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Karin Hofstetter - One of the best experts on this subject based on the ideXlab platform.
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Theory of transport processes in wood below the Fiber Saturation point. Physical background on the microscale and its macroscopic description
Holzforschung, 2011Co-Authors: Johannes Eitelberger, Staffan Svensson, Karin HofstetterAbstract:The macroscopic formulation of moisture transport in wood below the Fiber Saturation point has motivated many research efforts in the past two decades. Many experiments demonstrated the difference in steady state and transient moisture transport and the inadequacy of models derived for steady state transport when used to describe transient processes. A suitable modeling approach was found by distinguishing between the two phases of water in wood, namely bound water in the cell walls and water vapor in the lumens. Such models are capable of reproducing transient moisture transport processes, but the physical origin of the coupling between the two phases remains unclear. In this paper, the physical background on the microscale is clarified and transformed into a comprehensive macroscopic description, ending up with a dual-scale model comprising three coupled differential equations for bound water, water vapor, and internal energy, as well as a simplified microscale model for determination of the coupling term.
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prediction of transport properties of wood below the Fiber Saturation point a multiscale homogenization approach and its experimental validation part ii steady state moisture diffusion coefficient
Composites Science and Technology, 2011Co-Authors: Johannes Eitelberger, Karin HofstetterAbstract:In this publication a multiscale homogenization model for moisture transport in wood is developed and validated. The model aims at prediction of macroscopic transport properties of clear wood samples from their microstructure and the physical properties of a few microscale constituents. In the first part of this two-part paper, the theoretical background and fundamentals of the model were presented, and its specification for the estimation of macroscopic thermal conductivities was shown. In this second part the model is applied to steady state moisture diffusion below the Fiber Saturation point. The model starts on a scale of about 50 μm, where the wood cells form a honeycomb-like structure. In a first homogenization step the effective moisture transport behavior of the cell structure is determined from moisture diffusion properties of the cell walls and the (moist) air in lumens, respectively. Further homogenization steps account for the larger vessels that exist in hardwood species, the annual rings which are a succession of layers with different densities, and finally wood rays, that form pathways in the radial direction throughout the stem. The model validation rests on experiments as in the case of heat conduction: The macroscopic diffusion coefficients predicted by the multiscale homogenization model for tissue-specific composition data (input data set II) are compared to corresponding experimentally determined tissue-specific diffusion coefficients under steady state conditions (experimental data set). As for thermal conductivity, the good agreement of model predictions and test data underlines the suitability of the presented multiscale model.
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Modeling of Transient Moisture Diffusion in Wood below the Fiber Saturation Point
Defect and Diffusion Forum, 2011Co-Authors: Johannes Eitelberger, Karin HofstetterAbstract:During the last two decades the macroscopic formulation of moisture transport in wood below the Fiber Saturation point has motivated many research efforts. From experiments the difference in steady-state and transient transport processes is well known, but could not be explained in a fully physically motivated manner. In the following article, first the microstructure of wood is depicted, followed by a description of the physical background of steady-state and transient transport processes in wood, and thereon based mathematical formulations. For a correct macroscopic description of transient transport processes, three coupled differential equations have to be solved in parallel, which is done using the finite element method. The validation of the whole model by comparison of model predictions with experimentally derived values is currently in progress and will be published in near future.
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A multi-scale approach for simulation of transient moisture transport processes in wood below the Fiber Saturation point
Composites Science and Technology, 2011Co-Authors: Johannes Eitelberger, Karin Hofstetter, Sergey V. DvinskikhAbstract:Abstract This paper presents a new, physically-based model for transport processes in wood for conditions below the Fiber Saturation point. The macroscopic mathematical description of these processes involves three coupled differential equations: two mass balance laws for the bound water and the water vapor phase, and an energy balance equation. These governing equations and the corresponding boundary conditions are expressed in terms of the state variables bound water concentration, water vapor concentration, and temperature. Macroscopic material properties are estimated based on a multiscale approach in the framework of continuum micromechanics. The phase change between the two water phases and the thus resulting coupling between the differential equations cannot be suitably captured in a purely macroscopic description, therefore a microscale sub-model is presented. Numerical solutions of the model equations are derived by means of the Finite Element Method. Finally, the model is applied to prediction of moisture profiles in a wood sample under transient environmental conditions. Comparing these results with corresponding profiles obtained non-destructively by proton magnetic resonance imaging (MRI) validates the model and confirms suitability of the underlying physical assumptions.
