Physical Formulation and Numerical Modeling of High Frequency Heating of Wood

Abstract

Based on Maxwell's equations, a 2-D model for dielectric heating of wood has been developed. Its leading assumption (quasi-static state) can be made for high frequencies (13.56 MHz) systems and samples less than 40 cm large. As long as those conditions are met, the formulation is relevant for every soft or hardwood species for MC ranging from 0 up to 100% and temperature less than 100°C. As a result, a software has been developed. It can handle curved shapes and heterogeneous temperature and MC distributions found in realistic practical applications. Numerical strategy relies on a mesh generating freeware, the CVFE Method and the Bi-CG matrix solver. The 2-D model has proved very accurate in 1-D mode where an analytic solution is available for comparison. Application to various industrial processes has been done and led to the following practical results: shield effect of sapwood and gap effect in log heating, skin effect and inner gap effect during in-line processes, mean power density as a function of MC during stack heating.

Keywords:

• Wood
• Modeling
• RF/V drying
• Dielectric heating
• High frequency
• Numerical simulation
• CVFE

Acknowledgments

The author would like to warmly thank Professor Stavros Avramidis (Department of Wood Sciences, Faculty of Forestry, University of British Columbia, Vancouver, Canada) and Professor Jean-Louis Coulomb (LEG, University Joseph Fourier, Grenoble, France) for their technical information.