Study On Heat Transfer Of Solid-liquid Phase Change In Energy Storage Material With Porous Skeleton By Lattice Boltzmann Method

Fluid flow and heat transfer of solid-liquid phase change in porous media is widely existed in nature and various engineering fields, such as soil freezing and thawing, phase change energy storage, building energy conservation, electronic equipment cooling, crystal growth. It is very important to deeply study the heat transfer of solid-liquid phase change with porous skeleton. At the same time, there will be a mushy zone that is region of solid-liquid coexistence in the solid-liquid phase transition, studying the influence of mushy zone on the solid-liquid phase change heat transfer and flow is very helpful to discuss the mechanism of solid-liquid phase change with porous skeleton.Therefore, relevant research can provide the necessary theoretical basis for engineering application, and has important scientific significance and practical value.The enthalpy method is employed to solve coupled temperature and liquid fraction of phase change material in pore, and the mushy zone is simplified as porous medium at REV scale.Which means using enthalpy-porosity approach, the flow of liquid phase in mush zone use the Brinkmann-Forchheimer-Darcy percolation model, the solid-liquid phase change heat transfer and flow with porous skeleton are described as one region equation by liquid fraction, and the phase change is also treated as one region equation by enthalpy method at the same time.The mesoscopic numerical calculation method--lattice Boltzmann method (LBM) is adopted to solve the solid-liquid phase change heat transfer and fluid flow in porous media at pore scale. By choosing proper equilibrium distribution function and nonlinear source term, the LBM double distribution function (DDF) model of flow and heat transfer are established at pore scale, which the flow field and temperature field are discretized by D2Q9 lattice model. The validity of the LBM model of solid-liquid phase change of the energy storage material with porous structure was tested by the classical solution of several models.First of all, the LBM is used to solve the solid-liquid phase change in a square cavity without porous structure, which will help us to get the basic rules of phase transformation and the mushy zone, establish foundation for the numerical simulation and analysis of solid-liquid phase change with porous skeleton. Secondly, in order to explore the flow and heat transfer mechanism of the solid-liquid phase change in porous media deeply, the LBM is also used to solve the solid-liquid phase change with porous skeleton at the pore scale.Results of the solid-liquid phase change in a square cavity without porous structure shows:1) Under the influence of natural convection, the mushy zone is curved, and it has obvious effect on the velocity field, when the Fo number is larger than a certain value, the solid-liquid phase change enter into the quasi-steady state, melting rate can’t reach 1 at the point; 2) When phase change radius TR is smaller, the mushy zone is thinner, left wall average Nusselt number increases, but TR has almost no effect on melting rate; 3) As Stefan number reduces, phase change process is more slowly, the stable time of left wall average Nusselt number state is longer, but at same time, Stefan number has almost no effect on melting rate; 4) When Rayleigh number increases, the melting is faster and the melting rate is higher at the quasi-steady state, natural convection has the greater effect of left wall average Nusselt number, and if Rayleigh number increases to threshold, left wall average Nusselt number will decrease firstly, increase secondly and decrease to the stability; 5) Left wall average Nusselt number increases with the increase of Prandtl number, left wall average Nusselt number isn’t effected by Prandtl number when it is high at none mush zone, but mush zone will reduce this trend.Results of the solid-liquid phase change in a square cavity with porous structure shows:1) In the earlier stage, low porosity melting is slightly faster than high porosity, but high porosity melting rate and speed significantly higher than low porosity at the quasi-steady state, and the left wall average Nusselt number is smaller as porosity lower; 2) The pore structure has a great effect on flow of liquid phase in the pore, while little effect on the heat transfer and melting rate:the flow resistance of liquid phase in the pores is larger, the melting rate is lower at the quasi-steady state, and left wall average Nusselt number is smaller; 3)But when the porous geometry is different, the phase change is also affected by the heat transfer caused by the different geometry:left wall average Nusselt number of circular and square skeleton is significantly greater than the triangle skeleton, and the maximum velocity of melted liquid in the circular and square pores is significantly greater than triangle skeleton, the flow resistance of circular skeleton is small,which is conducive to the promotion of melting, but it is similar with high thermal conductivity of porous skeleton, the melting rate at the quasi steady stage is instead reduced because of promoting on heat transfer.