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Where Jw and Je represent the fluxes of total moisture and total enthalpy at the boundary. The pressure P∞ and temperature T∞ at the external drying surfaces are fixed at the kiln vacuum pressure and operating temperature. As one of the primary variables used for the computations is the average air density, modification of the boundary pressure condition (50) is required to form an appropriate nonlinear equation for this primary variable: (51) Introducing symmetry planes into the model (i.e., a quadrant of a board) reduces the overall computational times. We assume all fluxes of liquid, vapour, air, and heat are zero at the symmetry planes. Initially the board has some prescribed moisture content, with the pressure and temperature being constant throughout the board at the initial kiln pressure and temperature. The values for external pressure and temperature are essential boundary condition parameters for the development of an accurate deterministic wood drying model and were therefore, measured as part of this study. The methodology for which is detailed in section 3.3 Drying modelling - Measurement of kiln conditions and wood drying properties. 3.5.6 Numerical Solution Procedure The numerical procedure used to resolve the drying model has been extensively published, and the reader is referred to the most relevant literature for the finer details (Perré and Turner, 1999a, Perré and Turner, 1999b). In summary, to discretise the conservation laws, we implement the finite volume method on a structured mesh. Triangular meshes were calculated over symmetry plane dimensions using the software application GMSH version 2.4.2. Thereafter, we used an efficient inexact Newton method to resolve in time the complicated and often large nonlinear system that describes the drying process. Flux limiting is used to determine the spatial weighting schemes for all advection/convection terms in the equations in order to reduce numerical dispersion of the drying fronts, and the introduction of the fixed phase ensures that full saturation Sw = 1 is never reached at the surface of the medium. These are important advancements in the computational model that can enhance convergence of the nonlinear iterations and ensure accurate resolution of the drying fronts on relatively coarse meshes and are particularly useful for simulating the vacuum drying process (Turner and Perré, 2004). For this project, we performed drying simulations in the radial-tangential (R-T) plane. 3.5.7 Physical Properties The following physical properties are necessary scalar or tensor input variables for the drying model computations. The expressions and/or values provided have been used to simulate homogenous modelling of softwoods (Turner and Perré, 2004, Perré and Turner, 1999b), and their adaptability to the proposed hardwood model will be tested and modified where necessary. Some of the properties discussed are specific to the wood direction (Plate 20) termed longitudinal (up the stem of the tree parallel to the wood fibres), radial (from the outside of the tree towards the centre) and tangential (perpendicular to the radial direction). A backsawn board has the tangential direction along the wide face of the board and a quartersawn board has the radial direction along the wide face of the board. 40 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final report ε (P−P)+ρaRT=0. gv∞Ma Equation (51) must be resolved, along with the conservation laws, during the nonlinear iterations for every external boundary control volume within the computational domain.PDF Image | Evaluation of super-heated steam vacuum drying
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