On thermal diffusivity anisotropy of dry and wet woods

Abstract

Wood is hygroscopic, taking on moisture from the surrounding environment. Therefore, moisture content is one of the most important properties of wood. Knowing the thermal properties of wood as a function of moisture content is important for understanding how this material insulates, dries, and behaves when it is exposed to fire or during industrial processing. Here, we use laser spot lock-in thermography (LS-LIT) to measure the anisotropy in planar thermal diffusivity (D) of wood samples. It is well known that LS-LIT yields the correct values of D only along the directions of the principal axes of the sample, while the resistivity to heat diffusion (r = 1/D) is the recovered parameter along any other spatial direction. The difference between D and 1/r as a function of angular orientation is here defined as the excess curve of thermal diffusivities. We study how these parameters change for wet and air-dry samples of low- and high- density wood species, such as balsa (Ochroma pyramidale) and cocobolo (Dalbergia retusa), respectively.