Search

Drying is the most energy consuming process in the industrial transformation of wood. The current energy and climate crisis makes it imperative to adapt the process to energy availability, in terms of quantity, cost and temperature level. Wood drying schedules are historically based on practice. In the present work, a mechanistic drying model is used as a predictive tool to adapt conditions to specific situations. A multiscale computational model, Multi_Wood_DryS, has been combined with a probabilistic optimization code to propose tailor-made drying schedules that meet operators' expectations in terms of energy consumption, quality, drying time and cost. Optimizations of drying schedules of a stack of boards are proposed. The cost-optimized schedule is advantageous for all criteria with a 48% reduction in drying time. The resulting metamodel is a first step toward an intelligent controller for wood drying. [ABSTRACT FROM AUTHOR]

Most European hardwoods are nowadays under-utilized due to drawbacks such as low dimensional stability, durability and surface hardness. Seven hardwood species were thermally and/or chemically modified in order to improve these disadvantages. Heat treatment was carried out in air at atmospheric pressure at three temperatures, while two types of chemical modifications were tested, the first being based on furfuryl alcohol with tartaric acid, the second being based on succinic anhydride and glycerol. Then, modified woods were studied to determine weight loss due to thermal treatment, solution uptake, weight percent gain and swelling due to chemical modification processes. After that, main properties of chemically modified woods were characterized including density, equilibrium moisture content, dimensional stability and surface hardness. Effect of thermal pretreatment on subsequent chemical modification and on properties of modified woods obtained have also been evaluated. Combination of thermal modification with chemical modification had led to better improvements than each separately. Furfurylation treatment appeared to be more efficient than polyester modification. Most of the modified samples were denser, more stable and harder than native wood. These modified woods might compete with imported tropical wood for applications requiring high stability and hardness, such as joinery, decking or flooring, music instruments, handles for tools or kitchen utensils. [ABSTRACT FROM AUTHOR]

An X-ray density analyzer was used to determine the evolution of the moisture content (MC) profiles of a wood board during drying. Tests were performed on quartersawn boards of five wood species (oak, beech, Scotch pine, birch, alder) of two thicknesses (20 and 40 mm) dried at low temperature. The water vapor diffusion coefficient and liquid permeability of each wood species were determined by inverse analysis of the profiles using a computational model of coupled heat and mass transfer. These parameters were also measured by classical methods for validation purposes. The inverse method gives values close to the measured ones when using the evolution of the moisture profile, but also quite remarkably when using only the evolution of moisture at the core of the board. On the contrary, the method is less accurate when using only the average kinetics as experimental information. [ABSTRACT FROM AUTHOR]

In the wooden construction, structural elements are particularly exposed to external climatic conditions during the construction phase. To ensure their durability over their expected lifespan, the required drying time have to be known after a rain shower and before closing the timber structures. This question, involving coupled heat and mass transfer, can be investigated through numerical tools. The aim of this work is to provide experimental data through X-ray imaging to validate the prediction potential of computational models simulating transfer in porous media. Two complementary methods were developed to access the water migration in Norway spruce. The first one allowed to quantitatively measure spatial distribution of moisture content over time. It is based on a rigorous treatment of the X-ray beam attenuation. The second method, using a high-resolution 3D reconstruction, highlighted the role of wood rays on liquid migration within the porous network.

This review paper was written on the occasion of the 40th anniversary of the Drying Technology Journal. Interestingly, most of the progress made in the domain of drying modeling and simulation has occurred over the last 40 years. Indeed, 40 years ago, the comprehensive formulation of coupled heat and mass transfer in porous media was available; however, its potential use for computational simulation was only in its nascent stage. This is therefore an ideal time span in which to take a full historical context, to present the state-of-the-art and to give insights into its future trajectory. All aspects are deliberately presented in a pedagogical manner with many illustrations to make the text accessible to young scientists. However, for the benefit of experts, detailed formulations are provided in the annexes. The main points addressed in this work include: The description of the comprehensive macroscopic formulation, An historical perspective of the progress made in computational simulation, The contribution of modeling in parameter characterization, The mechanical aspects of drying: strain and stress induced by drying, including the case of large deformations, The impact of non-local equilibrium, The promising domain of the digital twin in parameter tuning and control/command. [ABSTRACT FROM AUTHOR]

