Three- dimensional temperature distribution of the packaged foods with different shapes during microwave heating
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摘要: 为了考察矩形、六边柱形、圆柱形三种几何形状样品在微波加热过程中的温度分布情况,以含水量为99%的凝胶琼脂为研究对象,依据Maxwell方程和传热方程,用有限元方法建立了三维温度分布模型。并通过考察各几何形状包装食品的表面、切面和内部三维等温面等情况,确定了各部位温度差异情况。结果表明:相同体积、不同几何形状的样品,由于各表面的形状不一样,进入样品的微波能量在内部形成不同的复杂聚焦区,温度分布有明显区别。微波加热60s后,矩形、六边柱形和圆柱形样品内部各出现4个、7个和1个热点区域,高低温差分别为23.87、21.92、25.10℃。实验值和计算值呈现良好的一致性。温度分布特性与样品的介电特性有关,对于含水量为99%的凝胶琼脂,在10~45℃范围内,频率为2450MHz的微波穿透深度在10~25mm之间。三维模型可较为清晰地描述不同几何形状包装食品的内、外部温度分度,为改善微波食品加热过程中的温度不均匀和优化包装食品质量提供基础数据和理论基础。Abstract: Temperature distribution characteristics of samples ( gelatin- agar with a moisture content of 99%) were studied with different shapes, such as rectangular, six- side cylindrical and the cylindrical, during microwave heating by experiments and simulations. Based on Maxwell's equations and heat transfer equations, models of three- dimensional temperature distribution were established using finite element method ( FEM) . The isothermal surface of the exterior surface, the intersecting surface and the interior surface of packaged foods with different shapes were investigated, and the correlated temperature differences among three surfaces were observed.Samples with the same volume, but varied in shapes, had distinct differences in the distribution of temperature, because the penetrated microwave energy samples formed various complex focusing areas. Upon microwave heating for 60 s, 4, 7 and 1 hot spots appeared in the rectangular, the six- side cylindrical and the cylindrical sample respectively, and the relevant temperature difference between the maximum and the minimum were 23.87, 21.92 and 25.10℃.The temperature distribution was concerned with the dielectric characteristics of certain samples.As to the sample, penetration depths of the microwave at 2450 MHz ranged from 10 to 25 mm, when the correlated temperature varied from 10 to 45℃. Based on three- dimensional models, the temperature distribution inside and outside of the packaged foods with different shapes could be described distinctly.
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