When the wood is heated in microwave electromagnetic field, the temperature of the whole section will be the same if the surface of the wood is not cooled. However, because of the heat loss and moisture evaporation on the surface of wood, the temperature inside the wood is actually higher than the surface temperature, thus forming a temperature gradient on the different thickness of the wood. Therefore, moisture movement in wood depends not only on moisture gradient, but also on temperature gradient during microwave drying. Especially when the internal temperature of wood is higher than the boiling point of water, super-atmospheric water vapor pressure is generated in the wood, which makes the water move from inside to outside more quickly.
Therefore, the rate of microwave drying is much higher than that of convective drying. Because wood itself is a medium, wood contains water, water dielectric constant is very large, is a very good material to absorb microwave. When the moisture content of wood is high, the temperature of microwave heating is also high. In addition, the dielectric constant of wood increases with the increase of temperature, so it can keep high efficiency in the whole process of heating and drying.
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