The aim of this article is to characterize the wood-water relationship and certain moisture transfer properties of Central African species for which very little data exists in the literature, i.e. Iroko, Padouk, Bibolo (Dibetou), Moabi, Doussié, Bilinga and Grenadilla. Moisture sorption isotherms were measured using the Dynamic Vapor Sorption method at 35°C. The Hailwood-Horrobin model was used to describe the sorption isotherm and the model parameters are given for each species. A rough estimation of the fiber saturation point could be made and is between 18.5% for Doussié and 26.5% for Bibolo. The water vapour diffusion coefficient in the radial direction of the wood was determined by the cup method at 35°C. Its value ranges from 1.72E-11 for Grenadilla to 1.71E-10 m2/s for Bibolo. It has been shown that the model proposed by Siau (1984) allows a relatively reliable estimation of the value of this parameter from wood density. The gas permeability was also measured on the same samples and ranged from 3.3E-19 m2 for Moabi to 3.78E-16 m2 for Bibolo. From these parameters, the apparent tortuosity and equivalent pore diameter for the mass flow were estimated for each species. [ABSTRACT FROM AUTHOR]

Wood permeability is a macroscopic property resulting from complex pathways at the cellular level. In this study, high-resolution X-ray CT scans were performed during longitudinal imbibition in Norway spruce to investigate the role of wood rays and tracheids network in liquid migration. A customized setup was designed to maintain the bottom face of a wooden cylinder in contact with water, without vibration, despite the sample rotation required during each scan. The results highlighted a contrasted role of latewood and earlywood in water migration. Liquid imbibition was fast in latewood, because the bordered pits are less prone to aspiration than in earlywood, due to their smaller diameter and larger cell wall thickness. In earlywood, the migration pattern was complex due to pit aspiration that isolate tracheids clusters. Indeed, the images revealed a significant contribution of wood rays in the migration pathways. Consequently, tracheids were not progressively filled in the longitudinal direction from one tracheid to the next, but they were filled laterally with water coming from wood rays at different heights of the cells. This explained entrapped air bubbles in the lumens and led to a 3D heterogeneous liquid distribution. [ABSTRACT FROM AUTHOR]

The improvement of durability and dimensional stability of wood properties via modification of the microstructure and wood–water interaction has been widely utilised. This study investigated polyester treatments, a possible alternative, using environmentally friendly chemicals such as malic acid to improve the beech wood (Fagus sylvatica) properties. The modified properties have been studied with four treatments using malic acid, glycerol, butanediol and succinic anhydride, mixing polycarboxylic acids and polyols. Results showed that the anti-swelling-efficiency (ASE) improved up to 70%, and the bulking coefficient improved around 23%, exhibiting an efficient penetration within the cell walls. The leaching rates (LR) of treatments and the extractables remained low, between 0.05 and 2.4%. The equilibrium moisture content (EMC) decreased by 50% for the four treatments, compared to untreated beech wood. [ABSTRACT FROM AUTHOR]

The study of moisture migration in Norway Spruce may reduce the discrepancy between measurements and simulations of transfers in buildings and thus be helpful for improving their performance and durability. For this purpose, X-ray imaging was used to study moisture migration in Norway spruce during imbibition at tissue level. Moisture content kinetics in earlywood and latewood, based on X-ray attenuation, was observed and measured through rigorous protocol. The imbibition process was studied in both longitudinal and radial directions. The results showed an easier migration of liquid in latewood than in earlywood in the longitudinal uptake. In the radial direction, the impact of compression wood as a barrier and the role of the anatomical structure in the liquid migration were highlighted by the direct visualization of filled tracheids. [ABSTRACT FROM AUTHOR]

The paper brings new insights into nonsymmetrical convective drying, a configuration in which stresses induce a global change of the section shape of a wooden board with five insulated faces. Two experimental devices that allow to test simultaneously “twin” wooden boards have been developed: the first board is free and its curvature is collected vs. time; the second one is constrained and the force required to keep it flat is collected vs. time. Convective drying tests have been performed on radial specimens of spruce, 5 and 15 mm thick at 45 and 65°C. The results are commented and compared with simulation runs. For this purpose, a 1-D mechanical formulation is embedded in the computational code of heat and mass transfer TransPore. The objective of the comparison between numerical and experimental results is a better understanding of the coupled phenomena and the identification of some transfer and mechanical parameters. [ABSTRACT FROM AUTHOR]