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A comprehensive model for transient moisture transport in wood below the Fiber Saturation point: Physical background, implementation and experimental validation
International Journal of Thermal Sciences, 2011Co-Authors: Johannes Eitelberger, Karin HofstetterAbstract:During the last three decades the macroscopic formulation of moisture transport in wood below the Fiber Saturation point has motivated many research efforts. From experiments the difference in steady-state and transient transport processes is well known, but it could not be explained in a fully physically motivated manner. In this article, we aim at enhancing the current understanding and improving the mathematical description of the moisture transport process in wood. For this purpose, we first present the microstructure of wood and then describe the physical background of steady-state and transient transport processes in wood, based on which we finally derive a suitable mathematical model. For a correct macroscopic description of transient transport processes, three coupled differential equations have to be solved in parallel, which is done using the finite element method. A comparison of model predictions for the sorption behavior of wood specimens with corresponding experimentally derived values yields very promising results and confirms the suitability of the assumptions underlying the model.
Johannes Eitelberger - One of the best experts on this subject based on the ideXlab platform.
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Theory of transport processes in wood below the Fiber Saturation point. Physical background on the microscale and its macroscopic description
Holzforschung, 2011Co-Authors: Johannes Eitelberger, Staffan Svensson, Karin HofstetterAbstract:The macroscopic formulation of moisture transport in wood below the Fiber Saturation point has motivated many research efforts in the past two decades. Many experiments demonstrated the difference in steady state and transient moisture transport and the inadequacy of models derived for steady state transport when used to describe transient processes. A suitable modeling approach was found by distinguishing between the two phases of water in wood, namely bound water in the cell walls and water vapor in the lumens. Such models are capable of reproducing transient moisture transport processes, but the physical origin of the coupling between the two phases remains unclear. In this paper, the physical background on the microscale is clarified and transformed into a comprehensive macroscopic description, ending up with a dual-scale model comprising three coupled differential equations for bound water, water vapor, and internal energy, as well as a simplified microscale model for determination of the coupling term.
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prediction of transport properties of wood below the Fiber Saturation point a multiscale homogenization approach and its experimental validation part ii steady state moisture diffusion coefficient
Composites Science and Technology, 2011Co-Authors: Johannes Eitelberger, Karin HofstetterAbstract:In this publication a multiscale homogenization model for moisture transport in wood is developed and validated. The model aims at prediction of macroscopic transport properties of clear wood samples from their microstructure and the physical properties of a few microscale constituents. In the first part of this two-part paper, the theoretical background and fundamentals of the model were presented, and its specification for the estimation of macroscopic thermal conductivities was shown. In this second part the model is applied to steady state moisture diffusion below the Fiber Saturation point. The model starts on a scale of about 50 μm, where the wood cells form a honeycomb-like structure. In a first homogenization step the effective moisture transport behavior of the cell structure is determined from moisture diffusion properties of the cell walls and the (moist) air in lumens, respectively. Further homogenization steps account for the larger vessels that exist in hardwood species, the annual rings which are a succession of layers with different densities, and finally wood rays, that form pathways in the radial direction throughout the stem. The model validation rests on experiments as in the case of heat conduction: The macroscopic diffusion coefficients predicted by the multiscale homogenization model for tissue-specific composition data (input data set II) are compared to corresponding experimentally determined tissue-specific diffusion coefficients under steady state conditions (experimental data set). As for thermal conductivity, the good agreement of model predictions and test data underlines the suitability of the presented multiscale model.
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Modeling of Transient Moisture Diffusion in Wood below the Fiber Saturation Point
Defect and Diffusion Forum, 2011Co-Authors: Johannes Eitelberger, Karin HofstetterAbstract:During the last two decades the macroscopic formulation of moisture transport in wood below the Fiber Saturation point has motivated many research efforts. From experiments the difference in steady-state and transient transport processes is well known, but could not be explained in a fully physically motivated manner. In the following article, first the microstructure of wood is depicted, followed by a description of the physical background of steady-state and transient transport processes in wood, and thereon based mathematical formulations. For a correct macroscopic description of transient transport processes, three coupled differential equations have to be solved in parallel, which is done using the finite element method. The validation of the whole model by comparison of model predictions with experimentally derived values is currently in progress and will be published in near future.
