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Coupled heat and mass transfer by natural convection in a fluid-saturated porous medium is a dynamic domain of research, due to many important engineering and geophysical applications, see the books by (Nield & Bejan, 2006), (Ingham & Pop, 1998; 2002; 2005), (Pop & Ingham, 2001), (Ingham et al., 2004) where a comprehensive account of the available information in the field is presented. In the present context, the following assumptions will be used: a) the fluid and the porous medium are in local thermodynamic equilibrium; b) the flow is laminar, steady and twodimensional; c) the porous medium is isotropic and homogeneous; d) the properties of the fluid and porous medium are constants; e) the Boussinesq approximation is valid and the boundary-layer approximation is applicable. Thermal diffusion (thermodiffusion or Soret effect) corresponds to species differentiation developing in an initial homogeneous mixture submitted to a thermal gradient, (Soret, 1880). The energy flux caused by a composition gradient is called Dufour (diffusion-thermo) effect. These effects are considered as second order phenomena, on the basis that they are of smaller order of magnitude than the effects described by Fourier’s and Fick’s laws, but they may become significant in clear fluids, according to (Eckert & Drake, 1972), but also in porous media in areas such geosciences or hydrology, see (Benano-Melly et al., 2001) and the references quoted there. Consequently, Dufour and Soret effects on convective flows in porous media have been analyzed in a growing body of literature in the last years. However, the majority of the papers issued in the journals, in the last period, do not offer unfortunately a physical basis to calculate Dufour and Soret coefficients. Restricting our considerations to porous media, let us refer further to the textbook by (Nield & Bejan, 2006) where some basic information are given on pages 42-44. According to the references quoted there • In most liquid mixtures the Dufour effect is inoperative, but that this may not be the case in gases (Platten & Legros, 1984). This fact was confirmed also by (Mojtabi & Charrier-Mojtabi, 2000), who found that in liquids the Dufour coefficient is an order of magnitude smaller than the Soret effect. • A not encouraging conclusion was drawn by these later authors, (Mojtabi and CharrierMojtabi, 2000), in the sense that for saturated porous media, the phenomenon of cross

Topics in Heat and Mass Transfer in Porous Media: Cross-Diffusion, Thermophoresis and Reactive Surfaces