RELATIVE PERMEABILITY RELATIONS - A KEY FACTOR FOR A DRYING MODEL

In the modelling of heat, mass and momentum transfer phenomena which o ccur in a capillary porous medium during drying, the liquid and gas fl ows are usually described by the generalised Darcy laws. Nevertheless, the question of how to determine experimentally the relative permeabi lity relations remains unanswered for most materials that consist of w ater and humid air, and as a result, arbitrary functions are used in t he drying codes. In this paper, the emphasis is on deducing from both numerical and experimental studies a method for estimating pertinent r elations for these key parameters. In the first part, the sensitivity of liquid velocity and, consequently, of drying kinetics in the variat ion of the relative permeabilities is investigated numerically by test ing various forms. It is concluded that in order to predict a realisti c liquid velocity behaviour, relative permeabilities can be linked to a measurable quantity: the capillary pressure. An estimation technique , based on simulations coupled with experimental measurements of capil lary pressure, together with moisture content kinetics obtained for lo w or middle temperature convective drying, is deduced. In the second p art, the proposed methodology is applied to pine wood. It is shown tha t the obtained relations provide closer representation of physical rea lity than those commonly used.