Research On Unsteady Physical Coefficient And Drying Properties Of Porous Materials

Because of the temperature gradient and moisture gradient driven in the porous materials in the drying process, the material internal water molecules was diffused to the surface by way of vaporization and migration, and then it will evaporate into the hot air in the form of convection water, the material volume, the percentage of each phase, porosity, and density changes with the reduction of internal moisture, and it shows a great non-steady-state properties; And these changes will have an impact on the next step of drying of porous material. Thus the porous material drying process involves heat and mass transfer and mechanical properties and other issues.In the study of mechanical properties and heat and mass transfer problems in the drying process, the physical parameters such as moisture effective diffusion coefficient, the effective thermal conductivity ware the key problem. So the study of its variation could provide a theoretical basis for master the drying characteristics of materials, quality assurance and dried, and the drying process developed the design-related drying equipment.This paper against the unsteady characteristics, it researched the moisture effective diffusion coefficient and the effective thermal conductivity of the spherical corn grain, sliced carrot particles, columnar apple particles, the main contents are as follows:(1)A unsteady moisture diffusion coefficient and the shrinkage model was built based on second Fick diffusion law, a theoretical research on unsteady water diffusion characteristics and tissue shrinkage law was studied in different shapes(spherical, cylindrical, flake) and in different materials. The results show that:The internal water was exists in the form of free water at the beginning of the hot air drying, the free water was evaporation and spread more easily, and the water effective diffusion coefficient is almost not changed; The porous material internal free water was become less and the combined water become more in the ratio of the whole water as the drying process march, so the moisture diffused hardly and the water effective diffusion coefficient is become smaller with the decrease of the ratio of water; The porous material’s internal moisture was no changed when it achieved the balance dry basis moisture at the final, the resistance of the prevent moisture diffusion was balance as the driving force of the moisture, the moisture diffusion coefficient was not changes effectively; The water effective diffusion coefficient was a gradually reduced process rather than fixed in the whole drying process. The outside layer was shrinkage first and the internal cell layer shrinkage last, and the shrinkage was larger than the internal cell layer rather than a synchronous shrinkage.(2)A effective thermal conductivity model was built based on Fourier law, a theoretical research on unsteady heat transfer law was studied in different shapes(spherical, cylindrical, flake) and in different materials. The results show that: The effective thermal conductivity was a diminishing value rather than a fixed value in the whole drying process, it because that the water reducing area some translate into shrinkage volume and the other into the internal pore, so the water volume percentage was decrease and the pore percentage was increase in the drying process, and the water coefficient of thermal conductivity was larger than the air coefficient of thermal conductivity. The effective thermal conductivity of porous was reduced along with the moisture reduced rapidly in the early drying process. Due to the drying rate decreased, the moisture diffusion was slow when the drying rate entered the stage of slow drying rate. When the internal thermal conductivity was no changed the drying process end.(3) A spherical porous material radial viscoelastic stress model was established see the porous materials as a viscoelastic body and combined with the heat and mass transfer regular and the layer shrinkage characteristics, based on the radial stress calculated the tangential stress distribution and variation of spherical porous material’s inside layers combined with the characteristics of the spherical geometry. The results showed that:The radial stress and the tangential stress appeared two peak stress and the first peak is greater than the second in the whole drying process. The radial stress was compressive stress at the beginning of the dry process, and change to tensile stress after the reverse; The tangential stress was tensile stress outside compressive stress inside, and tensile stress inside compressive stress outside; The change tendency of the curve of thermal stress and wet stress with time was the same as the total stress change trend, the wet stress value is greater than the thermal stress value at the same time, the moisture gradient in the process of drying on the drying stress is greater effected than the temperature gradient.(4)Finally it studied the coupling relationship of material shrinkage, diffusion, heat transfer and the mechanical properties combining with porous unsteady physical parameters model, the unsteady drying characteristics model, the results show that:The influence of the drying process for material drying characteristics was by the way of changing the physical parameters, the hot air temperature and rate were increased with water effective diffusion coefficient, the effective thermal conductivity coefficient increased, the water effective diffusion coefficient was increased with drying rate and drying stress increased, the effective thermal conductivity was increased with heat transfer rate increased; The drying rate of considering shrinkage was less than the drying rate not consider the shrinkage, it because that the water reducing area some translate into shrinkage volume and the other into the internal pore under the condition of considering the shrinkage, and the water reducing area was all translate into the internal pore under the condition of considering the shrinkage.