Simulation And Experimental Study Of Water-Thermal-Solute Transfer In Porous Media During Drying Process

Fruits and vegetables,as a typical porous medium,are rich in various essential nutrients for the human body and are crucial for a healthy diet in humans.Due to the high moisture content of fruits and vegetables,the quality and nutrient loss of dried products can be affected after drying treatment.Traditional drying models neglect the influence of solute migration on the loss of nutrients.In view of this,this study applied the theory of heat and mass transfer and drying kinetics methods to investigate the drying process of porous media in fruits and vegetables.A coupled drying mathematical model was developed,taking into account the heat and moisture transfer and solute migration.Drying experiments on apples were conducted to verify the accuracy and reliability of the proposed model.Using COMSOL Multiphysics numerical simulation software,the drying process of porous media in fruits and vegetables was simulated,providing new ideas for improving the drying quality of porous media.The main research work in this thesis includes:(1)A physical model of water,heat and solute transfer in porous media during hot air drying was constructed through analysis of the drying process of porous media.The structural characteristics and basic parameters of porous media were analyzed,and the drying process of porous media was described.Considering the pore structure,heat transfer,mass transfer,and quality changes inside the medium,a physical model of water-heat-solute transfer during porous media drying was established using the finite element software COMSOL Multiphysics based on the actual porous media model.(2)Based on the mechanism of water-heat-solute transfer in porous media drying,a coupled drying mathematical model considering heat and mass transfer and solute migration was established.The model was constructed by applying the equilibrium analysis method,taking into account the heat transfer mechanism,moisture transfer mechanism,and solute transport mechanism in porous media drying.Numerical simulations were performed using the COMSOL Multiphysics software to obtain the heat and mass transfer and solute migration characteristics during porous media drying.(3)Hot air drying experiments were conducted on apples under different drying conditions.During the drying experiment,water content,temperature distribution,water state distribution,and sugar content data of different parts were measured for apples under different drying conditions.The effective moisture diffusion coefficient of apple slices during hot air drying was calculated,and the drying kinetics model of apple slices during hot air drying was fitted.The Person correlation of drying indicators for apple slices during hot air drying was analyzed.The experimental results showed that the drying rate of apple slices increased first and then decreased with the progress of drying time,and the maximum rate appeared at around 40 minutes.The temperature of the slices showed a trend of initially increasing and then decreasing with time,with the fastest rate of change occurring in the first 60 minutes.The content of free water in the initial state of the apple was the largest,accounting for 85.6%of the total water,the content of bound water was 4.6%,and the content of water that was not easy to flow was 9.8%.The bound water and immobile water in the apple slices increased first and then decreased,while the free water gradually decreased.During the hot air drying process,sugar migrates with the progress of drying,and the sugar concentration at different positions showed an overall upward trend.The effective moisture diffusion coefficient Deff increased with the increase of drying temperature and the decrease of slice thickness.The fitting goodness R2 of the kinetic regression equation was greater than 0.98,thus indicating that the kinetic model could accurately predict the changes and distribution of moisture in apple slices under different drying conditions.(4)Based on the mathematical model and COMSOL Multiphysics numerical simulation method,reliability verification was carried out and simulation analysis was performed for different drying conditions and different pore structures.The simulated values of the drying curve,drying rate curve,temperature distribution curve,moisture distribution curve,and sugar distribution curve were compared with the experimental values,and the error between them was within ±10%,which demonstrated the high accuracy and reliability of the established mathematical model and numerical solution.The simulation results indicated that with the increase of hot air temperature or the reduction of apple slice thickness,the drying time was shortened and the drying rate was accelerated.The temperature and moisture content of apple slices both decreased from the outer layer to the inner layer.The change in sugar content of apple slices mainly occurred through diffusion from the inside to the outside during drying.The smaller the porosity of the apple slices,the smaller the effective heat capacity of the slices,and the faster the temperature rises.The smaller the initial sugar concentration,the slower the rate of sugar diffusion from the inside of the slice to the surface,resulting in a longer time required for the sugar concentration to increase.The model established in this paper elucidates the migration mechanism of nutrients,such as sugars,during the hot air drying process of porous media and reveals the laws of moisture flow and nutrient diffusion and transfer under different conditions during the hot-air drying process of porous media.The research provides a theoretical basis and technical support for improving the quality of porous media drying.