孔隙尺度下高含湿多孔介质干燥收缩变形的传热传质与应力应变机理及模型

The shrinkage and deformation of drying process not only lead to the collapse of pores in the material, the blockage of mass transfer path and the increase of energy consumption, but also make a serious deterioration in the quality of dried products. In view of the fact that the traditional drying shrinkage model can not fully consider the influence of the capillary force and microporous structure characteristics, the pore network theory and molecular dynamics method are applied to analyze the drying process of high moisture porous media, and a pore network model for drying of bi-directional fully coupled "thermal-wet-capillary- liquid-film-force" at pore scale is developed in this project. An experimental study on drying of high moisture porous media is conducted in order to validate this model, and the experimental data will be compared with the simulation results. The micro-imaging technology is used to construct the characterization function of porous media structure parameters, and the effect of the microporous structure characteristics on the process of drying shrinkage and deformation is explored by obtaining the rule of the topological structure transformed into the pore network and constructing the pore network model with adjustable skeleton size, variable pore structure and meltable "mass-heat -force" grid. Based on the molecular transfer characteristics in micropore, the microscale effect of capillary force and liquid film force on the process of drying shrinkage and deformation is taken into account by coupling the governing equations of heat-mass transfer and stress-strain on the micro-body of skeleton. The relationship between the drying shrinkage-deformation process and the microstructure and microscale effect will be revealed, the intrinsic microscopic mechanism of the drying shrinkage and deformation will be clarified and the "irregular deformation" phenomenon will be described effectively in this project model. The success of this project will lay a theoretical foundation for the saving energy of drying process and the analysis and optimization of drying quality.

干燥收缩变形不仅导致物料内部孔隙塌缩、传质通道受阻、能耗升高，同时也使得干后产品品质严重下降。本项目针对传统干燥收缩变形模型不能充分考虑毛细力、微孔结构特性的影响等问题，拟运用孔道网络理论和分子动力学方法分析高含湿多孔介质干燥过程，建立其孔隙尺度下的“热湿-毛细-液膜-力”双向全耦合孔道网络干燥模型。应用显微成像技术构造多孔介质结构参数表征函数，获得其拓扑结构用孔道网络表达时的规律；构建骨架尺寸可调、孔隙结构可变，“质热力”网格相融的孔道网络，探究微孔结构特性对干燥收缩变形的影响。基于微孔隙内分子传递特性，耦合骨架微元传热传质与应力应变方程，考虑毛细力、液膜力等微尺度效应对干燥收缩变形的影响。模型可揭示物料干燥收缩变形与微孔结构、微尺度效应之间的关系，阐明干燥收缩变形的微观影响机制，能有效描述与预测“不规则变形”现象。本项目的完成可为干燥过程的节能降耗、干燥品质的分析与优化奠定理论基础。

干燥收缩变形不仅导致物料内部孔隙塌缩、传质通道受阻、能耗升高，同时也使得干后产品品质严重下降。本项目针对传统干燥收缩变形模型不能充分考虑毛细力、微孔结构特性对干燥的影响等问题，运用显微成像技术构造了高含湿多孔介质结构参数表征函数，获得了其拓扑结构用孔道网络表达时的一般规律；基于提取的微孔网络，构建了高含湿多孔介质干燥收缩变形的孔道网络物理模型。运用分子动力学方法分析了多孔介质干燥微孔隙内的湿分迁移过程；选取骨架微元体，应用平衡分析法推导得到了孔隙尺度下关于质量和热量传递的离散控制方程；运用细观力学理论建立了关于骨架微元体受力变形的本构方程、平衡差分方程和几何方程，建立了高含湿多孔介质干燥传热传质与应力应变的双向全耦合数学模型。编程开发了模型求解软件；选用典型高含湿多孔介质苹果切片等进行了干燥试验，测量了干燥收缩变形的传热传质与应力应变特性试验数据，对模型进行了验证与修正。运行求解软件程序模拟了在各种情况下的高含湿多孔介质干燥及收缩变形过程；模拟得到的干燥数据如物料平均湿含量、平均温度变化曲线和收缩变形率曲线等反映了其干燥过程的实际情况；再现了干燥过程中的“非规则收缩变形”现象；孔道网络模拟的湿分场、温度场及应力应变场均呈现为不规则非对称变化规律，产生了明显的干斑、湿斑、非规则干燥前沿等；毛细应力和湿应力对多孔介质干燥收缩变形影响较大，其中毛细应力是引起非规则收缩变形的主导因素；孔隙结构参数对多孔介质干燥过程影响显著；配位数越大，毛细应力越大，干燥时间越长；孔隙直径分布呈现均一直径分布规律的物料产生的毛细应力大，其次为孔隙直径分布呈现正态分布规律的物料和试验物料分布。本项目的研究结果进一步丰富了干燥科学基础理论，揭示了物料干燥收缩变形与微孔结构、微尺度效应之间的关系；这对破解“不规则收缩变形”现象成因、改善干燥品质与效率、节能降耗、提高经济效益等都具有重要的理论与科学意义。