Simulation And Experimental Research On Drying Of Grain Packing Porous Media

In recent decades, the research on grain packing porous media has attracted more and more attentions. The traditional drying model can not reasonably explain the observed phenomenon during its actual drying processes, and traditional model generally gives an average moisture content and distribution of grain packing in macro scale, and can not give the microscopic scale moisture content information of pore skeleton itself, and these are precisely the necessary for the research on the quality of drying materials such as grain stress cracking and fissuring. In this paper, using the paddy pile as a typical representative, through the following aspects to analyze these issues:(1)Construction of multi-scale and multi-layer structural phyical model. By using the theory of multi-scale to divide the scale, and using the theory of pore network to build a physical model. There is a multi-physical structure inside the grains, including the seed coat(hull) 、 chaff and embryo, so establish a multi-layer structural and multi-scale model. The most common five structural parameters for describing physical model are presented, which are distribution law, distance between two nodes, the space distribution coefficient, model size number and the coordination number. The characteristic parameters of the particle packing porous media, which are porosity, particle size distribution, pore size distribution and pore coordination number. Then, the structural parameters of physical model and the characteristic parameters of the particle packing porous media are analyzed correspondingly. So, the pore network theory can be applied to the theoretical analysis of drying of the actual grain packing porous media.(2)Construction of multi-scale mathematical model. Based on physical model, the mathematical model of heat mass transfer for grain packing porous media drying is founded. Heat and mass transfer equation on the grain-scale is concluded. Then, heat and mass transfer equation and momentum equation on drier-scale is established. And also establish the heat and mass exchange equation between two scales.(3)Drying experiment. With paddy as experiment materials, using the self-made device to proceed the paddy pile drying. Measure and calculate the paddy pile of each structural parameter, and through the test to get the moisture content distribution curve of the drier-scale, temperature distribution curve and temperature distribution curve of the pore vapor phase, also to get the three measurement points drying curve of grain-scale and the temperature change curve of measurement point 7.(4)Solving mathematical models and computer simulation. Solving the established mathematical model, heat and mass equations of grain-scale uses finite element method to solve; heat and mass equations of drier-scale uses separate method to solve. Combined with VC++ and Matlab language to proceed program development and simulation.We can see that through simulation and experimental results of comparative analysis, the established model can explain the actual grain packing porous media drying. The temperature of paddys is lower than the surrounding pores vapor temperature during the drying process at the same time. During the drying process, the effect of different structural parameters is the greater of porosity and coordination number, the faster of the rate of drying, coordination number decrease lead to wet cluster phenomenon. During the drying process, the effect of physical characteristics of different organization is the embryo diffusion coefficient is very significant, followed by hull diffusion coefficient, the chaff diffusion coefficient is minimum.