Assessment of drying quality by use of Kirpichev number

Abstract

The present paper aims to determine the moisture content distribution of a wood board during drying by numerical modelling and to use a dimensionless group (invariant), termed Kirpichev number, which is able to describe the moisture content when there is a probability for the surface cracks to occur. The constant-rate drying period will be considered, when the mass transfer intensity is maximum. The system of partial differential equations governing the phenomenon consists of the unsteady energy and mass (moisture) equations and is considered in its 1D version. One way to tackle the problem is to use the analytical solution given by Luikov and the other is to use the numerical simulation performed with TORKSIM (version 5·0); our strategy is to combine these two ways in order to derive an appropriate hygrometric invariant. The proposed hygrometric Kirpichev number assesses the moisture content distribution of the body, being a measure of the inner stress magnitude. This invariant is related to the velocity, temperature and relative humidity of the drying agent. They are to be selected in such a way that they can determine, at all times of the drying process, a lower value than the maximum one of this invariant. By use of this method, an optimal domain of the drying schedules, with respect to the wood quality, can be recommended, and consequently the drying schedule with the maximum drying intensity can be chosen. In order to obtain the hygrometric Kirpichev number from the moisture content distributions, five drying schedules are applied to pine samples.