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A clear difference in fibre porosity, size, wall thickness and orientation is evident between species. For instance Corymbia citriodora is made up of fibres with very thick cell walls, and hence low porosity or lumen/wood tissue ratio, compared with Eucalyptus obliqua whose fibres have large lumens and thin cell walls. The cropped images were loaded into the Meshpore software and contours of the cell lumens were automatically generated for fibre porosity determination. Table 63 contains the fibre porosities and, measured, and calculated densities for each species sample. We measured distinct fibre porosity between species, as can be visually observed from ESEM images. Calculated density values were higher than the basic density values measured because the density calculation uses the published oven dry density of wood tissue and other voids were not taken into consideration such as vessels. Additionally, intra-wall voids could also contribute to a lesser extent. However, a strong correlation was observed between measured and calculated density (R2 = 0.973) as shown in Figure 57. Table 63 Average fibre porosity, measured and calculated density for each species Species E. obliqua E. marginata E. pilularis C. citriodora Measured basic density (kg/m3) 489.0 635.9 674.0 921.0 Calc density - fibre (kg/m3) Fibre porosity 945 0.37 1110 0.26 1080 0.28 1395 0.07 1500 1400 1300 1200 1100 1000 900 800 700 600 y = 1.040x + 425.496 R2 = 0.973 600 700 800 900 1000 Measured density (kg/m3) 400 500 Figure 57. Calculated versus basic density correlation 4.4.3 Gas permeability The average permeability results measured for each species in the longitudinal, radial and tangential directions are given in Table 64. The results highlight the dramatic anisotropy ratios of these species and the vast difference between species. As stated by Perré (2007a), “wood has dramatic anisotropy ratios: the longitudinal permeability can be 1000 times greater than the transverse permeability for softwoods, and more than 106 for hardwoods”. This is certainly the case for the species measured where the longitudinal to transverse anisotropy ratio was in the order of 107 for some species. No permeability values were measured for C. citriodora in the radial direction as, even after 24 hours, no noticeable measurement could be made indicating this species is highly impermeable in this direction (>10-23 m2). C. citriodora was the lowest permeability species measured, with the permeability order of the other species changing depending on wood direction. 99 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final report Calculated density (kg/m3)PDF Image | Evaluation of super-heated steam vacuum drying
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