| Summary: | The purpose of this thesis research was to better understand the nature of gas flow in
wood, and especially the phenomenon of non-Darcian air flow. Specifically, the objectives were to
evaluate the non-Darcian air flow due to (1) specimen length; (2) nonlinear flow; and, (3) slip
flow in wood, through a systematic investigation of the air flow phenomena in two softwoods and
two hardwoods. Throughout the thesis, over the experimental range, the hypothesis that Darcy's
law is not of universal application to gas flow in wood was shown to be true.
Firstly, non-Darcian behaviour due to specimen length seemed common in the studied
species. In the experimental range of specimen lengths, there was an existence of a certain length
above which the permeability values were nearly identical for the various lengths of the tested
species. These specimen lengths were found to be 140, 100, 60 and 40 mm for red oak
heartwood, red alder heartwood, ponderosa pine sapwood, and Douglas-fir sapwood,
respectively. When the specimen length was below a critical value for the different species
described above, permeability increased drastically with decreasing specimen length. The higher
the air permeability of a species, the greater was the critical specimen length. When the specimen
length is above a critical value for the different species described above, the pressure drop caused
by end effects due to the shape and condition of the specimen entrance is negligible.
Secondly, except for red oak heartwood, there was no evidence of non-Darcian flow due
to nonlinear flow in the studied species throughout the entire measured range of flow rates. For
red oak heartwood, when the lower flow rates are used (Q≤ 19.57 cm³/s), the test results for the
detection of nonlinear air flow were exactly the same as the specimen groups of red alder
heartwood, ponderosa pine sapwood and Douglas-fir sapwood. That is, both permeability
measurement and pressure-flow rate-relationship methods for the detection of nonlinear flow,
indicated the existence of linear flow components only within the specimen. However, when the
flow rates used were above 19.57 cm³/s, the test results showed that, the superficial specific
permeability at the mean pressure of 0.5xl0⁵ Pa decreased with the increase of the flow rates, and
the expression equation of pressure drop and flow rate at a given mean pressure of 0.5x10⁵ Pa
involved both a linear and quadratic dependence of the pressure drop on the flow rate, thus
demonstrating the presence of the nonlinear flow components in the specimen. The calculated
value of Reynolds' number in the range of 0.263 to 1.05 further suggested that, the nonlinear flow
found in the red oak heartwood at higher flow rates in this study was probably nonlinear laminar
flow due to the kinetic-energy losses occurred in the curved openings.
Finally, the test results indicated that the non-Darcian air flow due to slip flow existed in
all the studied specimen groups. The true permeability of red oak heartwood, red alder
heartwood, ponderosa pine sapwood and Douglas-fir sapwood was 20.91, 7.05, 0.51 and 0.068
μm³/μm, respectively. The average ratios of the superficial specific permeability at 0.5xl0⁵ Pa
mean pressure to the true permeability were found to be: red oak heartwood: 1.047; red alder
heartwood: 1.204; ponderosa pine sapwood: 1.292; and, Douglas-fir sapwood: 1.53. The slip
flow constant b was highest (0.265xl0⁵Pa) for Douglas-fir sapwood, higher (0.146xl0⁵Pa) for
ponderosa pine sapwood, lower (0.102xl0⁵Pa) for red alder heartwood, and lowest
(0.023xl0⁵Pa) for red oak heartwood. The radius (r) and the number (n) of average effective
openings were found to be: red oak heartwood: 17.432 μm and 0.066xl0⁶ per cm²; red alder
heartwood: 3.955 pm and 7.5xl0⁶ per cm²; ponderosa pine sapwood: 2.972 μm and 3.3xl0⁶ per
cm²; and, Douglas-fir sapwood: 1.552 μm and 3.6xl0⁶ per cm². === Forestry, Faculty of === Graduate
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