Finite Element Analyses of Two Dimensional, Anisotropic Heat Transfer in Wood

The anisotropy of wood creates a complex problem for solving heat and mass transfer problems that require analyses be based on fundamental material properties of the wood structure. Inputting basic orthogonal properties of the wood material alone are not sufficient for accurate modeling because wood is a combination of porous fiber cells that are aligned and mis-aligned in low- and high-density regions called annual rings. Modeling heat transfer requires the development of effective thermal conductivities as a function of those parameters associated with the rings. Effective 2D thermal conductivities were determined by modeling the wood structure at the cellular scale as a function of cell alignment and cell openness or density. Effective heat transfer coefficients were then applied to a macro scale wood board model. This macro-scale model was developed to study the transient heat transfer effects for any specific location, board dimension, and annual ring dimensions located on the cross section of a log. Using ANSYS Parametric Design Language program we were able to easily generate the geometrical specifications and enter the appropriate heat transfer coefficients determined from the micro wood cell model. Traditional initial and convective boundary conditions were applied as if a dry wood board were being heated in a convection oven. These were used to examine the transient heat transfer and temperature rise in the core of the wood board “cut” from various positions in a log. Significant differences in transient core temperature and heat conductive paths were observed for the various board configurations and annual ring geometry.

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