The description of the transfer between a porous medium and its surroundings is commonly made using transfer coefficients which are theoretically well described only under boundary layer hypothesis. Solving Navier-Stokes equations in the surroundings of the product in order to get information about the boundary conditions avoids the classical using of these transfer coefficients. After a brief description of the model, a simulation of convective drying of a rectangular piece of porous medium is proposed using a coupling method between a porous medium code and a CFD software. The analysis of the interfacial transfer coupled with the analysis of the temperature and moisture content profiles show the influence of the edges. We show that the global kinetics cannot reflect the local surface transfer phenomena: even at the so-called constant rate period heat is not entirely used for water evaporation with a portion for temperature increase within the product.