Study On Interfacial Transfer Mechanism Of Energy Storage Materials In Solid-liquid Phase Change With Porous Framework

Solid-liquid phase change technology is widely used in efficient utilization of thermal energy,improvement of building wall envelope performance and thermal management of batteries.Phase change heat exchanger and phase change heat pipe greatly enhance heat transfer efficiency and energy efficiency.Sealing phase change materials into building envelope can reduce energy consumption and improve building environment comfort.Electronic components and lithium battery systems using phase change materials for thermal management will have longer life and higher efficiency.The porous framework will be further filled for each phase change application system to improve the overall heat transfer performance of the system and strengthen the rigid strength of the mechanism.The solid-liquid phase transition process filled with porous skeleton was studied,and the progress of the whole process and the heat and mass transfer mechanism of the phase transition interface were explored,which can provide theoretical basis for the application and optimization of phase transition technology.The solid-liquid phase change process with porous skeleton filling is a typical unsteady,nonlinear,multi-scale and multi-field coupling problem.In this paper,based on the preliminary experiments and numerical calculations of the research group,according to the basic principles of non-equilibrium thermodynamics and phase transition in complex media,the problem is analyzed by combining theoretical analysis and numerical simulation.Based on the theory of porous media percolation and multiphase flow,a two-region model of paste zone was established.Porous media seepage model was used to analyze the low liquid content zone(large solid particles,partial fluid and solid matrix coexist).The multiphase flow model was used to analyze the high liquid content area(small solid particles coexist with the fluid).The double distribution function(DDF,density and temperature distribution function)of the hot lattice Boltzmann method was selected for simulation calculation,in which the discrete velocity model adopted D2Q9 model.The coupling of flow and heat transfer is realized by adding the lift term of thermal levitation into the velocity evolution equation.Flow-solid coupling is realized by increasing the thermal equilibrium distribution function and relaxation factor.The relation between temperature and liquid content is established by enthalpy method.Compared with the research results in the literature,the validity of the LBM model and the selected boundary conditions is verified.The porous skeleton was generated by the numerical reconstruction method(QSGS),and the influence of each skeleton parameter on the detailed structure of the skeleton was studied.The skeleton of porosity(n =0.7,n =0.8,and n =0.9)and thermal conductivity(? =5,? =10,? =20,and ? =50)and the phase-change material were combined to form a closed square cavity,and numerical simulation was carried out respectively.The results show that: 1)in the early phase of phase transition,the filled porous skeleton will improve the melting rate of the whole field and increase the thickness of the pasty zone;2)the smaller the porosity of the multi-skeleton is,the larger the proportion of the matrix in the whole field will be,which will enhance the thermal conductivity of the whole field and increase the melting rate in the early stage,while decrease the melting rate in the later stage;3)with the increase of the thermal conductivity of the porous skeleton,the Nu number of the corresponding average hot wall surface will also increase,and the influence of the high thermal conductivity will gradually weaken with the progress of the phase change process;4)filling porous skeleton in accordance with the law of phase change heat transfer can greatly improve the efficiency of phase change heat and increase the melting rate in the quasi-steady state stage;5)When the thermophysical properties of porous skeleton are quite different from that of the phase-change material,when the hot wall surface is subjected to sudden high temperature,there will be temperature difference between the skeleton and the phase-change material,resulting in local thermal non-equilibrium effect,which will bring certain influence on the phase-change process.The effects of Rayleigh number,Prandtl number and Stefan number on the solid-liquid phase change were studied.The results show that: 1)the greater the Rayleigh number,the greater the natural convection intensity,and the greater the bending degree of the paste area,and the wider it is near the lower wall surface;2)with the increase of Prandtl number,the average Nusselt number of hot wall surface increases,and the effect of filling porous skeleton will be weakened.3)the higher the Stefan number is,the more heat will be used for the phase change rather than stored as latent heat,which will accelerate the melting rate throughout the whole process,but is unfavorable for the phase change energy storage system.