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5.5 • • • • diffusion coefficient for C. citriodora was approximately 1:1 indicating low diffusion isotropy in this plane. This is evident in many other wood property ratios measured for this species such as shrinkage and permeability. The ratio of longitudinal diffusion to both radial and tangential directions is much greater for E. obliqua than C. citriodora, indicating E. obliqua’s greater propensity for drying in the longitudinal direction. The species investigated are highly impermeable compared to published data of exotic hardwood species and were at least a factor of 10 less permeable than the lowest published figure C. citriodora was the lowest permeability species measured, with the permeability order of the other species changing depending on wood direction. A fundamental technique was developed to increase the range of gas permeability measurements 4 orders of magnitude than previously measured using the same device. Drying modelling - Vacuum drying modelling Using the existing softwood drying model Transpore 2D, the model was re-coded and optimised in the Matlab environment, allowing peak performance with rapid convergence. The model is able to accurately simulate the heat and mass (water) transfer or flux in the radial and tangential wood directions. We conducted vacuum drying simulations for each species tested using the kiln drying and wood property parameters measured as part of this project. Additionally, we used the model to compare the conventional drying rate of C. citriodora using a known conventional schedule for this species. This was outside the proposed scope of this work. As a result of this modelling research the following conclusions can be made: • The drying of these species is primarily limited by diffusion in the radial-tangential plane. • Diffusion coefficient parameters seemed the most sensitive in regards to accurate heat and mass transfer modelling. • Initial, uncorrected, simulations resulted in faster drying rates than experimental results. • Accurate diffusion coefficient values have been characterised for each species in the radial, tangential and longitudinal directions. They were found to differ from theoretical calculations and have been successfully used to refine the model accuracy. • Experimental material was found to be entering the kiln with moisture gradients due to manipulation of the boards prior to kiln drying. This seems to coincide with simulation results after approximately the first 16 hours of drying. • The introduction of a pre-drying phase to simulate the initial setup of MC gradients prior to drying, and corrected radial and tangential macroscopic calculated diffusion coefficients resulted in a good agreement of simulated and experimental drying curves. • Depending on the species, the model accurately predicted drying time form green to 11% MC within 91 to 98% of the actual drying trials. • Using the model, including corrected diffusion coefficients, conventional kiln drying simulations of C. citriodora produced accurate determination of end-point MC after the same drying time as reported by industry. 117 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final reportPDF Image | Evaluation of super-heated steam vacuum drying
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