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Our aim here is to study the thermal diusion phenomenon in a forced convective flow. A system of nonlinear parabolic equations governs the evolution of the mass fractions in multicomponent mixtures. Some existence and uniqueness results are given under suitable conditions on state functions. Then, we present a numerical scheme based on a \mixed nite element" method adapted to a nite volume scheme, of which we give numerical analysis. In a last part, we apply an homogenization technique to the studied equations in order to obtain an ecient modelling of Soret eect and adsorption in a porous medium at a macroscopic scale. R esum e. On etudie un syst eme d' equations paraboliques non lin eaires mod elisant l' evolution des fractions massiques d'un fluide multiconstituant dans un ecoulement convectif forc e sous l'influence d'un gradient thermique. Des r esultats d'existence et d'unicit e sont donn es sous des hypoth eses relatives aux fonctions d' etat des equations. On propose ensuite une m ethode num erique de type \ el ements nis mixtes" aboutissant a un schema \volumes nis" dont on eectue l'analyse et la pr esentation des premiers r esultats. Nous appliquons enn une technique d'homog en eisation aux equations etudi ees dans le but d'obtenir une mod elisation macroscopique d ele des ph enom enes d'adsorption et d'eet Soret en milieu poreux.

An efficient model for Soret effect in porous media is determined. After having described a local effect that can be generated at a microscopic scale, we present an homogenization technique that will allow us to determine precisely the macroscopic Soret coefficient and other caracteristic lengths such as thermal diffusivity and mass diffusive coefficient. Numerical simulations have been made for several geometrical configurations and corresponding porous media influence is discussed.