Study On Heat-Mass Transfer And Stress-Strain Of Fruit And Vegetable Porous Media During Drying Process

The drying shrinkage-deformation of fruits and vegetables porous media is one of the most important factors that affect drying quality and drying efficiency of fruits and vegetables porous media drying.The drying shrinkage-deformation of fruits and vegetables porous media is closely related to the heat-mass transfer and stress-strain mechanism during porous media drying process.In recent years,with the quick improvement of people's living standard,the research on the drying quality of fruits and vegetables porous media drying has attracted more and more attention.The research on the numerical simulation of heat-mass transfer and stress-strain mechanism during fruits and vegetables porous media drying process has become a hot research topic at domestic and abroad.However,the traditional continuous simulation method ignore the influence of complicated pore structure of fruits and vegetables porous medium on the porous media drying process in the simulation process.Compared with the actual porous media drying process of fruits and vegetables,the obtained continuous simulation results are quite different.Meanwhile,the continuous simulation results can not give a reasonable explaination to the produced irregular drying shrinkagedeformation during the actual porous media drying process of fruits and vegetables.Therefore,in view of the aforementioned problems,the apple is selected as a typical representative of fruits and vegetables porous media in this thesis to conduct the hot-air drying experiment and the pore network model simulation of heat-mass transfer and stress-strain process for giving a deep understanding of the drying mechanism during the fruits and vegetables porous media drying process.The main research contents are as follows:(1)Pore network physical model for porous media drying of fruits and vegetables is constructed.The physical model of pore network is constructed by the theory of pore network drying to extract the real porous media.The porosity,pore diameter and coordination number parameters of actual apple porous medium are determined by experimental method,and corresponding to themodel parameters of pore network physical model for making sure that the pore network drying theory can be perfectly applied to the research and analysis of heat-mass transfer and stress-strain analysis for fruits and vegetables porous medium.(2)Heat-mass transfer and stress-strain mathematical model of porous medium drying process for fruits and vegetables is established.The mathematical model of heat-mass transfer and stress-strain is built through analyzing the mechanism of mass transport,heat transfer and drying stress-strain during porous medium drying process of fruits and vegetables.(3)Numerical simulation of mathematical model of fruit and vegetable porous media drying is conducted.The mathematical model of heat-mass transfer and stress-strain are solved through numerical method.In order to conduct the numerical simulation of heat-mass transfer and stress-strain mathematical model for the fruit and vegetable porous medium drying process,the pore network model simulation program is developed through combining the MATLAB with Visual C++.The simulation program includes pore network physical model construction module,mathematical model solving module and data processing and analysis module,which can achieve the real time display,data record and storage,data analysis of drying process for fruit and vegetable porous media drying.(4)Hot-air drying experiment and continuous simulation of porous medium drying for fruits and vegetables are investigated.Apple is chosen to conduct the hot-air drying experiment and continuous simulation.Temperature,moisture content and drying shrinkage-deformation of apple slices during the drying process are measured,the drying curve and shrinkage-deformation curve of apple slices are obtained through the analysis of experimental data.The traditional continuous simulation method is used to analyze the apple's humidity field,temperature field and the corresponding drying stress distribution during the drying process.(5)Analysis and discussion of pore network model simulation results with the continuous simulation results and hot-air drying experiment results are conducted.Through the comparison analysis between the simulation results and experiment results,the pore network model simulation method can well reflectthe actual drying process of fruits and vegetables porous media.Pore network simulation results show that capillary stress is the main cause of drying shrinkage-deformation of fruit and vegetable porous media.The influence of different pore network model structure parameters on the drying process of fruit and vegetable porous media is analyzed through the drying simulation experiment.The simulation results showed that the porosity of pore network model increases,the drying time is longer and the average temperature is lower,the capillary stress decreases with the increase of porosity,while moisture stress and thermal stress increase with the increase of porosity;the coordination number of pore network model increases,the drying time is longer and the average temperature is lower,the capillary stress increases with the increase of coordination number,while moisture stress and thermal stress decreases with the increase of coordination number;the drying rate of uniform pore diameter distribution is faster,the average temperature is higher,and the capillary stress,moisture stress and thermal stress are bigger,while the drying rate for material pore diameter distribution and normal pore diameter distribution is slower,the average temperature is lower,the capillary stress,moisture stress and thermal stress are smaller than the obtained stress value of uniform pore diameter distribution.The results of this study can lay a theoretical foundation for the further study of the porous media drying process of fruits and vegetables.And meanwhile,it can provide a theoretical reference for making the reasonable drying process parameters and the research of improving drying quality.