Based on the mechanism of moisture and heat transfer in wood during microwave-vacuum drying (MVD), a one-dimensional mathematical model to describe the process of wood MVD was established and verified by experiments in this research. The results showed that the process of MVD of wood experienced three distinct periods: (1) accelerating rate with rapid warming-up drying period, (2) a constant temperature and constant rate drying period, and (3) a heating-up with falling rate drying period. Compared with conventional hot air drying, the total drying process is almost governed by a constant rate period in vacuum-microwave drying of wood. The predicted temperature and moisture content in wood match well with the experimental data, the square of the relevant coefficient of the values of simulation and test is above 0.9, and the simulation precision of the change rule of the moisture is higher than that of the temperature.
INTRODUCTIONDrying is the most energy-intensive and time-consuming component of the lumber manufacturing process. Currently, over 80% of the produced lumber is dried in conventional and vent kilns, where hot and dry air is forced-circulated through banks of the stacked lumber. Although this operation is simple and inexpensive, the product quality is often low and the drying time is very long, especially for hardwoods.The application of volumetric heating can help to decrease gradients of temperature and moisture during drying and increase the rate of heat transfer in wood, because the energy is absorbed throughout the volume, and the heat is generated directly within the material being dried. Compared with radio frequency (RF), the wavelength of the microwave (MW) is shorter and its field is more uniform
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