【作者】 黄小丽；

【导师】 刘相东；

【作者基本信息】 中国农业大学 ， 农产品加工及贮藏工程， 2014， 博士

【摘要】 针对高水分早稻干燥时间长、能耗大的问题,开展高水分早稻过热蒸汽干燥的生物材料力学及干燥特性探索研究,为过热蒸汽应用于干燥高水分谷物奠定理论研究基础。本文主要研究内容及结论如下：1.在自行搭建的过热蒸汽循环干燥实验台进行高水分稻谷过热蒸汽薄层干燥实验,获得稻谷经验水分有效扩散系数Deff,数量级为10-9m2/s。2.对干燥实验所得样品进行接触应力实验,获得稻米力-变形曲线和弹性模量。当湿基含水率大于0.235kg/kg时,稻米受力随变形线性增加,呈现出橡胶态特性,可承受较大变形；当湿基含水率在0.16～0.235kg/kg之间,稻米力-变形曲线呈现弹塑性变形特性；当湿基含水率低于0.16kg/kg时,稻米明显变脆,呈现出玻璃态特性,只能发生小变形；干基含水率在0.25kg/kg以上时,稻米弹性模量较小,低于20MPa,而此含水率以下,弹性模量剧增至100MPa以上；不同温度的过热蒸汽干燥所得稻米弹性模量无显著差异,分段建立稻米弹性模量与含水率的关系式。3.将稻谷看作是由稻壳和糙米构成的三轴不等的弹性椭球体,基于热质传递原理和弹性力学理论,构建三维传热、传质、应力数学模型,利用COMSOL Multiphysics软件求解方程组,得到过热蒸汽干燥过程中稻米内部温度、水分和应力分布及变化情况。4.以常压下150℃过热蒸汽干燥高水分稻谷模拟结果为例分析,结果表明,模拟结果与实际总体相符。过热蒸汽干燥2分钟左右稻米内部温度基本均匀分布,并到达干燥介质温度；稻米内部水分分布不均匀,水分梯度大小排序：厚度方向>宽度方向>长度方向,但水分梯度值在三方向上变化趋势相同,随着水分下降先增大后减小,由表及里水分梯度依次减小；干燥中,稻米外层受到拉伸应力,内层受到压缩应力,应力随着干燥进行逐渐下降,应力(第一主应力)最大值数量级为106Pa,其中芯部受到的压缩应力值较大,表层受到的拉应力值较小。整个干燥过程中,稻米内部存在两个应力集中区：厚度方向的表层区和中心区。5与热风干燥(50℃,相对湿度22%,风速0.5m/s)高水分稻谷爆腰增率33%相比较,140～180℃过热蒸汽干燥,稻米腰增率明显下降,其值不超过12%。过热蒸汽干燥作为饲料用粮的高水分早稻具有推广应用价值。更多还原

【Abstract】 For the problem of long drying time and high energy consumption for early indica rice with high moisture, studies on the mechanical characteristics of biological material and drying dynamic of rice kernel during superheated steam (SHS) drying could provide a basis for study the application of SHS drying technique to high moisture paddy dehydration. The main research contents and conclusions of this paper are as follows:1. A series thin layer drying experiments on SHS drying paddy with high moisture were conducted on a self built SHS dryer. And the drying characteristics was analyzed and the experimental coefiBcient of moisture transfer in paddy Deff during SHS drying was obtained from experiments, with the magnitude10-9m2s.2. The force-deformation curves and elastic modulus of paddy subjected to SHS drying were gotten through mechanical experiments. Results showed that, when moisture content is beyond0.235kg/kg wet basis (w.b.), the force rice kernel subjected linear increases with the increase of the deformation, and rice can withstand large deformation; when the moisture content ranges from0.16to0.235kg/kg w.b., the force-deformation curve shows the characteristics as that of elastic-plastic material; when the moisture content is lower than0.16kg/kg w.b., rice kernel becomes brittle and just withstands small deformation; the elastic modulus of rice is below20MPa when the moisture content is higher than0.25kg/kg dry basis (d.b.), however, it dramatically increases and beyond100MPa while the moisture content is lower than0.25kg/kg d.b. There is no significant difference among elastic modulus of rice kernels dried with SHS with temperature range from135to180℃. At last, the relation between elastic modulus and moisture content of rice kernel was determined.3. Paddy was viewed as an elastic ellipsoid with two layers:hull and brown rice. Base on the principle of heat and mass transfer and the theory of elasticity, three-dimensional mathematical models of heat and mass transfer and stress were built.. Soft ware COMSOL Multiphysics was employed to solve the mathematical equations.4. Take the simulation results from drying condition at150℃at1atm for example. Simulation results were generally consistent with experiment results. Simulated results showed that, temperature in rice kernel becomes uniform and reaches to steam temperature in about two minutes, so thermal stress can be neglected; moisture in rice kernel is not uniform that induces moisture content gradient (MCG); the max of MCG appears in the short axis in direction of the thickness of rice kernel, followed by that in the width, and the minimum appears in the long axis in direction of the length; MCGs in rice kernel decreases from surface to center, and they firstly increase and then decrease during SHS drying; outer layer of rice kernel is subjected tensile stress while the inner is subjected compressive stress, and the tensile stress (the first principle stress) decreases with the decrease of moisture content while the compressive stress firstly increases and then reduces as the moisture content decreases during SHS drying; magnitude of stress is106Pa, and maximum compressive stress is higher than maximal tensile stress in rice kernel during SHS drying; there are two stress concentration zones in rice kernel:the tensile stress concentration zone in surface in direction of thickness and the compressive stress concentration zone in center.5. Compared to the additional breakage ratio33%of paddy dried with continuous hot air drying(temperature50℃, relative humidity22%, velocity0.5m/s), the additional breakage ratio of paddy subjected to SHS drying with the temperature of140～180℃at latm is reduced sharply, less than12%. Base on this study, SHS drying paddy with high moisture, which is as feed grain, has an application value.更多还原