Numerical Simulation Of Cross-scale Heat And Mass Transfer For Hot Air Drying Of Biological Porous Media

Hot air drying is one of the most important drying methods in industrial production.Biological porous media is one of the main drying objects.At present,the research on biological porous media drying mainly adopts macroscopic models such as drying kinetic model and continuous hypothesis of porous media heat transfer and mass drying model.The macroscopic drying model has a strong dependence on the apparent physical properties parameters of the material,resulting in limited accuracy of the calculation results and limited contribution to the study of drying mechanisms such as material microstructure and drying quality.To this end,wolfberry is used as research object to carry out"cell-material"cross-scale drying simulation research from the cell scale,and the change law of the drying process of biological porous media is comprehensively explored.The details are as follows.At the cellular scale,the actual cell structure of wolfberry is simplified to model cell,which composed of cell cavity,cell membrane,and cell wall.The coupling model of moisture transport and structural deformation is established,which considering the moisture migration caused by cell shrinkage.The accuracy of proposed model is validated by literature results.The cell scale moisture transport and structural deformation laws of wolfberry drying process are studied.The moisture transport capacity of different structures is different,cell cavity is the main resistance structure.The cell shrinkage rate is faster during the early drying period,and hardly shrink during the late drying period.Shrinkage has greater effect on moisture transport,compared to the model without shrinkage,the mass flow calculated by the shrinkage model is larger.The model that considering shrinkage is closer to actual physical process.Based on the idea of lumping equivalence,a up-scale equation is proposed,which from the moisture diffusion coefficient of the cell cavity and cell wall to the material scale moisture effective diffusivity of porous media matrices for transient processes.Cell scale model simulation results are used for comparison,the form of the moisture effective diffusivity equation for the porous media matrix of wolfberry is determined by the response surface method.The form of equation is described as follows:D_eff,w=7.721×10^-3exp(0.47 X_dm)ex p（-E/RT）.The coupling of the cellular scale drying model and the material scale porous medium drying model is achieved by the up-scale equation of the effective diffusivity of moisture,and the"cell-material"cross-scale hot air drying model of biological porous media is established.The accuracy of proposed model is verified by comparing hot air drying experiment and simulation results of dry basis moisture content and shrinkage rate data of wolfberry.On this basis,the changing laws of material temperature,moisture content and evaporation rate during the drying process of wolfberry are analyzed.The results show that the temperature,moisture content and evaporation rate of the material is greatly affected by shrinkage.The higher the hot air temperature,the lower the relative humidity,the faster the heat mass transfer speed in the material.The wolfberry drying process is recommended to use a fractional drying process in which the hot air temperature gradually increases and the relative humidity gradually decreases.