Modelling water absorption in wood

Abstract

A fibre-level model for the longitudinal absorption of liquid water in wood has been developed. The model is primarily intended for simulation of absorption in softwoods. Capillary suction is based on the lumen radius, which is a stochastic parameter. The average lumen volume (and thus radius) is assumed to vary linearly across the annual ring and thus account for the difference in earlywood/latewood. The number of open bordered pits between fibres is also a stochastic parameter. The water flow rate is determined by the capillary suction and the flow resistance between fibres, i.e. the number of unaspirated bordered pits between fibres. The resulting pressure field in the liquid phase is calculated and the calculation has to be updated each time a new fibre has been filled with water. In this way the absorption is determined in a stepwise manner. Some general results are presented and the results resemble those obtained in experiments. It is shown that the model can predict features that a pure diffusion-based model cannot predict. Finally, the model is used for the simulation of an experiment that showed some unexpected results and the model gives a reasonable explanation.

Keywords:

• Capillarity
• percolation
• simulation
• softwood
• water absorption model

Acknowledgements

The work presented in this paper has been financed by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, which is gratefully acknowledged.