苹果切片干燥收缩变形的孔道网络模拟及试验

为揭示果蔬干燥收缩变形的传热传质与应力应变的机理，确定果蔬微孔结构特性及内部毛细力等因素对其干燥过程的影响，该研究运用孔道网络方法、热质传递原理和细观力学理论等交叉学科知识，构建了孔隙尺度下果蔬切片干燥收缩变形的孔道网络模型，采用VC++开发孔道网络求解程序，模拟分析了果蔬切片的湿分场、温度场以及应力应变场等情况，并以苹果切片作为果蔬典型代表进行了热风干燥试验及模型验证。结果表明：湿含量、温度和收缩变形率的模拟值与试验值的相对误差小于10%，模型可有效模拟果蔬干燥热质传递与应力应变的收缩变形真实过程，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果蔬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；孔隙结构参数对果蔬干燥过程影响显著；孔隙率越大，干燥时间越长，毛细应力越小；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。研究结果为果蔬干燥品质及工艺优化分析提供了一定的理论基础。

Pore network simulation and experiments of drying shrinkage-deformation for apple slices

Abstract: This study aims to explore the heat-mass transfer and stress-strain of shrinkage deformation of fruits and vegetables, thereby determining the effect of capillary force on the drying process. Pore networks, heat-mass transfer, and mesomechanics were also selected to clarify the micropore structure and capillary force during drying. A coupled heat-mass transfer and stress-strain model was established for the drying shrinkage deformation of fruits and vegetables at the pore scale. Microscopic imaging was utilized to capture the slices of fruits and vegetables, and then to extract and characterize the parameters of the micro pore structure, finally to identify and label the capillary liquid mass. A program was also developed to simulate using the programming software Visual C++. The simulation program mainly included the three modules: building physical model, solving mathematical model, and data processing. The original file was input to generate the framework of the physical model, according to the coordinate values of nodes and throats. The components of the physical model, such as skeleton particles, holes, and throats, were represented with the object of a class in object-oriented programming (OOP). Therefore, all the attributes and relationships of components were encapsulated to ensure that the mathematical model module easily accessed each component and absorbed the channel structure information of the physical model in time. The mathematical model module was the core of the program to realize the calculation and huge solutions. The data processing module mainly realized the real-time display of calculated data, further obtaining the distribution contours for later Excel processing. Correspondingly, the distribution of moisture, temperature, and stress-strain in the apple slices were simulated in pore networks. An attempt was also made to clarify the effects of drying stress and microporous structure on drying shrinkage-deformation. Taking apple slices as the research object, a hot-air drying experiment was carried out to compare the experimental and simulation data. Results showed that the relative errors were less than 10% between the simulated and experimental values of moisture content, temperature, and shrinkage deformation rate. Consequently, the model can be expected to effectively simulate the real process of heat-mass transfer and irregular shrinkage deformation of fruits and vegetables in the drying process. The distribution fields of moisture, temperature, and stress-strain in the pore networks were in the irregular and asymmetric shape, resulting in obvious dry spots, wet spots, irregular drying fronts, and even asymmetric shrinkage-deformation. Capillary and wet stress posed significant effects on the drying shrinkage deformation of fruits and vegetables, where the capillary stress was the dominant factor for the irregular shrinkage-deformation. It demonstrated that there was a significant effect of pore structure parameters on the drying process of fruits and vegetables. Specifically, the drying time was longer and the capillary stress was smaller when the porosity of materials was larger. The capillary stress was greater and the drying time was longer when the coordination number of the model was larger. The capillary stress with uniform diameter distribution was larger, followed by the distribution of normal and experimental materials. The finding can provide sound theoretical support to better drying quality under the optimal processing parameters of fruits and vegetables.