Wood Drying Processes under Essentially Nonisothermal Conditions

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UDС

674.047

DOI:

10.17238/issn0536-1036.2018.2.88

Abstract

The wood drying technology is being developed mainly by improving the wood drying schedule based on modern methods of computer simulation and process optimization. A promising direction is the improvement of drying schedules in terms of their energy efficiency and ensuring the required quality of products. The scope of research is a theoretical analysis of heat and mass transfer when wood drying by schedules on the basis of the phenomenon of thermal moisture conductivity. Such regimes are characterized by the presence of a temperature gradient along the section of the drying assortment, which classifies them as essentially nonisothermal. A model based on the two-phase filtration equations, sorption isotherms, the Kelvin‒Cliperon‒Clausius thermodynamic equations could be used to analyze nonisothermal transfer processes, but it is associated with significant mathematical difficulties. The goal of research is to adjust solve simultaneous equations of heat and mass transfer under the essentially nonisothermal conditions. The methodological basis of the study is the physical model of the capillary-porous wood structure, taking into account the colloidal nature of the cell wall and moisture transfer model in wood capillaries. In capillary-porous bodies, to which wood can be referred, moisture exchange with the medium occurs due to the removal of liquid from the capillaries on the surface. When the liquid in the capillary is in the equilibrium over its meniscus, a polymolecular film is formed on the wall. Its thickness decreases upwards according to a definite law, which can be found if the disjoining pressure or saturation of the film vapor is known in the function of its thickness. To evaluate the contribution of the film transfer mechanism to the total moisture flow the Deryagin‒Nerpin system of equations can be applied. On this basis we carried out computer simulation of the processes of capillary moisture transfer in order to clarify the influence of thermal moisture conductivity on the overall process of heat and mass transfer in the colloidal capillary-porous body, which is wood. In the analysis of heat and mass transfer processes, the authors have obtained the formulas for determining the phase transformation number and the thermal-gradient coefficient for essentially nonisothermal wood drying schedules. The results of the work can be used in studying the wood caрillary – porous structure and determining the parameters of wood drying schedules. The fundamentally new conclusions of the study include the fact that the direction of the temperature gradient in the section of drying assortment inhibits (or increases) the flow of liquid moisture in the capillaries occuring on the basis of a film transfer mechanism, caused by the disjoining pressure. This phenomenon significantly affects the value of the phase transition number and causes some asymmetry of moisture flows when the direction of the temperature gradient vector changes. 

Authors

A.G. Gorokhovskiy, Doctor of Engineering Sciences, Professor
E.E. Shishkina, Doctor of Engineering Sciences, Associate Professor
E.V. Starova, Postgraduate Student
A.A. Mikov, Postgraduate Student

Affiliation

Gorokhovskiy A.G., Shishkina E.E., Starova E.V., Mikov A.A. Wood Drying Processes under Essentially Nonisothermal Conditions. Lesnoy zhurnal [Forestry journal], 2018, no. 2, pp. 88–96. DOI: 10.17238/issn0536-1036.2018.2.88

Keywords

thermal moisture conductivity, heat-and-mass transfer, phase transformation number, wood drying schedule, thermal-gradient coefficient

For citation

Ural State Forest Engineering University, Sibirskiy trakt, 37, Yekaterinburg, 620100, Russian Federation; e-mail: elenashishkina@yandex.ru

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Received on September 23, 2017