多孔材料非稳态物性参数及干燥特性研究

发布时间：2018-08-19 13:17

【摘要】：多孔材料在干燥过程中由于温度梯度和湿度梯度的驱动,导致物料内部水分子以汽化和迁移的方式扩散到表面,再以对流的方式将水分蒸发到热空气中,在此过程中随着内部水分的减少,从而导致物料体积、各相百分比、孔隙率和密度发生变化,表现出了极大的非稳态特性；而上述变化又对物料下一步的干燥产生影响。因此,多孔材料的干燥过程涉及传热传质和力学特性等问题。在多孔材料干燥过程中传热传质和力学特性等问题的研究中,物性参数如水分有效扩散系数、有效导热系数的求解是问题的关键。因此对其变化规律的研究,对掌握物料的干燥特性,保证其干燥品质,制定干燥工艺以及为相关干燥设备的设计提供理论依据。本文针对多孔材料干燥过程中表现出来的非稳态特性,对球状玉米籽粒、块状胡萝卜颗粒和柱状苹果颗粒的水分有效扩散系数和有效导热系数的非稳态变化规律及非稳态干燥特性进行了系统研究,主要研究内容有以下几个方面：(1)基于Fick第二扩散定律建立了多孔材料热风干燥过程中非稳态水分有效扩散系数数学模型和非稳态收缩模型,对不同形状(球状、圆柱状、片状)、不同物料热风干燥过程中内部各单元层非稳态水分有效扩散系数变化规律及组织收缩特性进行了研究。研究结果表明：在热风干燥初期由于物料内部水分多以自由水的形式存在,蒸发扩散较容易,水分有效扩散系数几乎不发生变化；随着干燥过程的进行,物料内部自由水分变少,结合水分所占比例增大,水分蒸发扩散难度增加,水分有效扩散系数随着水分比的变小而变小；最后物料内部水分达到平衡干基含水率后不再发生变化,阻碍水分扩散的阻力和水分扩散驱动力达到平衡,水分有效扩散系数不再发生变化：在整个干燥过程中水分有效扩散系数是一个逐渐减小的非稳态变化过程,而不是一个稳态定值。先失水的外单元层先收缩,内单元层滞后收缩,而不是同时收缩,外单元层收缩量大于内单元层。(2)基于Fourier热平衡定律建立了多孔材料热风干燥过程中非稳态有效导热系数数学模型,对不同形状(球状、圆柱状、片状)、不同物料热风干燥过程中内部各单元层有效导热系数非稳态变化规律及传热特性进行了研究。研究结果表明：在整个干燥过程中有效导热系数是一个逐渐减小的非稳态变化过程,而不是一个稳态定值,这是因为物料随着水分向外部扩散蒸发,水分减小所留下的孔隙一部分转化为收缩体积,一部分转化为内部孔隙,因此在干燥过程中水分所占体积百分比逐渐减小,孔隙所占百分比逐渐增大,而水的导热系数大于空气的导热系数。干燥初期物料内部干燥速率较大,水分迅速减少,物料有效导热系数迅速减小,当干燥速率进入减速干燥阶段后,由于干燥速率减小进而水分扩散较慢,物料内部导热系数变化曲线趋于平缓。(3)基于上述多孔材料非稳态传热和传质特性,将其视为粘弹性体建立了球状物料内部各单元层的质热传递耦合作用下的非稳态粘弹性应力模型,结合球状几何形状的特性,在径向应力的基础上推导出了切向应力分布和非稳态变化规律,并将粘弹性应力模型分离为温度梯度引起的热应力部分和湿度梯度引起的湿应力部分。研究结果表明：在整个干燥过程中球状物料的切向应力和径向应力出现两个应力峰值,且第一次峰值大于第二次。径向应力在干燥初期各单元层受到的力为压应力,在应力反向后变为拉应力,在整个干燥过程中物料内部呈现拉应力和压应力两种应力类型,而不是只呈现一种应力类型；切向应力干燥初期是外拉内压,在应力反向后变为内拉外压；热应力和湿应力随时间的曲线变化趋势和总应力变化趋势一致,在同一时刻湿应力值大于热应力值,表明在干燥过程中湿度梯度对干燥应力的影响大于温度梯度。(4)最后结合多孔材料非稳态物性参数模型、非稳态干燥特性模型,研究了物料收缩、扩散、热量传递和力学特性之间的耦合关系,研究结果表明：干燥工艺对物料干燥特性的影响是通过改变干燥过程中物性参数来实现的,水分有效扩散系数和有效导热系数随热风温度、速率的增大而增大,干燥速率和干燥应力随水分有效扩散系数增大而增大,热量传递速率随有效导热系数增大而增大；考虑收缩情况下的物性参数小于不考虑收缩的物性参数,这是由于考虑收缩情况下物料失去水分的体积一部分转化为物料内部孔隙,一部分转化为物料收缩的体积,而不考虑收缩情况下物料失去水分的体积全部转化为孔隙,而物料内部孔隙越大水分扩散和热量传递所受到的阻力越小。
[Abstract]:Driven by temperature gradient and humidity gradient, porous materials diffuse water molecules to the surface by vaporization and migration, and then evaporate water into hot air by convection. In this process, with the decrease of internal moisture, the volume of materials, the percentage of each phase, porosity and density are caused. Therefore, the drying process of porous materials involves heat and mass transfer and mechanical properties. In the study of the heat and mass transfer and mechanical properties in the drying process of porous materials, physical parameters such as effective diffusion of water are studied. The key to the problem is to find out the coefficient and the effective thermal conductivity.Therefore, the research on the variation of the coefficient can provide theoretical basis for grasping the drying characteristics of materials, ensuring their drying quality, formulating drying technology and designing relevant drying equipment. The unsteady-state variation and drying characteristics of water effective diffusion coefficient and effective thermal conductivity of grain, block carrot particle and columnar apple particle were systematically studied. The main research contents are as follows: (1) Based on Fick's second diffusion law, the unsteady-state water content in hot air drying process of porous materials was established. The mathematical model of effective diffusivity and unsteady shrinkage model were used to study the variation law of unsteady water effective diffusivity and tissue shrinkage characteristics of different shapes (spherical, cylindrical, flaky) and different materials during hot air drying. It is easy to evaporate and diffuse in the form of free water, and the effective diffusivity of water hardly changes; with the drying process, the free water in the material becomes less, the proportion of combined water increases, the difficulty of water evaporation and diffusivity increases, and the effective diffusivity of water decreases