Three dimensional models of pore structure and airflow distribution inside grain bulks

由于对天气的依赖，谷物冷却或干燥过程总是有失败的风险。气流阻力不均匀导致同一仓内高气流位置的谷物过干，低气流位置的谷物湿（变质）。影响气流阻力分布的两个主要因素是籽粒间的孔隙结构（孔隙的大小、形状、连通性、弯曲度以及孔隙对气流方向的取向）和流体的性质（气流速度和粘度）。孔隙结构受籽粒大小、形状、堆积、细粒、异物和碎粒（DFFB）数量的影响。已有的数学模型主要采用有限元法（FEM）模拟气流阻力，并假设每个单元内部连续，而没有考虑DFFB对孔隙结构的影响。因此，所建立的模型没有考虑储粒体内部的三维孔隙结构分布以及孔隙内风速对气流阻力分布的影响。这导致预测精度较低。目前还没有建立颗粒体内部孔隙结构的模型。采用离散元法（DEM）可以建立孔隙结构模型。为了充分利用有限元和DEM两种方法的优点，提出了一种将有限元和DEM模型相结合的药物片剂计算方法。在接下来的5年里，我将着眼于目标，通过考虑DFFB对孔隙结构和气流阻力的影响，开发储粮体内部的三维孔隙结构DEM和气流阻力分布FEM模型来填补这一空白。这两个模型将耦合在一起。为了验证这些建立的模型，我建议测定不同含水率和DFFB下的颗粒松散和压实孔隙率，模拟不同装载高度下的DFFB在仓内的分布，并测量不同风速下仓内的气流阻力分布。所提出的实验将采用新颖的技术来表征孔隙结构和测量气流阻力分布。拟议计划的长期目标是推进散装固体系统的知识与应用储存粮食生态系统。因此，本文的研究结果不仅为散装固体储存系统的设计和运行提供了合理的分析工具，而且有助于了解颗粒体的孔隙结构和流动。建立的气流分布模型有助于设计工程师正确布置通风系统，帮助粮主制定正确的通风策略。该项目将培养1名硕士、2名博士和5名本科生。学生所获得的知识和技能将对他们在工业界或学术界工作有所帮助

The grain cooling or drying process is always at risk of failure because of its weather dependency. The non-uniform airflow resistance results in over-dried grain at high airflow locations and wet (spoiled) grain at low airflow locations inside the same bin. The two major factors influencing the airflow resistance distribution are the pore structure among grain kernels (size, shape, connectivity, tortuosity of the pores, and orientation of the pore to the airflow direction) and properties of fluids (air velocity and viscosity). The pore structure is influenced by kernel size, shape, and amount of dockage, fines, foreign materials and broken kernels (DFFB). Published mathematical models mainly used Finite Element Method (FEM) to simulate airflow resistance and assumed continuum inside each element without considering the effect of DFFB on pore structure. Therefore, these developed models did not consider 3D pore structure distribution inside stored grain bulks and the effect of air velocity inside pores on airflow resistance distribution. This results in low prediction accuracy. No model of pore structure inside grain bulks has been developed. A pore structure model can be developed by using Discrete Element Method (DEM). To take advantage of both the FEM and DEM methods, a method to combine FEM and DEM model has been developed for pharmaceutical tableting. In the next 5 years, I will focus on objectives to fill this gap by developing 3D pore structure DEM and airflow resistance distribution FEM models inside stored grain bulks by considering the effect of DFFB on pore structure and airflow resistance. These two models will be coupled together. To validate these developed models, I propose to determine loose and compacted porosity of grains at different moisture contents and DFFB, to model the distribution of DFFB inside bins with different loading heights, and measure airflow resistance distribution inside grain bins under different air velocities. The proposed experiments will use novel technique to characterize pore structures and measure air flow resistance distribution.The long term objective of the proposed program is to advance the knowledge of bulk solid systems with applications to stored grain ecosystems. Therefore, the results from this proposed research will not only provide a rational analysis tool for the design and operation of bulk solid storage systems, but also contribute to the fundamental knowledge of pore structure and flow through grain bulks. The developed airflow distribution model will help design engineers to lay out the aeration system correctly and grain owners to conduct a right ventilation strategy. One MSc, two PhD and five undergraduate students will be trained through this proposed program. The knowledge and skills acquired by students will be beneficial to them in preparing them to work in industry or academia.***