Neutron imaging of moisture displacement due to steep temperature gradients in hardwood

Abstract

One-side heating of moist wood results in the propagation of a drying front, which forces moisture ahead. This wet forefront occurs where the temperature downstream drops below 100 °C. The dynamics of moisture displacement in hardwood (beech) during one-side heating, leading to steep temperature gradients, is documented by means of thermal neutron radiography. Wood samples with initial hygroscopic moisture content in the 12–14% range are put in contact with a foil at 150 °C or at 250 °C for 40 min in a line of neutron radiography setup. Neutron imaging documents the moisture redistribution, while thermocouples document the temperature gradient. As the temperature increases in the material, moisture is desorbed and displaced downstream of the heat source, resulting in a zone with increased moisture content ahead of the drying front. The redistribution of moisture content has specific patterns in the three orthotropic directions, with a larger moistened zone in longitudinal direction, and growth ring related features in the radial and tangential directions. The desorption zone undergoes shrinkage which is documented by the radiographs.