The bark-water relation and the moisture transport in bark are essential for the living tree but also for the industrial utilization of bark. Thus the bark-water relation of beech (Fagus sylvatica ) and spruce (Picea abies ) have been studied in the present work. The sorption isotherms were determined at 35°C for both wood and bark and the water diffusivity was measured under steady state conditions (cup method) and under unsteady state conditions, i.e. via sorption test. The sorption isotherms were calculated based on the Hailwood-Horrobin (HH) model. It was observed that beech wood and bark have a similar hygroscopic behavior, whereas spruce bark is more hygroscopic than spruce wood in all the relative humidity (RH) ranges. The radial diffusivity was in the range 1.2 ⋅ 10-11-2.2 ⋅ 10-11 m² s-1 for spruce and beech bark, respectively, when the driving force of the diffusion is the gradient of bound water. [ABSTRACT FROM AUTHOR]

Intermittent drying may be of interest in the future to address energy issues. Such drying conditions are also likely to help stress relaxation through mechanosorptive creep. The influence of applying oscillations of temperature and relative humidity during the drying of beech timber on time and drying stresses is discussed in the paper by means of nonsymmetrical and loaded drying. Experimental data were used to validate a numerical model in the case of intermittent drying. The model was then used to perform a numerical investigation of the possibility of using an intermittent energy source to dry wood. [ABSTRACT FROM PUBLISHER]

In this study, the drying of thin layers of the “Violet de Galmi” onion (a variety mainly grown in West Africa) is presented in this article, along with the reaction engineering approach (REA) modeling for a comprehensive understanding of the drying kinetics. The experiments were conducted on a lab-scale dryer to form thin layer of cylindrical onion slice. By performing this experiment, the standard activation energy is evaluated and modeled. The model is validated by simulating the drying rates under various drying conditions. The comparison of simulation and experimental data is found to be satisfactory. This approach allows the determination of the internal characteristics of the onion for the further studies such as design of solar dryer for onion. [ABSTRACT FROM PUBLISHER]

Sorption behavior of four tropical woods coming from Cameroon is studied using a Dynamic Vapor Sorption-Intrinsic apparatus. Sorption isotherms at 20°C and 40°C of Triplochiton scleroxylon (obeche), Entandrophragma cylindricum (sapele), Sterculia rhinopetala (lotofa) and Terminalia superba (frake also called limba) are compared to those in the literature with methods using saturated salts for lotofa and frake with satisfactory results. Experimental values are fitted using two sorption isotherm models. Sorption hysteresis is greatest in sapele and least in frake. When temperature increases, differences in the hysteresis of sorption among the woods decreases. Hystereses of all studied woods ranged from 0,5 to 3%. Our studied woods have lower equilibrium moisture content probably caused by the higher values of extractives content. The water sorbed molecules on multilayer during both adsorption and desorption at 20°C and 40°C is greater in the case of frake and lower in the case of lotofa. The water sorbed molecules on multilayer of sapele is greater than those of obeche. The predominance of multilayer over monolayer of our woods is from 0,2 to 0,4 relative humidity in adsorption phase, and from 0,3 to 0,4 relative humidity in desorption phase. [ABSTRACT FROM AUTHOR]

This study provides the sorption isotherm, its hysteresis and their mass transfer properties of four Central Africa Tropical woods widely used for building construction: frake (Terminalia Superba), lotofa (Sterculia Rhinopetala), sapele (Entandrophragma Cylindricum) and ayous (Triplochiton Scleroxylon). Characterization of these four species in particular and Central Africa tropical woods in general were necessary to develop conservation and treatment of wood after first transformation using the drying. Also, moisture transport on wooden material used such as wall buildings can be facilitating to found the thermal comfort. Measurements of isotherms were performed using a dynamic vapor sorption apparatus (Surface Measurement Systems) at 20 and 40°C with air relative humidity ranged from 0% to 90%. Mass diffusivity was determined in steady state using a specific vaporimeter. Air permeability was determined using a specialized device developed to measure over a wide range of permeability values. Permeability and mass transfer properties were determined in the tangential direction with a "false" quartersawn board (sapele and lotofa) and in the radial direction with a flatsawn board (ayous and frake). Samples of sapele, ayous and frake are heartwood when lotofa contains as well as heartwood than sapwood. Results obtained showed that the temperature effect on sorption behavior was quite low. We observed also a low difference between the sorption behavior of these different species and hysteresis of sorption decreases when temperature increases. Hailwood-Horrobin model's explains plausibly the experimental sorption isotherms data. Results on characterization of mass transfer properties showed that, in the steady state, mass diffusivity decreases exponentially when basal density increases. Mass diffusivity was higher in desorption than in adsorption phase. The gaseous permeability of these species was between than those of Australian hardwoods and temperate woods. It was difficult to define a relationship between permeability and mass diffusivity. [ABSTRACT FROM AUTHOR]

