36 Drying of Wood

0.4Capillary pressure (P c /P atm )0.30.20.1458 kg m 3 641 kg m 3Late woodMiddle woodEarly wood349 kg m 30(a) 0 0.2 0.4 0.6 0.8 1.0Saturation0.8Capillary pressure (P c /P atm )0.60.40.2656 kg m 3543 kg m 3SaturationLate woodEarly wood(b)00 0.2 0.4 0.6 0.8 1.0FIGURE 36.15 Capillary-function curves determined using image analysis. (a) spruce (Picea abies): the cells have thickerwalls and smaller radial extension in latewood part, hence the highest value of the capillary-pressure curve. One also has to beaware that full saturation is obtained with a lower amount of water in latewood (the porosity of this part is very small); (b)beech (Fagus sylvatica): because beech is a pore diffuse-porous hardwood species, no significant difference is observedbetween these parts. The low capillary pressure obtained for saturation values above 0.2 corresponds to the meniscus radiilocated in the vessel elements. The dramatic increase for low saturation values is due to the small lumen diameters of theparenchyma and fiber cells.the bound-water diffusivity as the bound moisture contentincreases. For this reason, the same trend is predictedfrom the calculations or measurements at themacroscopic scale; bound-water diffusion is alwayseasier for higher bound-water contents.When using the gradient of bound water as adriving force, the macroscopic flux readsf b ¼ r 0 D b rX b (36:16)This expression is consistent with the derivation of thesecond Fick’s law by using the mass balance equation:@X b@t¼r(D b rX b ) (36:17)For the sake of simplicity, the foregoing expressionshave always assumed isothermal conditions. However,this assumption fails for certain drying processesß 2006 by Taylor & Francis Group, LLC.