The roles of bound water chemical potential and gas phase diffusion in moisture transport through wood

Stanish, M. A.

doi:10.1007/bf00350694

Cited by 27 publications

(14 citation statements)

References 3 publications

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“…The water vapour diffusion coefficient D wv = ξ • D a is given by the reduction factor ξ to account for the porous structure of wood (e.g. Stanish 1986;Krabbenhoft and Damkilde 2004) and the diffusion coefficient of water vapour in air D a = D a ( p wv , T ) (e.g. Schirmer 1938;VDI-Gesellschaft 2006), which slightly depends on partial water vapour pressure p wv .…”

Section: Influence Of Sample Materialsmentioning

confidence: 99%

“…Wadsö 1993a;Eitelberger and Svensson 2012), and various modelling approaches were given in the literature (e.g. Skaar and Babiak 1982;Stanish 1986;Wadsö 1994a;Frandsen et al 2007;Eitelberger et al 2011;Krabbenhoft and Damkilde 2004;Olek et al 2005;Hill et al 2010a). However, the relevance of WVT on the sorption kinetics of such samples appears to still be undetermined (Himmel and Mai 2016).…”

Section: Introductionmentioning

confidence: 99%

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The Relevance of Water Vapour Transport for Water Vapour Sorption Experiments on Small Wooden Samples

Murr

2019

Transp Porous Med

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To characterise the sorption behaviour of wood or other hygroscopic materials with the use of water vapour sorption experiments, an accurate understanding of water vapour transport both to and through the sample material is necessary. Within the last decades, various modelling approaches on the sorption kinetics were developed, but there seems to be no general agreement on the relevance of water vapour transport. Using small amounts of grained wood, this study aims to estimate the influence of water vapour transport in sorption experiments on small sample sizes. A theoretical analysis based on a diffusion equation including a simplified sink/source term was carried out and compared with corresponding experiments. Nonlinear isotherm and temperature were shown to have a major impact on the effective diffusion of water vapour, whereas step size in humidity seems to mainly influence processes within the cell wall (e.g. relaxation and reorganisation of microstructure). Considering the paths of stagnant air above sample surface, the anomalous behaviour for a variation in sample thickness of grained wood can be explained. In conclusion, water vapour transport appreciably influences the sorption kinetics of small sample sizes and has to be considered in the modelling and interpretation of water vapour sorption experiments.

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Section: Influence Of Sample Materialsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The Relevance of Water Vapour Transport for Water Vapour Sorption Experiments on Small Wooden Samples

Murr

2019

Transp Porous Med

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“…The logical solution is to use the state of the air in equilibrium with the wood as the driving force. This idea was suggested that vapour partial pressure is driving force (Bramhall 1979) and that chemical potential is driving force (Kawai et al 1978, Stanish 1986). In addition to this change of driving force, it has to be established how the state point of the wood changes, i.e.…”

Section: Modelling Sorption Hysteresismentioning

confidence: 99%

Inclusion of the sorption hysteresis phenomenon in future drying models. Some basic considerations

Salin¹

2011

Maderas, Cienc. tecnol.

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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.

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“…These data were analyzed using the flux expressions given by Eqs. 24 and 33 (Stanish, 1986). Table 1 contains a summary of the experimental conditions and results together with the corresponding theoretical migration rates predicted by the above equations.…”

Section: Transport Phenomenamentioning

confidence: 99%

A mathematical model of drying for hygroscopic porous media

1986

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A mathematical model is developed to simulate the drying of hygroscopic porous media and, in particular, of wood. Drying rate experiments were performed using wood specimens and a nonhygroscopic porous ceramic solid and were simulated using the appropriate version of the drying model. Calculated model predictions are in very satisfactory agreement with experimental results. An examination of the relative impacts on drying of the transport mechanisms that comprise the model leads to meaningful interpretations of observed drying behavior. Controlling rate factors can be identified and different types of drying behavior specific to a given material or drying condition can be explained and understood through model simulation studies. Such capability can provide important guidance for drying process design and control. M. A. Stanish, G. S. Schajer, Ferhan KayihanWeyerhaeuser Technology Center Tacoma, Washington 98477 SCOPEThe drying of moist porous solids is a complicated process involving simultaneous, coupled heat and mass transfer phenomena. Accordingly, drying behavior can be influenced by a rather large variety of independent factors, including, for example, ambient conditions of temperature, air velocity, and relative humidity, and solid properties such as density, permeability, and hygroscopicity. Extensive characterization of drying behavior using a strictly experimental approach constitutes a formidable challenge due to the excessively large number of variables that must be considered. The task becomes more manageable, however, with the help of a reliably realistic mathematical model of the drying phenomenon. Our objective is to develop such a tool to simulate drying behavior and therefore allow us to extend the results of experimental drying investigations. In this way, the impact of the many variables on drying behavior can be examined and interpreted without having to resort to an extensive program of experimental testing.The approach taken here represents advancement of drying modeling and theory on several fronts. The mathematical description of the drying process is derived and maintained for the most general case; a comprehensive set of fundamental heat-and mass-transfer mechanisms is coupled with thermodynamic phase equilibrium expressions in a way that accommodates either hygroscopic or nonhygroscopic materials. Heat transfer by both conduction and convection is included together with mass transfer by gaseous diffusion and bulk flow of gas and liquid through the void space and by bound-water diffusion through the solid matrix. Bound-water migration is expressed in terms of the diffusion of sorbed water driven by the gradient in the chemical potential of the bound molecules. While this concept of bound-water diffusion was previously suggested by Siau (1983) and others (Kawai et al., 1978), in this development a new, uniquely explicit expression for bound-water flux is derived in terms of temperature and vapor pressure gradients. Combination of all of these transport mechanisms in a single generalized ...

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The roles of bound water chemical potential and gas phase diffusion in moisture transport through wood

Cited by 27 publications

References 3 publications

The Relevance of Water Vapour Transport for Water Vapour Sorption Experiments on Small Wooden Samples

The Relevance of Water Vapour Transport for Water Vapour Sorption Experiments on Small Wooden Samples

Inclusion of the sorption hysteresis phenomenon in future drying models. Some basic considerations

A mathematical model of drying for hygroscopic porous media

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