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Espoo, Finland

Johansson J.,Linnaeus University | Salin J.-G.,Romensvagen 12 A
Wood Material Science and Engineering | Year: 2011

The aim of this study was to improve our understanding of the capillary uptake of water in aspen sapwood and heartwood, which may help to explain the variable performance of aspen used outdoors. The study uses a percolation model developed for softwoods to examine capillary liquid absorption in aspen and compares predicted results with ones obtained experimentally using computed tomographic scanning. The study shows that there is an equilibrium saturation level where new flow paths are being found at the same rate as old ones are being blocked. This is seen as a plateau where the water content maintains a relatively constant height within the material. This makes it possible to model the capillary behaviour of aspen sapwood. In heartwood, however, the uptake of moisture seems to be mainly restricted to bound water, except for a short region in the sample wet end. This absorption is thus governed mainly by bound water diffusion. This improved understanding of the capillary behaviour of aspen may contribute to a future market expansion where aspen may be utilized to a greater extent outdoors and above ground. © 2010 Taylor & Francis. Source

Salin J.-G.,Romensvagen 12 A
Wood Material Science and Engineering | Year: 2010

This paper gives a historic overview of wood drying modelling during the past 30 years. Some of the problems encountered and their solutions are discussed. Finally, some remaining problems that require solutions in the future are presented. The modelling work increased strongly when personal computers became widely available. Numerical solutions of Fick's equations became easy and the first promising simulations of the drying process were presented. The models required numerical values for material parameters such as the diffusion coefficient and models were used for adapting values to have a good fit with test results. External heat and mass transfer was a subject for discussion for a long time. Unfortunately, a lot of work was devoted to the surface emission concept, which has turned out to be useless from a practical point of view. Another problem in this field was the apparent deviation from the analogy between heat and mass transfer. Only recently have these external transfer problems been solved, or at least understood. As moisture migration modelling had reached a reasonable level of accuracy, the focus turned towards the calculation of moisture-induced stress. This required modelling of mechanosorptive creep behaviour, which is a subject that is still not fully understood. Initially, only low-temperature, single board models were developed, but gradually other areas were included such as kiln-wide models, energy consumption, drying costs, deformations and temperatures above the boiling point. Areas that still require research and development include modelling free water behaviour in the capillary network, inclusion of sorption hysteresis, environmental impact and modelling discolouration. Finally, the importance of technology transfer in the form of easy-to-use models for kiln operators and embedded models in kiln control systems should be emphasized. © 2010 Taylor & Francis. Source

Salin J.-G.,Romensvagen 12 A
Maderas: Ciencia y Tecnologia | Year: 2011

Most drying simulation models describe the moisture migration in wood as a diffusion process, including free water flow at contents greater than the fibre saturation point. This means that wood is seen as a homogeneous material lacking internal structure. However, especially in softwood narrow sections, bordered pits, divide the free water phase into rather distinct units. It is thus quite clear that the flow of free water is governed by capillary forces and not by diffusion. A model has been developed that investigates how water filled units are emptied one by one in a drying process. Simulations with the model explain some experimentally seen features that cannot be obtained using solely diffusion type models. Water absorption by dried wood is generally assumed to be governed by capillary (surface tension) forces. An additional feature is that a considerable part of the bordered pits have been aspirated, i.e. closed, in the drying process and the number of possible flow paths is thus reduced. Thus the driving force for water flow is the capillary suction into the lumen. Modelling wood wetting by these principles also introduces some interesting specific features, such as a limit regarding the maximal achievable water saturation. In summary it is found that specific behaviour seen on a real macroscopic level originates from properties at the microscopic, fibre level. This clearly indicates that experience from fibre level models should be included in future drying and wetting simulation models. The work in this direction so far, has been promising. Source

Salin J.-G.,Romensvagen 12 A
Maderas: Ciencia y Tecnologia | Year: 2011

The sorption hysteresis effect, i.e. different wood equilibrium moisture contents (EMCs) in desorption and adsorption for the same relative humidity, is well known. However, quantitative sorption isotherms, in the form of tables or analytical correlations, are almost always given as the average of the desorption and adsorption curves. Consequently most drying simulation models use these average curves, and does not take into account the sorption hysteresis phenomenon. The equilibrium state of a wood sample is thus not a function of the relative humidity only, but depends on the moisture history also. This means that Fick's equations - with moisture content as a single driving force - are not valid any more. For a pure desorption process the state of the sample follows the desorption isotherm, but a problem arises when desorption is followed by adsorption - as for instance in the timber conditioning phase. It seems reasonable to assume that for each EMC point, on or between the desorption/adsorption isotherms, the moisture content change follows a unique path when the surrounding climate changes. This path - the so called scanning curve - does not need to be the same in desorption and adsorption. Some selected results and corresponding scanning curve suggestions are presented and discussed. Drying models with the sorption hysteresis phenomenon included should be developed for the analysis of experimental data and more generally for use as an improved tool in practical applications. Source

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