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A multi-scale approach for simulation of transient moisture transport processes in wood below the Fiber Saturation point
Composites Science and Technology, 2011Co-Authors: Johannes Eitelberger, Karin Hofstetter, Sergey V. DvinskikhAbstract:Abstract This paper presents a new, physically-based model for transport processes in wood for conditions below the Fiber Saturation point. The macroscopic mathematical description of these processes involves three coupled differential equations: two mass balance laws for the bound water and the water vapor phase, and an energy balance equation. These governing equations and the corresponding boundary conditions are expressed in terms of the state variables bound water concentration, water vapor concentration, and temperature. Macroscopic material properties are estimated based on a multiscale approach in the framework of continuum micromechanics. The phase change between the two water phases and the thus resulting coupling between the differential equations cannot be suitably captured in a purely macroscopic description, therefore a microscale sub-model is presented. Numerical solutions of the model equations are derived by means of the Finite Element Method. Finally, the model is applied to prediction of moisture profiles in a wood sample under transient environmental conditions. Comparing these results with corresponding profiles obtained non-destructively by proton magnetic resonance imaging (MRI) validates the model and confirms suitability of the underlying physical assumptions.
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A comprehensive model for transient moisture transport in wood below the Fiber Saturation point: Physical background, implementation and experimental validation
International Journal of Thermal Sciences, 2011Co-Authors: Johannes Eitelberger, Karin HofstetterAbstract:During the last three decades the macroscopic formulation of moisture transport in wood below the Fiber Saturation point has motivated many research efforts. From experiments the difference in steady-state and transient transport processes is well known, but it could not be explained in a fully physically motivated manner. In this article, we aim at enhancing the current understanding and improving the mathematical description of the moisture transport process in wood. For this purpose, we first present the microstructure of wood and then describe the physical background of steady-state and transient transport processes in wood, based on which we finally derive a suitable mathematical model. For a correct macroscopic description of transient transport processes, three coupled differential equations have to be solved in parallel, which is done using the finite element method. A comparison of model predictions for the sorption behavior of wood specimens with corresponding experimentally derived values yields very promising results and confirms the suitability of the assumptions underlying the model.
Roger E. Hernández - One of the best experts on this subject based on the ideXlab platform.
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distribution of the equilibrium moisture content in four hardwoods below Fiber Saturation point with magnetic resonance microimaging
Wood Science and Technology, 2015Co-Authors: Leandro Passarini, Cedric Malveau, Roger E. HernándezAbstract:The distribution of liquid and bound water in wood samples under equilibrium moisture contents (EMC) below Fiber Saturation point (FSP) was assessed by magnetic resonance (MR) microimaging. Two Amazonian hardwoods, huayruro (Robinia coccinea) and cachimbo [Cariniana domesticata], a plantation grown eucalyptus (Eucalyptus saligna), and a temperate species red oak (Quercus rubra) were studied. Desorption tests were performed at 21 °C from full Saturation state for huayruro, cachimbo, and red oak, and from green condition for eucalyptus. The EMC was reached under three desorption conditions [58, 76, and 90 % relative humidity (RH)]. MR microimages were obtained based on T 2 times and on 1H concentration. Scanning electron microscopy images helped us to interpret MR microimages. The results showed that wood structure plays a major role in liquid water drainage and in water diffusion. Eucalyptus saligna and red oak showed liquid water entrapped in parenchyma tissues, even below FSP (90 % RH). At this same RH level, all liquid water was, however, drained for cachimbo and huayruro. For these woods, bound water was not uniformly distributed in wood structure, concentrating it more in Fibers for both species. Huayruro showed the highest heterogeneity in hygroscopicity, which is explained by its particular wood anatomy.
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Magnetic resonance microimaging of liquid water distribution in sugar maple wood below Fiber Saturation point.
