Effect of Cyclic Humidity Exposure On Moisture Diffusion In Wood

Authors

  • Jing Ping Lu
  • R. H. Leicester

Keywords:

Moisture diffusion, cyclic humidity, surface attenuation

Abstract

It is known that moisture content changes in wood strongly affect the mechano-sorptive effects, which in turn contribute significantly to the deformation and strength under long duration loads. Earlier studies have shown that the moisture changes due to the daily and annual variation in the surrounding air tend to be confined to the edges of practical size members. Therefore, prediction of the moisture content near the boundaries of a timber section becomes an important issue. This paper studies the mathematical solutions of the diffusion equation for the finite thickness element considering the effect of surface resistance. By introducing a surface attenuation factor, a simple approximate solution may be obtained. Procedures for evaluating the coefficients K and D are also discussed, together with some data on these values for radiata pine (Pinus radiata). The surface emission coefficient K is a function of air velocity V. A procedure for assessing this effect of the air velocity is also given.

A numerical example shows that the proposed approximate solution accurately predicts the cyclic range of moisture content for an ambient cyclic boundary condition. The procedures described herein can be easily extended to include the analysis of two-dimensional and three-dimensional elements.

References

Bramhall, G. 1979a. Mathematical model for lumber drying. I. Principles involved. Wood Science 12(1):15-21.nBramhall, G. 1979b. Mathematical model for lumber drying. II. The model. Wood Science 12(1):22-31.nCarslaw, H. S., and J. C. Jaeger. 1959. Conduction of heat in solids. The Clarendon Press, Oxford, UK.nChomcharn, A., and C. Skaar. 1983. Dynamic sorption and hygroexpansion of wood wafers exposed to sinusoidally varying humidity. Wood Sci. Technol. 17: 259-277.nChoong, E. T., and P. J. Fogg. 1968. Moisture movement in six wood species. Forest Prod. J. 18(5):66-70.nChoong, E. T., and C. Skaar. 1969. Separating internal and external resistance to moisture removal in wood drying. Wood Science 1(4):200-202.nCrank, J. 1956. The mathematics of diffusion. The Clarendon Press, Oxford, UK.nHart, C. A. 1977. Effective surface moisture content of wood during sorption. Wood Science 19(4):194-201.nKouali, M., and J. M. Vergnaud. 1991. Modelling the process of absorption and desorption of water above and below the fibre saturation point. Wood Sci. Technol. 25:327-339.nLeicester, R. H., and J. P. Lu. 1992. Effects of shape and size on the mechano-sorptive deformations of beams. Proc. IUFRO S.502 Timber Engineering Meeting, 17-21 August, Bordeaux, France.nLeicester, R. H. 1994. Studies of mechano-sorptive effects. Part 1. Mechano-sorptive effects on strength and stiffness. Proc. CIB-W18B Workshop, July, Sydney, Australia.nLiu, J. Y. 1989. A new method for separating diffusion coefficient and surface emission coefficient. Wood Fiber Sci. 21(2): 133-141.nLu, J. P., and R. H. Leicester. 1993. Effects of environmental conditions on timber creep. Pages 555-562 in Proc. 13th ACMSM, July, University of Wollongong, NSW.nLu, J. P. 1994. Deformation and strength loss due to mechano-sorptive effects. Proc. Pacific Timber Engineering Conference, July, Gold Coast, Australia.nNewman, A. B. 1931. The drying or porous solids: Diffusion calculations. Trans. Am. Inst. Chem. Eng. 27: 310-333.nSiau, J. F., and S. Avramidis. 1996. The surface emission coefficient of wood. Wood Fiber Sci. 28(2): 178-185.nSimpson, W. T. 1973. Predicting equilibrium moisture content of wood by mathematical models. Wood Fiber 5(1):41-49.nToratti, T. 1992. Creep of timber beans in a variable environment. Doctoral dissertation, Helsinki University of Technology, Finland.n

Downloads

Published

2007-06-19

Issue

Number 1 / January 1997

Section

Research Contributions

License

The copyright of an article published in Wood and Fiber Science is transferred to the Society of Wood Science and Technology (for U. S. Government employees: to the extent transferable), effective if and when the article is accepted for publication. This transfer grants the Society of Wood Science and Technology permission to republish all or any part of the article in any form, e.g., reprints for sale, microfiche, proceedings, etc. However, the authors reserve the following as set forth in the Copyright Law:

1. All proprietary rights other than copyright, such as patent rights.

2. The right to grant or refuse permission to third parties to republish all or part of the article or translations thereof. In the case of whole articles, such third parties must obtain Society of Wood Science and Technology written permission as well. However, the Society may grant rights with respect to Journal issues as a whole.

3. The right to use all or part of this article in future works of their own, such as lectures, press releases, reviews, text books, or reprint books.