THE SIGNIFICANCE OF THE GAPS BETWEEN BOARDS IN DETERMINING THE MOISTURE CONTENT PROFILES IN THE DRYING OF HARDWOOD TIMBER

ABSTRACT

A numerical simulation (CFX 4.1) of the airflow patterns around timber boards has been used to assess the significance of gaps between boards in terms of the mass-transfer coefficients across both side and top faces of 50 mm square pieces of hardwood timber. These gaps, which are the distances between board edges in the streamwise direction, are inevitable consequences of both imperfect sawing and shrinkage, and are typically of the order of 1–20 mm. However, for the laminar flow conditions which are typical of the air velocities used in hardwood drying (0.5 m ⁻¹), the simulations suggest that the air in the gaps quickly becomes almost saturated, even for 20 mm wide gaps, since there is no net air flow through the gaps. This situation means that the effective mass-transfer coefficients from the narrow faces of the boards are likely to be less than 1% of those from the broad faces (which are exposed to the airflow). This in turn suggests that the moisture-content profiles in stacked timber will be considerably less two-dimensional than those for the drying of single boards during laboratory testing, unless precautions are taken to simulate typical kiln stacking arrangements.

Key Words and Phrases:

• timber drying
• CFD
• mass-transfer coefficients