Wood and Fiber Science, 2010Co-Authors: Roger E. Hernández, Claudia B. CáceresAbstract:Magnetic resonance (MR) microimaging was used to determine the distribution of liquid water in sugar maple wood ( Acer saccharum Marsh.). Two moisture desorption tests were applied using saturated salt solutions at 21°C. Desorptions were accomplished between 58 and 96% RH starting from the full Saturation state and from the FSP. Each moisture sorption condition at equilibrium was associated with a MR microimaging scan. Signal intensity (represented by false colors in the MR images) allowed visualization of the concentration of liquid water distributed into the wood structure. In most cases, the presence of liquid water was noticed in samples coming from the full Saturation state at moisture contents below FSP. This result shows the coexistence of liquid and bound water even at moisture contents below the FSP. The remaining liquid water in the wood appears to be located principally in the lumina of the least accessible libriform Fibers.
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Effects of extraneous substances, wood density and interlocked grain on Fiber Saturation point of hardwoods
Wood Material Science and Engineering, 2007Co-Authors: Roger E. HernándezAbstract:Abstract Samples of nine tropical hardwoods from Peru and sugar maple wood from Quebec were selected for moisture sorption and swelling tests at 25°C. These tests evaluated the Fiber Saturation point (FSP) by two methods: following adsorption over distilled water, and from the volumetric swelling intersection point. Cold-water and hot-water extractives, sequential cyclohexane, acetone and methanol extracts, ash content, wood density and interlocked grain were also determined on matched samples. The results indicated that adsorption tests over distilled water were not applicable for determining FSP in all wood species. Condensation of water vapor apparently occurred, even though temperature during adsorption was controlled to the nearest 0.01°C. The volumetric swelling intersection point method was judged more appropriate. FSP ranged from 15 to 25% for tropical hardwoods and was 30% for sugar maple wood. FSP was negatively correlated with wood density, acetone extracted fraction, interlocked grain and ash ...
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Changes in physical properties of tropical and temperate hardwoods below and above the Fiber Saturation point
Wood Science and Technology, 2006Co-Authors: Giana Almeida, Roger E. HernándezAbstract:Changes in physical and mechanical properties of wood were analyzed using sorption tests combined with dimensional measurements and perpendicular-to-the-grain tangential compression tests. In order to determine the influence of wood structure on these changes, three hardwood species (Fagus grandifolia, Brosimum alicastrum and Cariniana domestica) presenting different anatomical structures were studied. Two experimental techniques were used to perform moisture sorption tests at 25°C. The first technique used saturated salt solutions (from 33 to 90% relative humidity) and the second used the pressure membrane method (above 96% relative humidity). Special attention was given to the “Fiber Saturation region”, where changes in wood properties started to take place. Results showed that at equilibrium moisture content (EMC), radial, tangential and volumetric shrinkage, as well as changes in transverse strength occurred above the Fiber Saturation point (FSP). This behavior can be explained by the effect of hysteresis at Saturation on wood properties. This hysteresis indicates that loss of bound water takes place in the presence of liquid or capillary water, which contradicts the concept of FSP. The initial EMC at which bound water starts to be removed varied largely among the wood species.
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Changes in Physical Properties of Yellow Birch Below and Above the Fiber Saturation Point
Wood and Fiber Science, 2006Co-Authors: Giana Almeida, Roger E. HernándezAbstract:Two experimental techniques were used to perform moisture sorption tests at 25°C on specimens of yellow birch sapwood. The first used saturated salt solutions (from 33% to 90% relative humidity), and the second used the pressure membrane method (above 96% relative humidity). These sorption tests were combined with dimensional measurements and perpendicular-to-the-grain tangential compression tests. Results showed that at equilibrium moisture content, radial, tangential, and volumetric shrinkage, as well as changes in transverse strength, occur above the Fiber Saturation point. This behavior can be explained by the effect of hysteresis at Saturation on wood properties. This hysteresis indicates that loss of bound water takes place in the presence of liquid water, which contradicts the concept of FSP. The initial equilibrium moisture content at which bound water is removed from yellow birch was about 41%.
J.m. Vergnaud - One of the best experts on this subject based on the ideXlab platform.
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Process of absorption and desorption of water in a wood board, with 3-dimensional transport beyond the FSP
Wood Science and Technology, 1992Co-Authors: M. El Kouali, J. Bouzon, J.m. VergnaudAbstract:The process of absorption of water in a piece of solid wood, as well as the following stage of desorption is studied, when the water content is beyond the Fiber Saturation point. A model based on a numerical method with finite differences is built and successfully tested. This model takes into account a 3-dimensional transport of water controlled by diffusion, with three principal axes of diffusion and three various principal diffusivities. The model is able to predict the kinetics of absorption or desorption when the three principal diffusivities are known, as well as the operational conditions.
