An investigation of some problems in drying of Tasmanian Eucalypt timbers

A detailed investigation of the air flow through a timber stack with the flow both along and across the board is described. Measurements of velocity profile and turbulence level at various distances from the leading edge are presented to show the structure and development of the boundary layer flow under various free stream velocities. Existing theories on the laminar separation at entrance and the transition process from laminar to turbulent flow in the separated shear layer and the development of the boundary layer flow are examined and compared with experiments. Surface shear stress and friction factor are estimated from the measurement of the static head drop and boundary layer properties along the flow passage. These are used to predict heat and mass transfer coefficients with the Reynolds analogy between heat, mass and momentum transfers. A simplified model based on the concept of \free water\" and \"bound water\" sharply separated by a fibre saturation point (FSP) is proved to be a useful concept for conveying preliminary idea but not sufficiently accurate for a detailed understanding of the drying of Tasmanian eucalypt timbers. Experimental evidence of surface temperature changes during drying and the formation of dry patches has led to an approach based on detailed measurements of the boundary layer flow and a coupled heat and mass transfer model within the boundary layer. In the model the transfer of moisture within the wood is assumed to be a diffusion process in which the diffusion coefficient is independent of the moisture content and the driving force is the moisture concentration as in true molecular diffusion. The transfer of heat is modelled by considering the convective heat transfer over the board surface and the conduction heat transfer inside the board. The governing transport equations are solved numerically based on analytical solutions. The model has been validated by measuring moisture contents of slices of boards during drying. Successful drying of Tasmanian eucalypt timber from green to EMC with the test procedure outlined in the process of research is made for the quarter-sawn boards. The effect of edge drying on the overall drying behaviour of the whole board is shown and the analytic solution of the two dimensional diffusion equation is quoted. A literature survey on the mechano-sorptive effect is made as entry to the subject. Tests with the Tasmanian peppermint eucalypt timber are conducted under bending conditions and the stresses are applied in the direction across the grain. It is shown that nearly linear relationships could be fitted between the average moisture content and mechano-sorptive effect of the wood over the whole moisture content range but the slope of the line changes at about FSP. The slope of the line also appears to be dependent upon the initial moisture content and temperature of the wood. It is also shown that change of the loading direction from along grain to across grain results in a large increase in the mechano-sorptive effect."