Research On Numerical Simulation Of Mixed Convection In Porous Media Based On CLB Method

In nature,various engineering fields,and even organisms,there are phenomena of fluid flow and heat transfer.However,in the process of studying fluid flow,it was discovered that part of the flow actually occurs in a very complex porous medium.Although traditional numerical simulation methods can study the macroscopic motion of fluids,it is often difficult to reveal the flow mechanism inside porous media.As an emerging numerical simulation method,the lattice Boltzmann method is based on molecular kinetic theory.It not only has a clear physical background,but also has many advantages such as easy implementation of procedures and simple boundary conditions.Therefore,it is widely used in porous media and other fields.This paper is mainly based on the thermal cascade lattice Boltzmann model to simulate the flow and heat transfer of fluid in porous media on the scale of characterizing voxels.The main work content is as follows:(1)A thermal cascade lattice Boltzmann model based on the representative elementary volume scale is constructed.The flow field is solved by the isothermal cascade lattice Boltzmann model with D2Q9 lattice based on the generalized non-Darcy model,and the temperature field is solved by the temperature-based thermal cascade lattice Boltzmann model with D2Q5 lattice.By introducing the porosity and heat capacity ratio into the displacement matrix,the influence of the porous medium is incorporated into the thermal cascade lattice Boltzmann model.The effectiveness and practicability of this method are verified by the numerical simulation.(2)The thermal cascade lattice Boltzmann model based on the representative elementary volume scale established above was used to simulate the natural convection in the porous square cavity.The verification of the grid independence of the natural convection in the porous square cavity and the analysis and comparison of the simulation results with the literature double-verify the correctness and accuracy of the model.The study analyzed the effects of Darcy number(Da),Rayleigh number(Ra),Prandtl number(Pr),and the temperature difference ?T between hot and cold walls on natural convection in a square cavity filled with porous media.The study found that when the number of Darcy-Rayleigh number Ra*is constant,the larger Da number and a smaller Ra number promote more convection disturbances in the cavity,which makes the flow and heat transfer process in the porous medium more intense,which is intuitively manifested as the gradual dilution and thinning of the velocity and thermal boundary layer near the vertical wall.When the Pr number is less than 5,it also affects the flow in the cavity;when the physical properties of the fluid do not change with the temperature difference,the change of the pure temperature difference does not affect the natural convection in the cavity.(3)After that,the mixed convection in the two-dimensional porous media channel is simulated,and the non-equilibrium extrapolation format is used to deal with the boundary conditions.First,the correctness of the model is verified by analytical equations,and then the effects of nonlinear resistance,porosity ?,and disturbance velocity on mixing convection in two-dimensional porous media channels are analyzed and compared.We get that when Reynolds number Re<0.1,the influence of nonlinear resistance is negligible.However,when Re>0.1,the influence of non-linear resistance will gradually become obvious,and it will increase exponentially with the increase of Re number.When the Ra*is small,the average Nusselt number of the hot wall does not change significantly with the porosity.Only when the Ra*number is large,the effect of the porosity change on the results is more obvious.The heat capacity ratio ? only affects the heat transfer process in an unstable state,and the smaller the response time,the shorter the response time(the temperature field reaches the steady state faster).In the steady state,the average Nu number of different values is the same(Nuave=3.22);The disturbance speed can promote the mixing and convection of the fluid within a certain range,but when the disturbance speed is too large,it may adversely affect the flow and heat transfer of the fluid.