Nonisothermal moisture movement in wood

In order to analyze the effect of temperature gradient on moisture movement during highly intensive drying, such as microwave-vacuum drying, the profile of the temperature and moisture content in sealed wood whose opposite faces were subjected to temperature gradient for a short time was measured. The ratio of the moisture content (MC) gradient to the temperature gradient (dM/dT) was calculated and the factors influencing moisture movement under nonisothermal conditions were discussed. The results indicate that moisture moved in wood from the warm surface to the cold one even if opposite faces of the sealed wood assembly were exposed continuously to different but constant temperatures for a short period. The moisture content on the cold surface was higher than that on the warm surface. The moisture content gradient opposite to the temperature gradient was established, and the dM/dT was below 0.9%/°C. The temperature in the sample and the distance from the hot surface of the sample was strongly linearly correlated. With an increase in temperature, initial moisture content and experimental time, the dM/dT was significantly increased. __________ Translated from Journal of Beijing Forestry University, 2005, 27(2): 96–100 [译自: 北京林业大学学报, 2005, 27(2): 96–100]     

Nonisothermal diffusion model based on irreversible thermodynamics

Summary A comparison is made between a nonisothermal diffusion model derived from irreversible thermodynamics and three other equations that have been applied to several series of nonisothermal experiments.     

Application of a thermodynamic model to experiments on nonisothermal diffusion of moisture in wood

Summary Five series of nonisothermal diffusion experiments originally analyzed by two theoretical equations were reanalyzed using a model derived from irreversible thermodynamics.     

Modeling moisture absorption process of wood-based composites under over-saturated moisture conditions using two-part equations

The objective of this study was to investigate the moisture absorption process for wood-based composites subjected to over-saturated moisture conditions. Two stages are comprised in the moisture transfer process at the over-saturated moisture conditions, an initial stage which is the moisture transfer process mainly under fiber saturation point (FSP), and a second stage which is the moisture transfer process beyond the FSP. A model was developed based on two-part equations to describe the process, from which three coefficients (k ₁ , k ₂₁ , and k ₂₂) can be used to quantitatively describe the moisture transfer process under the conditions. Two different wood-based composites, wood fiberboard and wood fiber/polymer composites (polymer content: 30%), were used to test the model at four different ambient temperatures (30, 45, 62, and 80°C). It was shown that the two-part equation can accurately describe the moisture absorption process under over-saturated moisture conditions. The moisture absorption rate in the initial stage was about 30–60% greater than that in the second stage for most of the cases evaluated in this study. The higher the temperature, the greater moisture absorption parameters were obtained. At both moisture absorption stages (below FSP and above FSP), the calculated activation energy for the moisture absorption rate of wood fiberboard was very close to that of wood fiber/polymer composites.     

An irreversible thermodynamics model for unsteady-state nonisothermal moisture diffusion in wood

Summary A model that predicts heat and moisture transfer through wood in the hygroscopic range and which is based on the principles of irreversible thermodynamics, was evaluated with unsteady-state nonisothermal moisture desorption experimental data. The model predicted the phenomenon of thermal diffusion during the initial stages of desorption and results in a very good simulation of the desorption curve and the center's temperature change with time.Symbols C_p specific heat of air (= 0.24 cal/g K @ 70 °C) - C_T specific heat of wood, cal/g K - D transverse diffusion coefficient, cm²/s - E_b activation energy, cal/mol - E_o heat of vaporization, cal/mol - E_L differential heat of sorption, cal/mol - G specific gravity of wood - H relative humidity, % - h_T convective heat transfer coefficient, cal/cm² s K - h_c convective mass transfer coefficient based on the concentration of moisture in wood, cm/s - h_v convective mass transfer coefficient based on the concentration of moisture in the air in equilibrium with the wood surface, cm/s - K_M coefficient for diffusion due to moisture gradient, g/cm s % - K_T transverse thermal conductivity coefficient, cal/cm K s - M moisture content, % - P_o saturated vapor pressure, atm - R universal gas constant, cal/mol K (= 82.056 cm³ atm/mol K) - t time, s - T temperature, K - x distance, cm Greek Letters evaporation or condensation criterion - wood density, g/cm³ - _W water density (=1), g/cm³ - _a air density, g/cm³ Department of Wood Science and Forest Products Virginia Polytechnic Institute and State University Blackburg, Virginia 24061-0503     

