Temperature Evolution in Poplar (Populus nigra) Tension Wood and Normal Wood during a Conventional Drying Process

In this paper, temperature evolution through tension wood and normal wood in poplar (Populus nigra) under a convective drying condition was investigated. Flat-sawn boards with green dimensions 80×40×25 mm were dried at constant dry-bulb temperature of 60°C and relative humidity (RH) of 50% to a final moisture content of about 8%. They were coated on four surfaces using aluminum foil bonded with polyurethane (PU) glue to confine moisture movement along the board thickness. The measurement of board temperature was carried out at 2.5, 5, 7.5, 10 and 12.5 mm along the board thickness every 20 minutes by means of 1 mm-thermocouples. The pattern of temperature profile was observed to be almost similar for both tension wood and normal wood. However, a slightly steeper temperature gradient occurred in the normal wood compared to the tension wood. In both types of woods, the surface temperature rose progressively from the initial value to the dry-bulb temperature but the core temperature remained at an almost constant value as the wet-bulb temperature even at the end of drying.