【作者】 刘显茜；

【导师】 陈君若；

【作者基本信息】 昆明理工大学 ， 机械设计及理论， 2010， 博士

【摘要】 热风对流干燥生物多孔材料,伴随着水分的扩散蒸发,常常会出现体积减小,组织收缩现象。生物多孔材料体积减小、组织收缩致使材料表面与热空气之间的对流传质系数和物料内部的水分有效扩散系数减小,影响了生物多孔材料干燥水分进一步扩散和蒸发。本文针对生物多孔材料热风干燥非稳态收缩及其对传热传质的影响进行了系统研究,主要研究内容如下：首先,基于连续介质理论,建立了生物多孔材料干燥组织非稳态收缩递推模型,将物料干燥过程中整体体积收缩随其平均干基含水率变化的一维线性关系应用于物料各离散单元,研究了生物多孔材料干燥过程中的组织非稳态收缩问题。结果表明,物料颗粒在干燥过程中随着水分的对外扩散、介质干基含水率降低,物料组织收缩并不是由外到内同步收缩,而是先失去水分的外部先收缩,失水多的外部收缩幅度大于失水少的内部,没有失水的部位不收缩。并从数学证明和数值计算两方面验证了生物多孔材料干燥组织非稳态收缩递推模型的正确性。然后,基于生物多孔材料干燥过程中组织非稳态收缩,提出了生物多孔材料局部水分有效扩散系数与局部水分扩散面收缩变化的组织收缩-水分有效扩散系数模型,对生物多孔材料干燥过程中局部水分有效扩散系数的变化规律进行了研究。结果表明,干燥过程中,生物多孔材料内部局部水分扩散面收缩,局部水分有效扩散系数减小；局部水分扩散面不变,局部水分有效扩散系数不变；组织孔隙分布均匀的材料,各部分水分有效扩散系数相等。最后,基于生物多孔材料干燥过程中体积收缩和对流传质面减小现象,研究了物料特征尺寸、形状以及热空气温度、相对湿度、风速等对干燥特性的影响。结果表明,整个物料干燥过程应划分为外部干燥条件控制的第一干燥阶段和物料内部水分扩散控制的第二干燥阶段。而经典干燥理论干燥过程恒速干燥阶段和降速干燥阶段的划分只是特定形状和材质的物料在一定干燥条件下才出现的干燥现象。物料外层单元水分活度能够判别物料干燥过程是处于第一干燥阶段还是第二干燥阶段,是表征物料干燥特性的唯一特征参数。而物料临界平均干基含水率不能作为物料干燥过程是处于第一干燥阶段还是第二干燥阶段的判据,不是表征物料干燥特性的特征参数。更多还原

【Abstract】 Volume decreasing and tissue shrinking of bio-porous materials often occur accompanied with the water removal during the convective drying under the hot air. The convective mass transfer coefficient between the surface of the material and the hot air and the water diffusion coefficient will decrease with the decrease of the volume and tissue shrinkage. And these will affect water further diffusing from the internal to the external and evaporating from the surface of the material to the hot air. The transient tissue shrinkage of bio-porous material under convective drying and its effect on the heat and mass transfer were explored in this dissertation.Firstly, a transient shrinkage recursion model was proposed for bio-porous material on the base of the continuous media theory and the relationship between the shrinking volume and average moisture content of the particles. The validity of the proposed model was also tested by mathematical reasoning and numerical calculation. The problem of the transient tissue shrinkage was studied based on this model. As suggested in the result, the external of the bio-porous material firstly appeared contraction due to the water loss, rather than synchronous contraction from the outside to the inside with the wet sub-external migration and the humidity’s decrease during the drying process. There was no shrinkage of the parts without loss water. Furthermore, the external shrinked more greatly than the internal due to more water loss in the external.Secondly, a model of tissue shrinkage-water effective diffusion coefficient was also proposed for the relation between the local water effective diffusion coefficient and the change of the local diffusion area based on the transient tissue shrinkage of the bio-porous material. The changing mechanism of the local water effective diffusion coefficient of the bio-porous material was investigated based on the model proposed. The results showed that the local water effective diffusion coefficient decreased with the shrinkage of the area of the water diffusion. And the local water effective diffusion coefficient didn’t change if the area of the local water diffusion remained constant. Furthermore, the local water effective diffusion coefficients were equal due to the uniform of the porosity in the bio-porous media.Finally, the effects of the characteristic dimension and the shape of the bio-porous material, and the effects of the temperature, the relative humidity and the velocity of the hot air on the characteristics of the drying were explored based on the shrinkage of the volume and the decrease of the convective mass transfer area between the material and the hot air. The results showed that the whole drying process should be divided into the first drying process controlled by the external drying condition and the second drying process controlled by the internal water diffusion of the material, rather than the constant drying rate process and the falling drying rate process described in the classical drying theory. Furthermore, the characteristic parameter judging whether the drying process is in the first drying phase or in the second drying phase was the external element’s water activity rather than the critical average moisture content.更多还原