DETERMINATION OF STATIC AND DYNAMIC DIFFUSION COEFFICIENTS OF MOISTURE IN PARTICLEBOARDS

The static and dynamic diffusion coefficients are important coefficients todescribe the moisture transfer processes in particleboard.In this paper,the formula ofculculating the static and dynamic diffusion coefficients were deduced.At first,the staticdiffusion coefficients of four kinds of particleboards were determined by using diffusion cupmethod.The results demonstrated that the static diffusion coefficients parallel to panelsurface were 10-20 times as large as that of perpendicular to panel surface for test boards.To determine both dynamic diffusion coefficients and surface emission coefficients ofmoisture in particleboards in one experimental period,specimens in four different thickness-es of each kind of particleboard were used in the experiment.Then the method ofregression was used and the dynamic diffusion coefficients and surface emission coefficien-ts were determined based on the slope and intercept of the regressive line.     

Biomass equations for four shrub species in subtropical China

Estimation of shrub biomass can provide more accurate estimates of forest biomass and carbon sequestration. We developed species-specific biomass regression models for four common shrub species, Chinese loropetal (Loropetalum chinense), white oak (Quercus fabri), chastetree (Vitex negundo var. cannabifolia), and Gardenia (Gardenia jasminoides), in southeast China. The objective of this study was to derive appropriate regression equations for estimation of shrub biomass. The results showed that the power model and the quadratic model are the most appropriate forms of equation. CA (canopy area, m²) as the sole independent variable was a good predictor of leaf biomass. D ² H, where D is the basal diameter (cm) and H is the shrub height (cm), is a good predictor of branch and root biomass, except for V. negundo var. cannabifolia and the root biomass of L. chinense. For total biomass, D ² is the best variable for estimation of L. chinense and G. jasminoides, and D ² H is the best variable for estimation of Q. fabri and V. negundo var. cannabifolia. Although variables D ², D ² H, and H are the preferred predictors for biomass estimation, CV (canopy projected volume, m³) could be used alone to predict branch, root, and total biomass in shrub species with acceptable accuracy and precision.     

An investigation of the external and internal resistance to moisture diffusion in wood

Summary An attempt was made to resolve the resistance to moisture diffusion in wood into its components, namely, the external and internal resistances by using Newmann's solution of Fick's second law. The effect of specimen thickness, moisture content, and temperature on the coefficients were also investigated.This research was supported by the National Science Foundation     

Research on the effect of microwave pretreatment on moisture diffusion coefficient of wood

Based on the pathway of moisture diffusion in wood below FSP, an improved moisture diffusion model has been proposed. The theoretical solution of the mathematic model shows that the diffusion coefficient for microwave pretreated wood was increased no more than 3% compared with that of wood without being treated even under extreme condition. Experiments indicate that the pit membrane structure can be effectively destroyed during microwave pretreatment, but the moisture diffusion coefficient can not be significantly improved. In practice, the effect of microwave pretreatment on moisture diffusion coefficient can be ignored. The drying rate of wood during the sequent conventional drying can not be significantly improved by microwave pretreatment.     

时间分数阶扩散方程的并行高效Legendre谱方法

研究了两类时间分数阶扩散方程的并行高效Legendre谱方法,分数阶导数分别代替标准的扩散方程的二阶空间导数和一阶时间导数。空间方向采用高效的Legendre谱方法,时间方向使用了基于Fourier级数展开的Laplace数值逆,并对其参数进行了优化。给出了两类时间分数阶扩散程的数值格式和数值例子,与其他方法比,该方法数值结果更优。     

Dependence of the water vapor diffusion coefficient of aspen (Populus spec.) on moisture content

