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In the modelling of heat, mass and momentum transfer phenomena which occur in a capillary porous medium during drying, the liquid and gas flows are usually described by the generalised Darcy laws. Nevertheless, the question of how to determine experimentally the relative permeability relations remains unanswered for most materials that consist of water and humid air, and as a result, arbitrary functions are used in the drying codes. In this paper, the emphasis is on deducing from both numerical and experimental studies a method for estimating pertinent relations for these key parameters. In the first part, the sensitivity of liquid velocity and, consequently, of drying kinetics in the variation of the relative permeabilities is investigated numerically by testing various forms. It is concluded that in order to predict a realistic liquid velocity behaviour, relative permeabiIities can be linked to a measurable quantity: the capillary pressure. An estimation technique, based on simulations coupled with experimental measurements of capillary pressure, together with moisture content kinetics obtained for low or middle temperature convective drying, is deduced. In the second part, the proposed methodology is applied to pine wood. It is shown that the obtained relations provide closer representation of physical reality than those commonly used. Nomenclature AV (AV)j aw By C Cp D D.C.Fm g ha hb hb averaging volume j phase volume within the averaging volume AV water activity resistance factor in the effective diffusitivity coefficient of vapour in the medium mass fraction of the vapour in the gaseous phase constant pressure heat capacity [J kg -1 K-1] diffusivity [m 2 s -1] convective drying condition assumed to remain constant during the overall process total moisture mass flux [kg m -2 s -1] gravity vector [m s -2] intrinsic averaged enthalpy of dry air [J kg-X]: h~ = Cp~(T -T~) specific averaged enthalpy of bound water [J kg-1]: hb = ht -Hb intrinsic averaged enthalpy of bound water [J kg-l]:hb = hz -p-~ jo Hbd(pb) 304 F. COUTURE ET AL. hi h~ h~ Hb H o I.C. J k K k~ L n P q Q RH T Tinfh T~ t S U v W z intrinsic averaged enthalpy of free water [J kg-l]: hz = CpL(7" --Tr) intrinsic averagedenthalpy of solid [J kg-1]: h~ = Cp~IT -T~) intrinsic averaged enthalpy of vapour [J kg-1]: h~ = Hv 4-Cp, (T -T~) heat of desorption [J kg -~] latent heat of vaporisation at the reference temperature T~ [J kg -1 ] Initial conditions of the medium flux intrinsic permeability [m 2] volumetric mass rate of evaporation [kg m -3 s -1] relative permeability thickness of the medium [m] exterior normal unit vector pressure [Pa] source term total heat flux [W m -2] external relative humidity [%] temperature [K or ~ C] wet bulb temperature [K or ~ reference temperature [K]: Tr = 273.16 K time [s] saturation conserved quantity velocity [ms -11 moisture content (in dry basis) space variable [m] Greek Symbols 6z space step [m] e porosity ej volume fraction for the phase j: ej = (AV)s/AV heat source [W m -3] ), effective thermal conductivity [W m -...

The forest industry operates in a dynamic and global market where change and competition are the rule rather than the exception. The color of wood is one of the most attractive features for the modern wood industry. Even when wood is chosen for its structural qualities, attractive and decorative colors are usually an important factor. In many applications, particularly in furniture, decorative products, decorative veneers and fl ooring, accurate matching of the color of different samples is required. Wood attributes and properties are important because they have a direct bearing on market opportunities and consumer acceptance for many types of manufactured wood products. The aim of this review is to identify causes of wood discoloration and advances in drying technology to overcome this problem. Wood discoloration is a complex phenomenon, mainly affected by heat, light, physiological and biochemical reactions, as well as from attack by microorganisms.

Production of carbon aerogels by CO 2 supercritical drying of resorcinol-formaldehyde (RF) hydrogels followed by pyrolysis in inert atmosphere has been extensively described in the scientific literature, since their introduction by Pekala in 1989 [1]. Supercritical conditions suppress the liquid-vapor interface, avoiding shrinkage and cracking of the material during solvent removal and preserving the porous texture. As supercritical drying remains difficult to apply at an industrial scale because of its expensive and potentially dangerous character, other softer drying techniques have been tested in order to produce an aerogel-like mesoporous texture: freeze-drying [2], vacuum drying [3], microwave drying [4], solvent exchange followed by freeze drying [5] or drying under nitrogen in tube furnace [6],... Quite surprisingly, it appears that conventional convective drying, with controlled air temperature, velocity and humidity, has never been used in order to produce RF xerogels. As this technique is well known and largely used in the industry, it seemed interesting to study its suitability: would this technique enable us to obtain porous RF (and carbon after pyrolysis) xerogels?Hydrogels have been prepared following the method described by Job et al. [3]. The molar ratio R/F and the dilution ratio D, as defined by these latter, were fixed at 0.5 and 5.7 respectively. Three initial values of the pH of the precursors solution were chosen: 6. 6.5 and 7 (±0.05). Cylindrical samples were obtained by casting 5 ml solution into sealed glass moulds (∅ = 22 mm) and putting them for gelation in an oven at 85°C during 72 h. After gelation, the samples had a mass comprised between 4.2 and 5.1 g. They have been dried in a classical convective rig, with air at ambient humidity, at a temperature of 70°C and a superficial velocity of 2 m/s, i.e. quite severe drying conditions. Fig. 1a shows the drying curves, i.e. the evolution of mass with time, for the three samples. In each case, mass stabilization occurs after approximately 4 to 5 hours, indicating the end of the drying process. In comparison with vacuum drying, the drying duration is about 40 times shorter [3]. The three samples reached a final mass close to 1.45 g and kept their monolithic form.Volumetric shrinkage was negligible when pH = 6 (orange xerogel), reached 21% when pH = 6.5 (light brown xerogel) and 60% when pH = 7 (dark brown xerogel). At high pH, a large shrinkage is observed even with supercritical drying, because of the 'polymeric' character of the gel [7]. The drying kinetics are represented in Fig. 1b by plotting the drying flux (kg/m²s) vs. the water content W (kg/kg), expressed on a dry basis, as commonly encountered in the drying literature. A

Porous carbon xerogels can be obtained by convective drying of resorcinol (R)-formaldehyde (F) hydrogels, followed by pyrolysis. Drying conditions have to be carefully controlled when crack-free monoliths with well-defined shape and size are required. The knowledge of the mechanical properties of the RF xerogels and their evolution with water content is essential to model their thermo-hygro-mechanical behaviour during convective drying and avoid mechanical stresses leading to deformation and cracking of the sample. The shrinkage behaviour and the mechanical properties of RF xerogels obtained with R/C ratio ranging from 300 to 1500 were investigated. R/C greatly influences the shrinkage and mechanical properties of the wet gel, on the one hand, and the mechanical and textural properties of the dried gel, on the other hand. The smaller the R/C, the higher the shrinkage, the stiffening, and the viscoelastic character of the xerogels. Water content has an influence on both the stiffness of the gels and the viscoelastic response. Generally, samples lose their mechanical viscous character and become more rigid when they are dried. Finally, mercury porosimetry measurements showed that the gels exhibit a marked lowering of 2 their stiffness upon compression, interpreted as a result of the heterogeneity of the microstructure.

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