Research Of Heating Mechanism For Larix During Radio-frequency Vacuum Drying

Conduction, convection and radiation can bring heat influxes in conventional kiln drying, because of this manner of heating, drying rate is slow; it is faster that removing water from surface than inner layer, so water is unevenly distributed. As a new method of drying radio-frequency vacuum drying has advantage that is beyond kiln drying. Frequency drying produce heating from interior of wood, it is really just like produce heat by wood-self. Temperature gradient is from inner toward outer, and so dose pressure gradient, because water change into vapor and pressure gradient in wood is bigger than outside. These two gradients are both beneficial to wood drying, which can overcome the shortcoming of conventional drying. The main content of the study including:1. Learn about elementary theory of dielectric heating, including polarization mechanism of wood in high-frequency electric field. As an electrolyte wood in high-frequency electric field exist four polarizations:electron polarization, ionic polarization, dipole polarization, interface polarization. Electron polarization can be rapidly established, polarization of electron in inner layer come out within range of X-Ray; polarization of electron in outer layer come out within range of ultraviolet ray; normally, dipole polarization of solid come out within range of audio and radio frequency; interface polarization come out in inhomogeneous medium, it take longer time than dipole polarization, however, stiil in audio and radio frequency.2. Learn about dielectric parameters and the relationship among them, we measured and calculated dielectric constant and loss tangent. Dielectric constant is influenced by moisture content, wood grain and frequency of electric field. The study show that:Moisture content in wood is the most important factor, the relationship between MC and dielectric constant can be described as exponential relationship from MC=8%to the fiber saturation point (FSP), above FSP it is linear relationship; The dielectric property showed decreasing trends when frequency raised. In high moisture content it is distinct, in low moisture content it is not distinct. Permittivity across longitudinal direction is bigger than that across transverse direction, but the difference across tangential direction and that across radial direction is small.Base on these dielectric parameters we calculated the absorbed power of wood in high frequency dielectric field, the power can be used improve temperature and make water change into vapor, so we can obtain conclusion about drying rate and drying quality by using this parameter. The result showed that:more moisture content which wood contained more power it can be absorbed; with increasing of frequency, the friction of polarized molecules increased, the electric energy of consumption increased, drying rated also increased; influence of temperature is varied, when MC is higher, polarized molecules is mainly free water, influence of low temperature is not obvious, on the contrary, high temperature make polarized molecules move intense, polarization can not be established easily, absorbed power decrease. Absorbed power increased along with temperature when moisture content is low.3. After study on dielectric characteristics and absorbed power, we discussed temperature distribution of wood in high frequency dielectric field. Besides the influence of dielectric properties, vacuum degree is another factor. The result suggested that:higher moisture content is, higher increasing rate of temperature is; effect on temperature of vacuum degree is not obvious; thickness of wood increase need more energy to improve temperature, so speed is slow whether drop or increase temperature; temperature on upper surface increase the most rapidly, inner layer is more and lower surface is slow, but inner layer is the slowest when temperature dropped.It can be control the act ON and turn off OFF in the radio frequency. Temperature increased with heating time. Temperature affected by Moisture content. Based on dielectric heating, the more MC wood contain the higher temperature would be. But more water need absorb more energy change into water, so temperature depend on both of them. In this study, temperature which MC=20%in wood is lower than temperature when MC=20%.