Summary The dependence of the diffusion coefficient of water in aspen (Populus sp.) on moisture content over the range of 0 to 18 percent moisture content at 43 °C was determined. The method requires a series of adsorption experiments and application of a numerical method for solving the diffusion equation with a moisture-content-dependent diffusion coefficient. The diffusion coefficient was found to increase exponentially with moisture content. The numerical method includes variable boundary conditions, as defined by the surface emission coefficient, so that the effect of surface equilibrium moisture content can be examined. The experimental moisture content-time curves can be predicted by the numerical method and showed an average deviation of 0.2 percent moisture content with the largest deviation being 0.7 percent moisture content. Practical implications and uses of the information derived from the numerical method are also discussed.This research was partially funded under the USDA Competitive Grant program     

Distribution of the equilibrium moisture content in four hardwoods below fiber saturation point with magnetic resonance microimaging

Wood Science and Technology - The distribution of liquid and bound water in wood samples under equilibrium moisture contents (EMC) below fiber saturation point (FSP) was assessed by magnetic...     

Determination and use of moisture diffusion coefficient to characterize drying of northern red oak (Quercus rubra)

Summary Diffusion analysis can be used to estimate the time required to dry lumber. However, more accurate calculations require additional information on the relationship between the diffusion coefficient as a function of moisture content and on the effects of temperature, equilibrium moisture content, board thickness, and air velocity on drying time and moisture gradients. The primary objectives of this study were (1) to determine the diffusion coefficient of northern red oak (Quercus rubra) as a function of moisture content and (2) to compare experimentally determined sorption times and moisture content gradients with those calculated by the diffusion model. The diffusion coefficient was found to increase approximately exponentially with moisture content over a range of 6 to 30 percent at 43°C. This relationship was similar in both adsorption and desorption tests. Experimental adsorption and desorption times and desorption moisture content gradients were compared with finite difference solutions to the diffusion equation. Practical uses of solutions to the diffusion equation are illustrated for kiln drying wood that has first been predried to below the fiber saturation point. Drying time is also related to variable kiln conditions and board thickness.This research was partially funded under the U. S. Department of Agriculture Competitive Grant programThe Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time     

Modeling the kinetics of moisture adsorption by wood

Summary Modeling of the kinetic of moisture adsorption by wood has been studied by using cubic samples. The model is based on an explicit numerical method with finite differences. Experiments have been carried out either for determining the data necessary for calculations (diffusivity, amount adsorbed at equilibrium) and for testing the validity of the model. Two different experiments have been done in case of the longitudinal adsorption: the one by increasing the relative humidity of the atmosphere following a discontinuous step by step process. The other by determining the kinetic adsorption of moisture by samples previously equilibrated under the same conditions when they are contacted with atmosphere at various R. H. Good correlations are obtained between calculated values and experiments in both cases. Although the actual paper is concerned with constant diffusivity, the model is capable to use concentration-dependent diffusivities.This work has been done with the help and support of the French C.T.B. (Wood Technical Center), 10 Av. St.-Mandé, Paris     

Wood Moisture Content Measurement by X-Ray Exposure Method

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Assessing the impacts of four alternative management strategies on forest timber and carbon values in northeast China

Forest management strategies, reflecting the cognition and the demands of the human population on forests, have significant effects on the forest structures and functions. Nowadays, numerous forest management strategies have been introduced and implemented worldwide for a long time. However, our knowledge about the impacts of alternative management strategies on forest multipurpose management practices is still insufficient. Therefore, the overall goal of this study quantitatively assessed the impacts of four alternative forest management strategies on forest timber and carbon values in a large forest area in northeast China, as an example. Four alternative forest management strategies: no intervention management (NIM), classical timber management (CTM), multi-purpose management (MPM), and spatial-constraints management (SCM), in conjunction with different management objectives and regulations, were quantitative assessed using optimization methods. The results of numerical analysis showed that implementing eco-friendly forest management strategies such as imposed in MPM and SCM strategies may be economic inefficiencies, mainly due to significant decreases of the joint benefits from forest timber and carbon values (approximately $18.75 and $22.36 million per year) have been observed under the current carbon trading market of China (namely $20 per ton of carbon) when the ecological- and spatial-oriented constraints were further integrated into the typical CTM strategy. However, both MPM and SCM strategies were quite meaningful for the restoration of forest resources in northeast China, in which an additional of 13.6 and 16.2 thousand tons of carbon were sequestrated during the 50 years simulated horizon. Therefore, forest decision makers should evaluate the potential effects carefully prior to altering their forest management strategy in practices.