Simulation Of Grain Drying Stress Based On Glass Transition Theory

In the hot air drying process of rice kernels,the temperature and moisture gradient existing inside the rice kernel,which cause the uneven local deformation in the rice kernel and thus,the drying stress would be produced.When the drying stress exceeds the strength limit of the grain itself,there will be a stress crack(fissuring).Therefore,knowledge of the drying stress of rice grain is the basis for studying the mechanism of rice fissuring.In view of the phenomenon of fissuring in hot air drying of rice,its mechanism is still not clear.The image processing technology and Solidworks software were applied in this work to construct a three-dimensional body-fitted grid for the rice kernel.New measuring technology(TPS)was used to measure the heat conduction coefficient of the rice kernel.The "inversing" method was used to measure the effective diffusion coefficient of rice kernel.The mechanical experiment was used to calculate modulus of elasticity.Finally,the Comsol Multiphysics software was applied to solve the mathematical model.The simulation results were compared with the experimental data and the difference was less than 8%,indicting a high model accuracy.The temperature and water distribution gradients in the rice kernel along the radial direction(short axis)were larger than that ones along the axial direction(long axis),and the duration time of water gradient existence was longer than temperature gradient existence.The moisture gradient in the radial direction from the outer surface to the center 1/3 was larger than that in the other parts of the radial direction.Based on the analysis of the glass transition phenomenon of the rice grains,it was found that the temperature is the main factor affecting the glass transition rate.The higher the temperature,the faster the transformation rate.With the increase of temperature,the rubbery state maintained longer,and the beginning of the change appeared in the surface and tiny point of the rice kernel.At the same time,it was expected that the cracks appear in the drying stage in the low temperature drying,while cracks appear in the cooling stage in the high-temperature drying.The maximum tensile stress of rice surface is about 1.7MPa,which is in the reported range of rice stress,and the stress value is similar to that of acoustic emission test.The stress concentration area of rice kernel is the middle of surface and the central part of rice kernel,which was consistent with the cracking phenomena observed in experiments—the crack extended from the surface inward.The results provide a basis for revealing the mechanism of rice fissuring and optimizing its hot air drying process.