Numerical and experimental study of heat and mass transfer in microwave drying of hardwood.

摘要

The microwave drying of hardwood was studied using both numerical models and experimental tests. Red maple (Acer rubrum L.) and white oak (Quercus alba) species were selected due to the large differences in their permeability characteristics. Two different drying procedures, fast and slow microwave drying (according to both the drying time and temperature achieved in the wood) were included in this thesis study.;A 2-D comprehensive heat and mass transfer model was developed to simulate the free liquid, vapor, and bound water movement, heat transfer, and vapor pressure in fast microwave drying. The results showed that, for white oak, moisture movement was easily impeded and high internal vapor pressure occurred; for red maple, water and vapor could move smoothly, and low internal vapor pressure occurred in the samples. The internal vapor pressure was affected by sample size (length and thickness). In fast microwave drying wood moisture transport was mainly in a vapor state; in addition free water played an important role, especially in permeable species.;In the study of slow microwave drying a non-isothermal moisture diffusion and heat transfer model was used. The results showed that the moisture gradient controlled the mass transfer; and the temperature gradient in the wood increased the moisture transfer. The effect of temperature gradient on moisture transfer became less meaningful as drying time progressed.