基于孔道分形网络的木材水分迁移干缩效应机制

In order to truly describe the water transfer process in wood, it is necessary to consider the effect from pore structure and its changes during water migration. At present, most models using “Whitaker Volume Average Law” were studied without scale and structural effect. The features in project are: wood pore structure and its change rule of shrinkage deformation distribution is investigated. The constitutive relation between deformation distribution and pore structure is established under different moisture content. The characteristics of wood pore structure under different moisture content are analyzed by the pore fractal network. The corresponding relation between the parameters of pore fractal network and the characteristics of pore structure is studied and the geometrical model of wood pore structure is constructed. The diffusion mechanism is given to the geometrical model and the mathematical model of moisture transfer in the pore network is established and solved. The simulation results are compared and analyzed with traditional models and the actual drying results to explore the influence of internal pore structure on moisture migration and to establish its inner link and macro water changes. Three approaches from experiments, mechanism analysis and numerical simulation are used to reveal that the mechanism of shrinkage effect on water transfer in wood. The project will improve the basic theory of relationship between wood and water, which has a scientific guiding significance on technology optimization in wood drying and heat treatment, etc.

要真实描述木材内部水分迁移过程，必须考虑木材孔隙结构及其在水分迁移过程中变化的影响。目前大多数研究均采用Whitaker体积平均法则，忽略尺度和结构效应进行模型研究。本项目特色是：研究木材孔隙结构及其干缩形变分布的变化规律，建立不同含水率下形变分布与孔隙结构本构关系；运用孔道分形网络技术，对不同含水率下木材孔隙结构进行特征分析，研究其与孔道分形网络参数间的对应关系，构建木材孔隙结构几何模型；将扩散机制赋予几何模型，构建干燥过程中孔道网络内水分传递数学模型，并对其进行数值求解；将模拟结果与传统模型结果、实际干燥结果进行对比分析，探讨内部孔隙结构对水分迁移的影响规律，建立其与宏观水分变化的内在联系。通过上述试验、机理分析与数值模拟3种手段，揭示木材水分迁移的干缩效应机制。本项目将完善木材—水分关系基础理论，对木材干燥、热处理等工艺优化具有科学指导意义。

本项目采用实验与模型联合手段，主要围绕木材微观孔隙结构与宏观热、质、形变3者关系开展了以下研究工作：1）基于分形理论，采用压汞仪与XCT技术对人工林落叶松与柞木等树种孔隙结构进行联合精细化表征，在精确解析结构参数基础上，确定了孔隙结构参数与分形特征参数之间联系；2）将分形理论与蒙特卡洛算法相结合，提出一种快速且相对准确的木材等效导热系数模型，成功预测了木材横纹导热性能并揭示了导热系数与结构参数的本构关系；3）基于木材3维真实孔隙结构，将Navier-Stokes方程引入木材结构体的分形孔道网络内构建了介观尺度下木材水分迁移模型，采用控制容积与有限元法对其进行数值求解，成功预测了木材早、晚材及不同部位的渗流行为，并建立了孔隙结构及分形参数与渗透率的关系，解析了微观孔隙结构及其变化对宏观水分迁移的影响规律；4）采用数字图像相关技术监测了常规干燥过程中木材横纹干燥应变规律，结合木材干燥动力学（实验与模拟数据）与流变学理论成功预测了弦、径向干缩应变及其分布随水分的变化规律，最终解析了干缩与水分变化耦合关系的内在机制。本项目进一步完善了木材与热质迁移及形变关系的基础理论。项目执行期间，共发表学术论文8篇，其中SCI收录论文5篇；获得授权发明专利1件，实用新型专利3件；项目组成员1人晋升为副教授；培养了3名硕士研究生。