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Modeling the process of absorption and desorption of water above and below the Fiber Saturation point
Wood Science and Technology, 1991Co-Authors: M. El Kouali, J.m. VergnaudAbstract:The process of absorption of water along the tangential direction of the wood is studied by immersing the sample in water. The transport of water is then obtained below the Fiber Saturation point at the beginning of the absorption and above this Fiber Saturation point during the process. The potential which drives the transport of the bound-water and free-water through the wood has been considered by testing a diffusional transport model. The transient diffusion with a constant diffusivity has been found to describe not only the process of absorption but also the process of desorption with diffusion of water through the solid and evaporation from the surface. Analytical solutions have been successfully used to describe the stage of absorption during a time of four hours at the end of which an equilibrium of absorption is attained, as well as the following stage of desorption. A model based on a numerical method with finite differences has been found to describe the process of absorption and desorption in various cases, and especially when the equilibrium of absorption has not been attained.
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Modelling the process of absorption and desorption of water in two dimensions (transverse) in a square wood beam
Wood Science and Technology, 1991Co-Authors: H. Mounji, J. Bouzon, J.m. VergnaudAbstract:The process of absorption and desorption of water by two-dimensional transport in wood was studied at a moisture content beyond the Fiber Saturation point. The transverse directions through a square wood beam were considered. A numerical model capable of describing the process was constructed and successfully tested. It can predict by calculation the kinetics of absorption when wood is immersed into water and the kinetics of desorption when it is exposed to air. The model gives a fuller insight into the process with the profiles of moisture content developed through the cross-section of the wood at any time.
Donald S Stone - One of the best experts on this subject based on the ideXlab platform.
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a solution thermodynamics definition of the Fiber Saturation point and the derivation of a wood water phase state diagram
Wood Science and Technology, 2016Co-Authors: Samuel L Zelinka, Samuel V Glass, Joseph E Jakes, Donald S StoneAbstract:The Fiber Saturation point (FSP) is an important concept in wood–moisture relations that differentiates between the states of water in wood and has been discussed in the literature for over 100 years. Despite its importance and extensive study, the exact theoretical definition of the FSP and the operational definition (the correct way to measure the FSP) are still debated because different methods give a wide range of values. In this paper, a theoretical definition of the FSP is presented based on solution thermodynamics that treats the FSP as a phase boundary. This thermodynamic interpretation allows FSP to be calculated from the chemical potentials of bound and free water as a function of moisture content, assuming that they are both known. Treating FSP as a phase boundary naturally lends itself to the construction of a phase diagram of water in wood. A preliminary phase diagram is constructed with previously published data, and the phase diagram is extended to a state diagram by adding data on the glass transition temperatures of the wood components. The thermodynamic interpretation and resulting state diagram represent a potential framework for understanding how wood modification may affect wood–moisture relations.
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A solution thermodynamics definition of the Fiber Saturation point and the derivation of a wood–water phase (state) diagram
Wood Science and Technology, 2016Co-Authors: Samuel L Zelinka, Samuel V Glass, Joseph E Jakes, Donald S StoneAbstract:The Fiber Saturation point (FSP) is an important concept in wood–moisture relations that differentiates between the states of water in wood and has been discussed in the literature for over 100 years. Despite its importance and extensive study, the exact theoretical definition of the FSP and the operational definition (the correct way to measure the FSP) are still debated because different methods give a wide range of values. In this paper, a theoretical definition of the FSP is presented based on solution thermodynamics that treats the FSP as a phase boundary. This thermodynamic interpretation allows FSP to be calculated from the chemical potentials of bound and free water as a function of moisture content, assuming that they are both known. Treating FSP as a phase boundary naturally lends itself to the construction of a phase diagram of water in wood. A preliminary phase diagram is constructed with previously published data, and the phase diagram is extended to a state diagram by adding data on the glass transition temperatures of the wood components. The thermodynamic interpretation and resulting state diagram represent a potential framework for understanding how wood modification may affect wood–moisture relations.