In this work, the effects of moisture content variations produced by oscillating drying conditions have been studied by means of non-symmetrical drying (flying wood) and loaded drying (3-point bending test). The oscillating conditions are intended to produce a mechano-sorptive creep that is able to reduce drying stresses. However, our experimental data seem to prove that the activation of the mechano-sorptive creep by oscillations of the moisture content is very limited. Besides, the simulations showed that noisy conditions due to the kiln regulation already activate the mechano-sorptive creep, even for non-oscillating conditions, and could confuse the results. Contrary to the experimental observations, the implementation of classical mechano-sorptive models in the drying simulation model is far too optimistic, which means that these models are not suitable when the material is submitted to numerous oscillations. [ABSTRACT FROM AUTHOR]

Energy consumption during timber drying has become an increasingly important issue, alongside conventional concerns such as quality, cost, and drying time. This paper proposes a detailed analysis of the energy consumption evolution in conventional kiln drying using a sophisticated modelling approach. The formulation of energy consumption proposed by Perré et al.[ 1 ] was embedded in a multiscale computational model to consider kiln drying of a stack of boards. The drying of each board in the stack is simulated using a full version of a heat and mass transfer code (the 1-D version of TransPore), which allows all boards to be different and facilitates consideration of wood variability, in addition to the effect of position within the stack. For simple configurations, the energy consumption predicted by this complex computational tool is in agreement with the global approach.[ 1 ] Moreover, this sophisticated model is able to account for the standard deviation of the final moisture content (MC), which allows a compromise to be struck between quality, drying time, and energy consumption. In addition, this model can also be applied to test innovative, energy-saving drying strategies with an equivalent drying quality, i.e. sorting boards of similar properties before drying, or sorting boards at the end of drying to re-dry those boards that have a final MC that is still too high. [ABSTRACT FROM AUTHOR]

Wood drying is an essential process in the wood industry. During drying, the stress developed in the boards can produce several defaults such as deformations and cracks. The use of oscillating drying conditions should reduce the drying stresses in boards by activating the mechanosorptive creep. The best way to apply this concept remains an open question in the scientific community. In this paper, the effect of oscillating conditions on variations in moisture content across the board thickness was studied using an analytical model and a numerical code, TransPore. In addition, mechanical consequences of variations in moisture content produced by oscillations were studied by non-symmetrical drying (flying wood) and loaded drying (cantilever beam test). Beech wood, the second most commonly kiln-dried hardwood after oak in France, was chosen for this study owing to its elevated shrinkage coefficient, and hence its elevated risk of drying defaults. [ABSTRACT FROM AUTHOR]

Copyright of Maderas: Ciencia y Tecnología is the property of Universidad del Bio-Bio and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This article proposes a dual-scale computational model of wood drying in a batch lumber kiln. The stack may consist of a large number of boards (typically 100) arranged in layers. Each single board of the stack is simulated using one module of TransPore , a comprehensive computational model for heat and mass transfer in porous media. Timber variability is taken into account by a Monte-Carlo method. Such a dual-scale model allows the drying simulation of a 100-board stack to be completed in less than 30 s on a PC with a 2.8-GHz Xeon processor. Sample simulations are presented to depict the great potential and prospects of this new tool. [ABSTRACT FROM AUTHOR]

The purpose of this study is to investigate the effect of the position of the solar collector absorber and stagnant air relative to the airflow path on the thermodynamic properties of traversing air and the overall thermal performance of the collector. A numerical model was developed and validated using five different absorber design positions. Numerical results are validated using experimental data obtained from Epinal (France). The model was subsequently used to parametrically study the influence of these absorber positions on the density, temperature, velocity variations of the airflow through the collector and the thermal efficiency of each case scenario. The study showed the importance of stagnant air in solar collector thermal performance. Also, it showed that when one absorber is used, it is better to locate the absorber on the top of the airflow path than below the airflow path. Sandwiching the airflow path between double absorbers with the upper absorbers having stagnant air between it and the plexiglass cover and the down absorber having stagnant air between it and the bottom of the collector can produce a thermal efficiency of 31% which is 2% higher than the closest case and 13% higher than the worst-case design. [ABSTRACT FROM AUTHOR]