Diffusion through Isotropic Rectangular Parallelepipeds

Rectangular parallelepipeds (a polyhedron with six faces all of which are parallelograms) are sometimes used instead of sheets, making this sub-chapter of practical interest. The sheet has been considered as a rectangular parallelepiped whose one side, the thickness. 

S-shaped curve with R = 1 D = 10 cmVs L = 0.1 cm. Dotted line curve, with infinite R L = 0.1 cm D = 0.25 x 10 cm /s. Straight line (full line), tangent to the experimental 

Some materials are anisotropic, e.g., wood [3] or stretched polymers. In fact, the following equations can also be used for those materials, provided that the right diffusivity associated with each direction is put in the corresponding function. The problem with three axes of diffusion is resolved whatever the value of the coefficient of convection h, finite or infinite. 

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