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3.5 Drying modelling - Vacuum drying modelling The vacuum drying model was developed and implemented as part of this authors PhD project offered by QUT. At the time of running model simulations, not all vacuum drying trials were completed. For model validation, one vacuum drying trial was chosen per species. We chose the trial that, to date, had produced the best-dried quality outcome in the quickest drying time (most optimised). The vacuum drying trials chosen were the E. obliqua trial MES8, C. citriodora trial SPG9, E. marginata trial JAR10 and E. pilularis trial BBT11. As the anatomical configuration of wood is complex, one must write transport equations at the macroscopic scale. This leads to the definition of empirical laws of migration that can be demonstrated to a large extent by averaging over representative volumes (Perré, 1996). At this level, we observe the porous medium as a fictitious, continuous medium. By using this approach, we can write most fluxes as the product of a coefficient times the driving force, both for the porous medium and the surrounding air. The conservation of liquid, water vapour, air and enthalpy enable a set of equations governing transfer in porous media to be derived (Whitaker, 1977). These equations have subsequently been used to model the softwood drying process (Perré, 1996, Perré and Turner, 1999b), where their ability to describe several different drying configurations has been proven. The most recent application of the model describes the drying of single boards applied to the growth rings of softwood (Perré and Turner, 2008). This model, known as TransPore 2D describes the drying process for both homogeneous and heterogeneous cross sections, where density and transport differences due to the presence of growth rings accounted for. As the species investigated in this study are relatively homogenous, the majority of the transport model equations derive from the homogenous model reference as summarised: (41) (42) (43) (44) 38 3.5.1 Liquid conservation ∂(εwρw +εvρv +ρw)+∇⋅(ρwvw +ρgvgρbvb)=∇⋅(ρgDeff∇ωv) ∂t 3.5.2 Energy conservation ∂ (ε ρ h +ε (ρ h +ρ h )+ρ hb +ρ h −ε P ) wwwgvvaab0sgg +∇⋅(ρwhwvw +(ρvhv +ρaha)vg +ghbρbvb) =∇⋅(ρgDeff (hv∇ωv +ha∇ωa +Keff∇T)+Φ ∂t 3.5.3 Air conservation ∂ (εgρa)+∇⋅(ρavg)=∇⋅(ρgDeff∇ωa) ∂t The Generalised Darcy’s Law gives the gas and liquid phase velocities: vl=−Klkl∇φ μl Where l = w, g ∇φ=∇P−ρg∇χ , llll 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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