Mechanism Research Of Heat And Mass Transfer During Convective Drying Of Deformable Porous Media

Based on the continuum theory, two-dimensional 2-way coupled thermo-hydro-mechanical mathematical model (THM model) has been developed to simulate the hot air convective drying process of deformable porous media on basis of Fickian diffusion theory, Fourier's law of heat conduction for heat and mass transfer processes and thermoelasticity mechanics for the deformation behavior of porous media.Considering the thermal-stress and hydro-stress, elastic mechanics constitutive equations are applied to study the deformation behavior of porous media; the position of interface is demarcated according to the criterion of the critical dry basis moisture content, which divide the porous medium into dry zone and wet zone. The THM models for constant rate drying period and falling rate drying period are established respectively according to the heat and mass transfer mechanism of different zones with considering the displacement of solid skeleton during the drying process at the same time. The corresponding definite conditions are described in detail. The transient model, composed of a system of partial differential equations, is solved by finite difference methods. In which the unsteady items are discretized using the explicit finite difference scheme, first order partial derivative in spatial terms are discretized using central difference quotient, and second order partial derivative are discretized using difference quotient between first-order forward difference quotient and first order backward difference quotient. A series of nonlinear equations is obtained. The computational procedure was programmed using C language.Taking potatoes and carrots as deformable porous media of experimental research object, the convection drying experiments are carried out. The average dry basis moisture content, the internal temperature of porous media and the deformation caused by dehydration are measured in convective drying process. The experimental results show that the convective drying process of hygroscopicity deformable porous media can be divided into constant rate drying period and falling rate drying period, in which the falling rate drying period can be further divided into the first falling rate drying period and the second falling rate drying period. The effects of drying conditions on the drying rate are:the higher air temperature and air velocity, the smaller the moisture content, the greater the drying rate. In the process of drying, two dry samples have obvious shrinkage deformation, the influence of drying conditions on the deformation is not obvious, the deformation is closely related to the dry basis moisture content, the relation can be fitted as a quadratic polynomial.After the grid independence tests of the THM model, the drying process parameters such as average dry basis moisture content, temperature and deformation of the numerical results are compared with the experimental data. The relative errors between numerical results and experimental data are within+/-10 percentage points. A good agreement between the simulation results and the measured results proved that the established mathematical model and the numerical solution method has higher reliability and accuracy.In order to obtain the temporal and spatial change of state parameters in drying process, convection drying processes of deformable porous media under the PID temperature control drying conditions and constant temperature drying conditions are further simulated respectively on the basis of reliability and accuracy for the established mathematical model and the numerical solution method, and the heat and mass transfer and drying-induced stress-strain in drying process are also analyzed. Drying began, the surface temperature of the porous media soared to the wet bulb temperature corresponding to drying medium conditions under the action of convective heat transfer between dry medium and the surface of porous media, dry basis moisture content in the surface of porous media quickly reduced to critical dry basis moisture content, and meanwhile deformable porous media started to shrink, this stage is described as the constant rate drying period; as surface temperature of porous media continue to rise, the internal temperature of porous media begins to increase under the action of heat conduction of solid skeleton, the dry basis moisture content dropped accordingly, shrinkage deformation became violent, this stage is described as the falling rate drying period; at the end of drying process, the temperature of the porous media rise to near the drying medium temperature, while the dry basis moisture content down to equilibrium moisture content corresponding to drying medium.The temperature and moisture content show a gradient inside porous media during drying, and the short reverse convective heat transfer processes appear between the porous media and the drying medium under the PID temperature control drying conditions; the interface between wet zone and dry zone, i.e., the evaporation interface changes dynamically. The moment when the evaporation interface appeare in the calculation area for the first time can be used as time transition criterion for the constant rate drying period and falling rate drying period, and when the evaporation interface appeare for the last time can be used as time transition criterion for the first falling rate drying period and the second falling rate drying periods; during the most of the drying processing time, the porous media bear the normal stresses are compressive stresses, and also suffer shear stress. At the beginning of the drying, compressive stress and shear stress rapidly reach to its maximum value, then decrease and tend to be stable. But the value of the shear stress is far less than the value of compressive stress. Thus the irregular shrinkage deformation of the skeleton of porous media happens to the opposite direction of moisture migration under the combined action of compressive stress and shear stress. Although the extent of deformation is not affected by the drying conditions, drying stresses-in particular, the maximum of the stress-are strongly affected by the drying conditions. The maximum values of compressive stress and shear stress increase with the increase of air velocity and the decrease of air relative humidity, and the influence of air temperature on the stresses is not obvious.This work should help in developing an understanding of the relationship between mass and heat transfer, shrinkage, stress, strains and physical degradation.