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.