Modeling the dielectric properties of wood

Abstract

A method for calculating the complex dielectric permittivity of an anisotropic wood structure at microwave frequencies is presented. A numerical model for describing the 3D wood structure containing fibers, rays, vessels and cracks with changeable dimensions and material composition is built. This model is introduced into an efficient solver that calculates the effective dielectric constant of any 3D structure of dielectric materials. Using our numerical model we succeeded in theoretically reproducing the results of recent measurements of the dielectric permittivity of wood, in various directions and various moisture contents. The qualitative agreement is realistic, reproducing all the trends of the changes in ɛ as the direction of the electric field and the moisture content are varied. The quantitative agreement is practical and reliable for engineering calculations with an average deviation of ±10% in ɛ′ and ±5% in ɛ′′. As microwave processing of wood involves internal temperatures as high as 150°C and pressures of up to 5 atm, the dielectric properties of wood were also calculated with the same numerical model by simulating high internal temperature and pressure. A comparison between the calculated and measured values shows once again how accurate the model reproduces the empirical study.