Problems and solutions in wood drying modelling: History and future

Abstract

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.