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sensitivity of all the drying ‘driving force’ parameters (porosity, permeability etc.) we concluded that the diffusion coefficient was by far the most sensitive in regards to the moisture content and pressure simulated outputs. This conclusion led to the comparison of the measured diffusion coefficients with those calculated during the simulations. We did this by calculating the radial and tangential diffusion coefficients under the same temperature and humidity conditions that we measured diffusion coefficients. Table 71 shows the measured and calculated diffusion coefficient results and measured/calculated ratios for each species. Due to differing cutting patterns for each species only the radial diffusion coefficients for C. citriodora, E. marginata, and E. pilularis and tangential coefficients for E. obliqua were analysed, where the direction for each species pertains to perpendicular drying to the wide board face; the predominant drying direction. The results indicate that, except for E. pilularis a discrepancy exists between the measured and calculated diffusion coefficients, where the measured values are approximately 50 to 70% lower than the calculated values depending on the species. The reason for this discrepancy requires further investigation, however personal communication with Perrè (2010) suggests that extractive content could be a significant factor. Table 71. Measured and calculated diffusion coefficient results under the same environmental conditions Diffusion Coefficient (m2/s) C. citriodora E. marginata E. pilularis E. obliqua Measured 1.43E-11 2.58E-11 2.67E-11 4.08E-11 Calculated Ratio 2.63E-11 0.54 4.78E-11 0.54 2.62E-11 1.02 6.96E-11 0.59 We established that by correcting the diffusion coefficient calculations, using the ratios shown in Table 71, simulated and experimental core MC outputs more closely matched for each species. The correction factor was achieved by simply multiplying the tangential and radial macroscopic diffusivities described in the model by the corresponding correction ratio for each species, effectively correcting the calculated values. Figure 76 shows an example of the MC simulation results for C. citriodora after the MC correction and using the diffusion coefficient corrections. A close match of MC core simulated and experimental outputs resulted. 0.4 0.3 0.2 0.1 0 0 50 100 150 Time (hrs) 200 250 Model - surface Model - core Experiment - core Figure 76. Transpore 2-D moisture content simulation results for C. citriodora using measured diffusion coefficient and adjusted MC 112 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final report Moisture content (%/%)PDF Image | Evaluation of super-heated steam vacuum drying
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