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首页> 外文期刊>Acta Horticulturae >Calculation of biot numbers in food drying.
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Calculation of biot numbers in food drying.

机译：食品干燥中的比奥数计算。

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摘要

This paper describes the calculation of mass and heat transfer Biot number for food drying processes, dimensionless numbers relating internal to external resistances to either mass or heat fluxes. Four case studies are covered: wheat drying kinetics, solving the unsteady state diffusion equation by implicit finite differences for this low-moisture product, and three types of fruit pectic gels a high moisture product: sucrose-added, apple pectic gel, tomato pectic gel and low-calorie, apple pectic gel (LCAPG). In wheat drying, the mass transfer Biot number varied from low to high values during the process if the grain started drying at high moistures, but only showed high values if starting drying at practical, lower moisture contents. For the first two gels, an analytical solution considering internal-external control was fitted to the drying curves in order to optimize mass transfer Biot numbers and diffusion coefficient, for an average thickness calculated by a shrinkage relationship. Mass transfer Biot values ranged from 1 for tomato pectic gel, to 2 for sucrose-added, apple pectic gel, showing that, on average, both internal and external resistances are important and that internal control could not be assumed from the beginning of drying. The LCAPG was solved with the most complex model: finite differences for the unsteady state diffusion equation considering shrinkage, and simultaneous solution of the macroscopic energy balance to predict the temperature curve. Mass transfer Biot numbers varied from almost zero at the start of drying to some 2000 at the end, while the heat transfer Biot number only varied from 0.25 to 0.5. Prediction of heat and mass transport in solids are quite different phenomena, so the heat transfer results should not be translated to predict the mass transfer behavior.
机译：本文描述了食品干燥过程中传质和传热比奥数的计算，无量纲数与内部对质量或热通量的外部阻力有关。涵盖了四个案例研究：小麦干燥动力学，通过这种低水分产品的隐式有限差分求解非稳态扩散方程，以及三种类型的果胶为高水分产品：添加蔗糖的苹果果胶，番茄果胶和低热量的苹果果胶（LCAPG）。在小麦干燥中，如果谷物在高水分条件下开始干燥，传质的毕奥特值在过程中会从低值变化到高值，但只有在实际的较低水分含量下开始干燥时，才显示高值。对于前两种凝胶，将考虑内部-外部控制的分析溶液拟合至干燥曲线，以优化传质比奥特数和扩散系数，以获得通过收缩关系计算的平均厚度。传质Biot值的范围从番茄果胶到1，到蔗糖加苹果果胶到2，这表明平均而言，内部和外部阻力都很重要，并且从干燥开始就不能假定内部控制。 LCAPG用最复杂的模型求解：考虑收缩的非稳态扩散方程的有限差分，以及宏观能量平衡的同时求解以预测温度曲线。传质毕奥特数从干燥开始时的几乎为零到干燥结束时的大约2000，而传热毕奥特数仅在0.25至0.5之间变化。固体中传热和传质的预测是完全不同的现象，因此不应转换传热结果来预测传质行为。

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