In this study, the vital role of fluid type in diffusion through a complex soil-like three-dimensional porous medium under gravity force was investigated by simulations performed using D3Q27 model of Lattice Boltzmann Method (LBM), with Double Relaxation Time (DRT) procedure as well as second-order discretization version of LBM. The elaborate porous medium was constructed using Image Processing technique by a detailed program written in MATLAB language. The hydrogen gas and water vapor were two fluids utilized in this project and because of small velocity of fluid, the Darcy law (with inclusion of gravity) was used extensively. Also, the satisfaction of continuity equation in different cross sections of porous medium was examined for both fluids; velocity and pressure contours were utilized also in this regard. The critical point of value of gravity acceleration g in LBM scale for different fluids was described and gLBM was calculated for 13 fluids to emphasize the vital role of fluid type in LBM simulations. For being confident regarding the role of gravity, the value of gravity acceleration was set to zero intentionally (|g|=|gLBM|=0) in some other simulations. Because of complex nature of porous medium, the inclusion of Knudsen diffusion phenomenon in calculations of pressure change was necessary. After coupling of Darcy equation with Knudsen diffusion according to valid scientific resources, the calculation of permeability and mean pore diameter of porous medium was accomplished through special three-dimensional fitting by MATLAB. The interesting concept of Specific Surface Area (SSA) was introduced, too.

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