Three-dimensional Numerical Study Of Solid-liquid Phase Transition In Non-uniform Porous Media Cavity By LB

Phase change energy storage materials are widely used because of their high energy storage density,stable chemical properties and low cost.Solid-liquid phase change materials usually have large latent heat storage capacity and small volume change before and after the phase change,which are easy to encapsulate and store and transport,but there is an inherent drawback of low thermal conductivity,which makes the heat storage and discharge efficiency very low and limits the application of phase change materials.The addition of a metal skeleton with high thermal conductivity can effectively solve such problems,so it is of great significance to carry out the study of phase change processes in skeletons containing porous media.In this thesis,the complex skeleton structure with various gradient porosity and gradient pore density is generated by four-parameter random growth method(QSGS),and the solid-liquid phase change process of skeleton containing non-homogeneous porous media is studied by three-dimensional lattice Boltzmann method based on the two-region model of"porous media-multiphase flow"constructed by the pore scale and enthalpy method.The study was carried out to obtain the migration evolution of the non-homogeneous porous media skeleton on the heat and mass transfer of phase change and the paste zone.The porosity of the metal skeleton has an important influence on the flow heat transfer and the migration of the paste zone of the phase change process.The decrease of skeleton porosity enhances the thermal conductivity,increases the heat conduction and thickens the paste zone,while the skeleton enhances the flow hindrance,weakens the liquid phase zone mobility,accelerates the melting rate of the phase change material and shortens the required complete melting dimensionless time.The direction of gradient change in porosity has a significant effect on the heat transfer of phase change flow within the skeleton of porous media.Among them,the porous skeleton is dense at the high temperature wall in the X direction negative gradient,with strong thermal conductivity,high heat transfer efficiency and fast melting rate,and the paste area is thin and less inclined.The porosity of the skeleton at the bottom of the square cavity in the Z direction negative gradient is small,and the porous skeleton is dense,which strengthens the bottom heat transfer and improves the accumulation of the phase change material under the natural convection,and the melting rate is accelerated.The magnitude of porosity gradient change increases,and the melting rate of phase change material is accelerated in the negative gradient in X direction,and the melting rate is accelerated in the negative gradient in Z direction.The dimensionless parameters Rayleigh number and skeleton to phase change material thermal conductivity ratio(Ra and R_s )have an important influence on the phase change flow and heat transfer process.the larger Ra is,the stronger the natural convection effect,the greater the inclination of the paste area,the faster the melting rate,the enhanced heat transfer capacity of the hot wall surface,the shorter complete melting time and the influence of positive and negative gradients in X direction.R_s reflects the size of skeleton to phase change material thermal conductivity ratio,the larger R_s is,the faster the melting rate.The larger the R_s ,the stronger the thermal conductivity of the skeleton,the larger the heat conduction,the faster the melting rate,and the melting rate in the X-direction negative gradient is influenced by R_s more than the X-direction positive gradient.The gradient change of pore density has a significant effect on the heat transfer of phase change flow within the skeleton of porous media.The difference of melting rate is mainly influenced by the size of the skeleton pores on the flow,the low pore density of the skeleton has a weak obstructing effect on the flow,the flow of liquid phase is concentrated,so that the mainstream area is easier to form,and then accelerate the convective heat transfer;the melting rate of positive gradient in X direction is faster than that of negative gradient,the flow is concentrated at the change of pore density and the flow velocity is greater at the low pore density;the porosityεincreases,the difference of temperature and velocity between positive and negative gradients in the heat transfer direction increases,and the whole field flow of negative gradient is more uniform.