Some considerations in heterogeneous nonisothermal transport models for wood: a numerical study

Abstract

This study compares a number of coupled heat and mass transfer models and presents numerical comparisons of phenomenological coefficients between the four models (Stanish, Perre, Pang, and Avramidis) that are most frequently used in the literature to describe wood-drying processes. The USDA sorption isotherm, the Hailwood-Horrobin model, was adopted to calculate the relations between moisture content in wood and water vapor pressure at any temperature. Due to different assumptions about the driving forces of heat and mass transfer, coefficients in each model represent different values for moisture content and temperature and are closely related to each other. In the case of isothermal mass transfer, the moisture diffusion coefficient in the transverse directions from the Stanish and Pang models increased with decreasing moisture content. This contradicts the Avramidis and Perre models and numerous experimental results. Thermal diffusion effects on the drying process may not be predominant because the nonisothermal state is relatively short. Therefore, the Perre model, which does not consider the thermal diffusion effect, has been used successfully in the drying simulation. However, it may be erroneous in certain cases when the nonisothermal state prevails over the system, such as building physics. The Pang model cannot explain the phenomena of thermal diffusion and moisture thermodiffusion. It might be reasonable to modify the thermal diffusion of the Avramidis model, which is lower than that of the Stanish model. The apparent heat diffusivity was higher than the true heat diffusivity.