基于玻璃化转变理论的稻谷籽粒干燥应力数值模拟及爆腰声发射法研究

Fissuring reduces rice quality and its economic value. To prevent fissuring, the low-temperature drying processes of paddy rice are often applied and this technology causes a low productivity and high energy consumption of the dryer, which hiders the wide application of artificial drying in China. This project aims at developing a high temperature drying technology for paddy that yields higher quality, improved productivity and lower energy consumption relative to current low-temperature drying processes. To realize this aim, a series of fundamental studies will be conducted on the generation mechanisms, prediction, dynamic and nondestructive testing, and fine control of fissuring by considering the glass transition process of paddy rice during process. We establish the rice drying stress model based on the glass transition theory and improve the model validation by applying the body-fitted coordinates mesh and dynamic mesh technologies. By carrying out numerical simulation of the hot air drying process, the spatial and temporal distribution of internal stresses in rice kernel during the drying process will be obtained; thus where and when fissures will form can be determined according to the internal work-fracture energy assessment criterion. The model will allow prediction of fissuring during the drying process, and identifying the micromechanical damage mechanisms leading to breakage of the rice kernels. We will propose a new dynamic, nondestructive testing method to quantitatively determine the rice fissures using the acoustic emission method. By studying the acoustic emission patterns and characteristics of rice fissuring during the drying process , it will be possible to develop an early warning system for fine control of the drying process. By combining the new stress model with the acoustic emission method, We will study the mechanism and function of the high-tempering in preventing rice fissuring, which will permit better design and optimization of the high temperature fast drying technology for paddy rice. Finally, small scale experiments of the proposed high-temperature drying method for paddy rice will be conducted to validate the model and test the control scheme. The actual drying effects will be also evaluated.

稻谷爆腰降低稻米品质和经济价值。为了抑制爆腰，当前稻谷往往采用低温干燥，导致干燥机生产效率低、能耗高，限制了我国稻谷机械化干燥的推广。课题拟发展一种稻谷高温快速干燥新工艺，以实现稻谷高效、保质干燥。为此，课题从玻璃化转变角度，研究新工艺所涉及的籽粒爆腰产生机理、模拟预测、动态无损检测和精细控制。建立基于玻璃化转变理论的干燥应力模型，采用适体坐标网格和动网格技术提高模拟精度；模拟热风干燥过程中籽粒内部应力的时空分布，并根据内功-断裂能评定标准预测爆腰形成时间及位置，阐明爆腰产生的细观损伤机制。提出了一种基于声发射的籽粒爆腰的动态无损检测方法；通过研究稻谷干燥裂纹的声发射模式和特征，探索基于声发射的稻谷爆腰预警机制及干燥过程精细控制方法。结合数值模拟和声发射法，研究高温缓苏抑制稻谷爆腰机理和条件，以突破稻谷低温干燥限制，设计和优化稻谷高温快速工艺；进行稻谷高温干燥小试实验，评价实际干燥效果。

针对当前我国稻谷低温干燥工艺生产效率低，能耗高缺点，课题拟在玻璃化转变理论基础上，发展一种稻谷高温快速干燥新工艺，以实现稻谷高效、保质干燥。为此，首先建立基于玻璃化转变理论的稻谷籽粒干燥应力模型。采用图像法建立稻谷籽粒三维适体网格，进行热湿传递系数、弹性模量和破裂载荷等模型参数的实验测定，以提高模型精度；应力模型包含玻璃化转变和干燥应力子模型，不仅可以模拟籽粒内部热湿传递过程，还可模拟籽粒内部玻璃化转变以及应力累积过程，为全面，细观分析稻谷干燥应力裂纹（爆腰）提供数据支持。再者，我们提出了一种基于声发射的籽粒爆腰的动态无损检测方法。建立稻谷干燥和机械压缩试验声发射检测装置，测量并分析了稻谷干燥、应力和声发射信号之间关联。结果表明：谷粒在干燥和受载过程产生的声发射信号能够被有效检测；干燥过程中声发射信号时序特征与稻谷干燥爆腰规律关联密切。. 结合数值模拟和声发射法， 研究不同干燥工艺条件下稻谷玻璃化转变过程，爆腰规律和机理。发现从玻璃化转变角度，将稻谷干燥工艺分为：低温干燥（干燥温度<35 度）,稻谷在干燥全过程处于玻璃态，不易爆腰。中温干燥（35 -45 度），稻谷在干燥过程发生橡胶态向玻璃态转变，易爆腰。高温干燥（>45 度）, 稻谷在干燥阶段处于橡胶态，不易爆腰。稻谷爆腰存在一个临界含水率17%；当水分低于17%，中温干燥条件下，稻谷爆腰发生在干燥和冷却段；而高温干燥条件下，爆腰主要发生在冷却段，并集中在冷却阶段7小时内。高温缓苏可以减缓稻谷籽粒橡胶态向玻璃态转变速率，从而抑制稻谷爆腰，实验条件下实现了50度恒温干燥-60度缓苏工艺下稻谷干燥爆腰率增值为1.1%，低于国家标准的3%。因此，我们从理论和实验证实了稻谷高温干燥工艺可行性。.