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On the activation energy of diffusion of water in wood
Authors:A J Hunter
Institution:(1) CSIRO, Division of Forestry and Forest Products, Private Bag 10, 3168 Clayton, Vic., Australia
Abstract:Summary The diffusion equation for water in wood is expanded in terms of temperature and moisture gradient on the assumption that the driving force for the diffusion of water in wood is the partial pressure of water vapour. An analytic expression is then developed for the activation energy of diffusion in terms of enthalpy and entropy changes associated with the sorption process. The expression is compared with another published curve and some similarity was observed.Symbols C water concentration, kg/m3 - D diffusion coefficient for water vapour in wood with vapour pressure as the driving potential, kg/ms Pa - Dc diffusion coefficient for water vapour in wood with water concentration as the driving potential, m2/s - Dcinfin a constant value of Dc, m2/s - E activation energy of diffusion, J/kg - F flow density, kg/m2 s - f Deltah/l - h specific enthalpy, J/kg - L l/R T - l latent heat of vapourization of free water, J/kg - ls latent heat of vapourization of sorbed water, J/kg - p partial pressure of water vapour, Pa - ps pressure of water vapour at saturation, Pa - R specifc gas constant for water, J/kg K - r relative humidity - s specific entropy, J/kg K - w dry basis moisture content - x length coordinate, m - beta a constant temperature equal to 6,800 K - zeta -phgr/ln r - rhovw density of wood (dry mass/moisture volume) at a given moisture content, kg/m3 - phiv Deltas/R - L style as 2 lines above - Delta free water relative to sorbed water The author is grateful to the Editorial Board in relation to the use of (4)
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