Three-dimensional Numerical Study Of Solid-liquid Phase Transition In Porous Media Cavity Based On LBM

Medium and low temperature phase change energy storage technology can effectively solve the problem of uneven distribution of energy between supply and demand.Solid-liquid phase change energy storage materials have the advantages of high enthalpy of phase change,small volume change and good thermochemical stability,and have been widely used in assembly building walls,fuel cells,aerospace and other fields.Traditional solid-liquid phase change materials generally have problems such as low thermal conductivity and low heat storage and discharge efficiency,and adding a high thermal conductivity skeleton to the phase change materials can effectively solve such problems.Exploring the solid-liquid phase change process in the three-dimensional square cavity containing the skeleton is of guiding significance for the practical application of energy storage technology in engineering.In this paper,the solid-liquid phase change process in the square cavity of the skeleton containing porous media is studied by numerical simulation,Based on the enthalpy method,a two zone model of"porous media-multiphase flow"is constructed to describe the flow and heat and mass transfer characteristics in the mushy region of solid-liquid phase change.The Brinkmann-Forchheimer-Darcy model is used for the low liquid phase rate region near the solid phase change material,and the multiphase flow model is used for the high liquid phase rate region near the liquid phase change material.The three-dimensional lattice Boltzmann method is chosen as the numerical calculation method,and the double distribution function model(DDF)and D3Q19 discrete velocity model are used.By reasonably selecting the equilibrium distribution function and boundary conditions,a three-dimensional LBM model for solid-liquid phase change in porous media skeleton is established based on the pore scale.The porous media skeleton was generated using the four-parameter stochastic growth method(QSGS).Compared with the pure phase change condition without skeleton,the addition of porous media skeleton with a porosity of 0.8 suppressed the natural convection effect in the cavity,the inclination of the paste zone became smaller and thicker along the skeleton direction,the complete melting time became shorter,and the temperature uniformity of the whole field was improved.The effects of dimensionless characteristic parameters on the solid-liquid phase transition process and the evolutionary migration of the paste zone within the skeleton-containing square cavity were investigated:1)the larger the Ra number,the stronger the natural convection effect in the cavity,the larger the tilt of the paste zone,the shorter the complete melting dimensionless time of the phase change material,and the larger the Ra number,the larger the Nu_avgof the hot wall surface;2)the larger the Pr number,the stronger the diffusion effect of the phase change material,the shorter the complete melting The shorter the dimensionless time,the larger the Nu_avgof the hot wall surface;3)the larger the Ste number,the smaller the latent heat of phase change of the phase change material,the less heat required for melting,the faster the melting rate,the shorter the dimensionless time for complete melting,the smaller the Ste number,the larger the latent heat of phase change of the phase change material,the heat is not easily transferred to the inside of the square cavity,the larger the temperature gradient of the hot wall surface,and the larger the Nu_avg.Homogeneous porous media skeletons with different porosity(?=0.9,0.85,0.8,0.7)and gradient porous media skeletons with different structures(X-directional positive gradient,X-directional negative gradient,Z-directional positive gradient and Z-directional negative gradient)were generated using the QSGS method to analyze the effects of skeletal thermal conductivity,porosity and different gradient skeleton structures on the solid-liquid phase change and paste zone,and the results showed that:1)the porous media skeleton thermal conductivity,the greater the effective thermal conductivity of the composite phase change material,compared with the no skeleton condition,the porous media skeleton and phase change material thermal conductivity ratio R_s=20,50,100,the complete melting time was shortened by 22%,30%and 32%,respectively,the greater the thermal conductivity ratio R_s,the greater the hot wall surface Nu_avgduring melting;2)the smaller the porosity,the more skeletal,the greater the effective The larger the thermal conductivity,compared with the no skeleton condition,?=0.9 when the complete melting time increased by 11%,the porosity?take 0.85,0.8 and 0.7 when the complete melting time shortened by 11%,32%and 57%,respectively,the smaller the porosity in the early phase transition,the larger the value of hot wall Nu_avg,in the middle and late phase transition,?=0.7 condition hot wall Nu_avgvalue decreased;3)X-directional positive gradient,X-directional negative gradient,Z-directional positive gradient and Z-directional negative gradient complete melting dimensionless time are 0.184,0.17,0.175 and 0.154,respectively,the Z-directional negative gradient skeleton structure complete melting time is shortened by 9%compared with the uniform skeleton structure,the remaining three structures increase the complete melting time,and the Z-directional negative gradient skeleton structure makes the temperature distribution of the whole field more uniform.