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应用网络分析仪和同轴探头技术,研究含水率为13%~22%的玉米在频率10~3 000 MHz,温度25~85℃下的介电性能。结果表明:玉米的介电常数和介电损耗因数随频率升高而降低,随温度和含水率的增加而增大。高温低频条件下,在双对数图中高含水率玉米的介电损耗因数与频率呈现明显的负线性关系,表明离子导电引起介电损耗是主要的加热机理。玉米射频加热时容易引起“热逸散”现象,因此均匀的电磁场分布至关重要。微波、射频在玉米中的能量穿透深度随水分、温度和频率的升高而降低。射频频段的电磁能比微波频段有更深的穿透深度,因此玉米射频加热适合于大容器厚层物料,而微波加热时可采用小容器薄层物料。
Using network analyzer and coaxial probe technology, the dielectric properties of maize with moisture content of 13% ~ 22% at frequency of 10 ~ 3 000 MHz and temperature of 25 ~ 85 ℃ were studied. The results showed that the dielectric constant and dielectric dissipation factor of maize decreased with increasing frequency, and increased with the increase of temperature and moisture content. Under the conditions of high temperature and low frequency, the dielectric loss factor and frequency of corn with high water cut showed a significant negative linear relationship in double logarithmic graph, indicating that the dielectric loss caused by ion conduction is the main heating mechanism. RF heating corn easily lead to “heat escape ” phenomenon, so uniform distribution of the electromagnetic field is essential. The penetration depth of microwave and radio frequency in corn decreases with increasing moisture, temperature and frequency. Radio frequency electromagnetic energy than the microwave band has a deeper penetration depth, so corn RF heating suitable for large container thick material, and microwave heating can be used when the small container of thin material.