Study On Heat And Mass Transfer Inside Rice And Mechanism Of Rice Fissuring Using Finite Element Method

In this thesis, the mechanism of heat and mass transfer within a grain kernel in drying or during moisture adsorption has been discussed thoroughly. The shape of the rough rice was assumed as a prolate spheroid. In a cylindrical coordinate system, finite element simulation of the transient moisture distribution and stresses distribution in the rice kernel during drying and the moisture adsorption were presented. The results indicated that, in the drying process, the surface layer of the rough rice loses moisture quickly, and the inner moisture diffusion is slowly. The drying process induced compressive stresses in the inner portion and tensile stresses in the outer portion of the rice. During the moisture adsorption, the outer portion of the rough rice was equilibrated rapidly, but the moisture content in the central portion was increased slowly. The moisture adsorption induced compressive stresses in the outer portion and tensile stresses in the inner portion of the kernel. The fissuring of rice is more likely to be initiated at the center of the kernel. It is concluded that the rough rice may be fissured after drying or during moisture adsorption.Through the experiments, the rough rice fissuring mechanism has been studied. The rough rice may be fissured pre-harvest, in the drying process,after drying and in the moisture adsorbing environment. Results indicate that the mature rough rice will induce fissuring by the moisture adsorption in the field, when the moisture content of rough rice reaches 31%. The rough rice mainly fissured after drying. The moisture gradient caused uneven shrinkage is the main reason of rice fissuring after rapid drying. During the moisture adsorption, the rice fissuring is primarily influenced by the temperature and relative humidity of air. Rough rice is more resistant to fissuring than brown rice.