Liquid Flow in Porous Media

The drying in a porous medium containing a certain percentage of moisture is a thermal process, many materials can be dried simply by heating to a temperature higher than the boiling point of the liquid. These processes are very common in many industrial manufactures, typically being the final stage of a sequence of operations. Typically the removal of the moisture from the porous solids also involves the transport of the liquid phase, through mechanisms that may be different depending on the microstructure of the system. During the drying processes in porous media different transport mechanisms are involved. The vapor diffusion in the pores is regulated by Fick's law, where vapor diffusivity depends on the tortuosity of the pores. The transport of gas generated by pressure gradients follows the Darcy???s law. Another possible mechanism which has recently aroused interest is the flow of liquid along the edges of a pore with a rectangular section, whose geometry contributes to increase the drying rate compared to capillaries with circular cross section (Chauvet et al., 2009, Prat 2007). The increase in the speed of drying found in a cell filled with a high density of silica spheres with respect to the case of an empty cell of equal volume is also attributed to the same effect (Shaw 1987, Yiotis et al. 2005). The diffusive flow of steam can increase (Laurindo et al. 1998) due to the presence of mechanisms of condensation/evaporation through remote areas of liquid, which entails a lowering of the vapor pressure (dependent on the curvature in accordance with the Kelvin equation), although so far has not been given any direct experimental evidence. The gradients of surface tension can also be induced by concentration gradients in the multi-component liquid mixtures. In conclusion, some fundamental questions about the physic of porous media still need an answer. How is the water distributed inside the pores and what is the dynamic interface in the process of drying? What are the mechanisms that regulate the drying depending on the material tested? What is the interaction between the mass and heat transfer? Is there a control mechanism that can be used in order to improve the efficiency of drying? Finding an answer to these questions would represent a significant step forward in the comprehension of transport phenomena in porous media, and the starting point for the development of predictive models of the drying process that can be applicable in several industrial fields.