental And Simulation Study On Heat Transfer Enhancement Of Spherical Phase Change Heat Storage Unit

Heat storage is an important link in clean energy storage,industrial residual heat recycling and other technologies,and latent heat energy storage has become a research hotspot because of its higher heat storage density than conventional heat storage.From the perspective of enhanced heat transfer in the heat storage unit,the mechanism of the effect of using pin-fin and carbon-based materials on the melting characteristics in the unit is not clear,therefore,based on the study of the heat transfer performance of the spherical phase change heat storage unit,strengthening the spherical phase change heat storage unit based on both material modification and unit structure is the main research content of this paper.Firstly,expanded graphite（EG）is used as a reinforcing matrix for heat transfer and mixed with myristic acid（MA）to prepare a composite phase change material（CPCM）,and melting experiments are conducted inside a spherical phase change heat storage unit.From the experimental results it can be concluded that during the MA melting process,the presence of gravity causes a relative movement of the melted material,which leads to an increase in the melting phase of the PCM in the T₃ region.The analysis of the experimental process for the addition of CPCM shows that the addition of EG material also has a positive effect on the melting of the PCM,mainly due to the shift in the heat transfer during the melting of the material.Secondly,the transient numerical simulations are performed using Fluent 19.2 software with the experimental bench as the simulation object,and the deviation of the measured temperature points between the simulated and experimental values is within 5%.In order to investigate the effect of EG mass fraction and Stefan number（Ste）on the melting of the heat storage unit,CPCM with EG content of 1wt.%～6wt.%is selected in the paper.The results show that the temperature cloud diagram tends to be concentric ring-shaped as the EG content increases,especially in cells with EG content≥4wt.%.EG enhances the heat conductivity of the material while reducing the effect of natural convection within the cell.The melting rate of the material increases with the increase of EG mass fraction and Ste number.The addition of EG has a positive effect on the overall melting inside the spherical cell,and the optimum ratio of EG addition inside the spherical cell is 4wt.%for the EG addition conditions analyzed in the paper.In addition,the results were subjected to dimensionless analysis and fitted to obtain generalized correlation equations on the liquid phase rate with the Fo number,Ste number and Gr number,leading to more generalized conclusions within the scope of the study.Finally,based on the simulation study of the spherical heat storage unit,a physical model of the heat storage unit with the addition of needle-shaped pin-fin is constructed.The velocity vector diagram of melting and the temperature cloud diagram are analyzed to derive the mechanism of the influence of the pin-fin on its melting.The results show that the melting time increased by15.6%,18.2%,and 21.2%for the case without added pin-fin compared to the case with added pin-fin 2,4,and 6,respectively.For the different spatial arrangement of the pin-fin,the melting time of the working condition at position B is shortened by 4.4%overall compared to that at position A.The addition of the pin-fin not only increases the heat transfer area,but also can enhance the local natural convection effect.The dimensionless analysis shows that the strengthening effect of pin-fin addition is not obvious at the working condition of EG mass fraction≥4wt.%.Finally,RSM is used to comprehensively evaluate and optimize the heat performance of the spherical phase change heat storage unit.The effects of different interaction factors are analyzed by ANOVA.With the shortest effective heat storage time and the maximum effective heat storage amount as the joint optimization objectives,the optimal effective heat storage time is predicted to be 1414s and the effective heat storage amount is 14612.3J with an expectation value of 0.90.