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An investigation of some problems in drying of Tasmanian Eucalypt timbers

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Wu, Qinglin (1989) An investigation of some problems in drying of Tasmanian Eucalypt timbers. Research Master thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (Research Master)
Keywords: Lumber, Eucalyptus, Eucalyptus
Copyright Holders: The Author
Copyright Information:

Copyright 1989 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (M.Eng.Sc.)--University of Tasmania, 1990. Bibliography: leaves 192-197

Date Deposited: 04 Feb 2015 23:39
Last Modified: 11 Mar 2016 05:55
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