with the decrease of water ratio; finally, the effective diffusivity of water in the material becomes smaller. The effective diffusion coefficient of water does not change any more. The effective diffusion coefficient of water does not change any more. During the whole drying process, the effective diffusion coefficient of water is a gradually decreasing unsteady state change process, not a steady state constant value. (2) Based on Fourier heat balance law, a mathematical model of unsteady effective thermal conductivity of porous materials during hot air drying was established, and the effective thermal conductivity of different shapes (spherical, cylindrical, flaky) and different materials during hot air drying were calculated. The results show that the effective thermal conductivity in the whole drying process is a gradually decreasing unsteady state change process, not a steady state constant value, because the material evaporates with the diffusion of water to the outside, and the water decreases. In the initial stage of drying, the drying rate of the material is higher, the moisture content decreases rapidly, and the effective thermal conductivity of the material is higher than that of the air. When the drying rate enters the stage of deceleration drying, the change curve of thermal conductivity becomes gentle because of the decrease of drying rate and the slow diffusion of moisture. (3) Based on the unsteady heat and mass transfer characteristics of the porous materials, the coupling of mass and heat transfer among the cell layers in the spherical materials is established as a viscoelastic body. The unsteady viscoelastic stress model under the action of temperature gradient and humidity gradient are separated from the viscoelastic stress model. The results show that the viscoelastic stress model is composed of the thermal stress part caused by temperature gradient and the wet stress part caused by humidity gradient. The tangential stress and radial stress of the spherical material appeared two peaks during the drying process, and the first peak value was greater than the second one. In the initial stage of shear stress drying, the tensile and internal pressures change into the tensile and external pressures after the stress reverses; the curves of thermal stress and wet stress change with time are consistent with the total stress, and the wet stress value is greater than the thermal stress value at the same time, indicating that the humidity gradient is related to the drying stress during the drying process. (4) Finally, the coupling relationship between material shrinkage, diffusion, heat transfer and mechanical properties was studied by combining the unsteady physical parameters model of porous materials and the unsteady drying characteristics model. The results show that the influence of drying process on material drying characteristics is through changing physical parameters during drying process. The effective diffusion coefficient and effective thermal conductivity of water increase with the increase of temperature and rate of hot air, drying rate and stress increase with the increase of effective diffusion coefficient of water, and heat transfer rate increases with the increase of effective thermal conductivity. It is due to the fact that the volume of the material losing moisture is partly converted into the internal pores of the material and partly into the volume of the material contracting under the consideration of shrinkage, while the volume of the material losing moisture is completely converted into the pores without considering shrinkage, and the greater the internal pores of the material, the smaller the resistance of water diffusion and heat transfer.
【学位授予单位】：昆明理工大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB383.4

【相似文献】