Numerical simulation of coupled heat and mass transfer in wood dried at high temperature

摘要

The mutual effect between heat and mass transfer is investigated for wood dried at high temperature. A numerical model of coupled heat and mass transfer under the effect of the pressure gradient is presented. Based on the macroscopic viewpoint of continuum mechanics, the mathematical model with three independent variables (temperature, moisture content and gas pressure) is constructed. Mass transfer in the pores involves a diffusional flow driven by the gradient of moisture content, convectional flow of gaseous mixture governed by the gradient of gas pressure, the Soret effect and phase change of water. Energy gain or loss due to phase change of water is taken as the heat source. Numerical methods, the finite element method and the finite difference method are used to discretize the spatial and time dimension, respectively. A direct iteration method to solve the nonlinear problem without direct evaluation of the tangential matrix is introduced. The local convergence condition based on the contraction-mapping principle is discussed. The mathematical model is applied to a 3-D wood board dried at high temperature with the Neumann boundary conditions for both temperature and moisture content, and the Dirichlet boundary conditions for gas pressure.