Numerical simulation and experiment of four-way ventilation mixed flow drying section for rice

Abstract: Rice is one of the most significant cash crops, with a yield of over 200 million tons each year in China. After harvesting, the rice grain can be dried in time to prevent mold, sprouting, and other defects, particularly for the high yield and quality. Hot-air drying has been the primary approach for drying rice. However, the uniform drying cannot be fully met using one-way ventilation with vertical box-type hot-air dryers. The uniform distribution of airflow can depend mainly on the structure and arrangement of the angle tubes in the drying section. In this study, four-way ventilation was proposed for the mixed-flow drying section, in order to improve the uniformity of airflow distribution in the grain layer for high rice quality. The double-side inlet variable diameter air ducts were also designed in this case. A mathematical model was then established to couple the heat and moisture transfer within the rice layer using the CFD platform, according to the porous media heat and mass transfer theory. FLUENT software was also used to simulate the static flow field, temperature, and humidity distribution in the drying section. The simulation results showed that the best reducer angle of the double-side inlet air duct was 0.85°, fully meeting the industrial requirement of uniform air distribution in the drying section. The wind velocity non-uniformity coefficient of the section below the inlet air duct was reduced by 6.11 percentage points after optimization. The uneven distribution of flow field, temperature, and moisture were obtained along the longitudinal direction of the inlet air duct using double-side inlet variable diameter air ducts in the drying section. An experimental bench was also utilized to verify the simulation. The three-factor five-level quadratic orthogonal rotation combination and parameters optimization were carried out with the temperature of hot air, the air velocity, and the initial moisture content as the experimental factors, while the drying rate, the crackle-added ratio, and the taste value as the experimental index. Design Expert 8.0.6 was utilized to analyze the experimental data for the regression equations and response surface plots. The optimal combination of operating parameters was obtained as follows. The optimal hot-air temperature, the inlet air velocity of the air duct, and the initial moisture content were calculated as 43℃, 4.1m/s, and 18.2%, respectively, where the drying rate, crackle-added ratio, and taste value were 1.116 %/h, 1.7%, and 80.33 points, respectively. Anyway, the experimental parameters were better consistent with the mathematical modelling. The drying experiments showed that the moisture non-uniformity of the rice was less than 0.7%, indicating the better drying performance of four-way ventilation mixed flow drying section for rice. The drying section with optimal parameters can be expected to apply for the double inlet variable diameter air ducts during rice drying. This finding can provide an effective reference to optimize the drying process after rice harvesting.