Numerical simulation of coupled heat and moisture transfer and yellowing of stored rice during storage in silo

Abstract: A theoretical model was proposed to implement the heat transfer, moisture content, and momentum in the cylinder-shaped silos, where rice was stored. A yellowing model of rice was also developed, according to the kinetics of color change. A field experiment was then carried out to evaluate the present model. A systematic investigation was then made on the temporal and spatial distribution of temperature, moisture content, and yellowness of stored rice within the concrete silo in the Nanchang area of China during storage without and with mechanical ventilation in the whole year. The simulation results show that the temperature of bulk rice near the inner wall of the silo had fluctuated with the local atmospheric temperature, and the mean temperature of bulk rice in the silo was rising during sealed storage without mechanical ventilation. Since the grain was a very good insulator, the center of the silo stayed much warmer with the longer storage. The temperature of bulk rice was more than 35.0℃ in the most area of the silo, due to the heat transfer between the silo with the local atmosphere and the exothermic heat of the grain respiration in the silo, where the rice was very easy to turn yellow during the sealed storage without mechanical ventilation. Moisture migration also occurred in the rice storage silo, resulting from some differences in grain under air temperatures. The cold air on the outside of the bin (cold air was much heavier) pushed the warm air into the center of the silo in autumn, where the warm air moved upward and eventually encountered the cool air at the top. The currents of natural convection were formed by the displacement of hot air by the dense cold air. The moisture content of grain bulk migrated to the vicinity of the grain surface by the natural air currents, resulting in the condensation and crusting in the surface of the bulk grain. The opposite process was found in spring and summer, particularly with the condensation and spoilage at the bottom of the bin. Therefore, mechanical ventilation was used to alternately alleviate the natural air currents and the moisture migration for the sealed storage. In this case, the higher temperature area of the grain bulk was observed near the south wall, and the grain surface on the south side, due to the solar radiation, where the rice was relatively yellowish. The internal temperature of grain bulk dropped significantly during ventilation, particularly after a period of sealed storage. The yellowness of rice was relatively lower than before for the sealed storage during ventilation over the whole year, indicating that the temperature posed a predominant effect on the color change. Meanwhile, the ventilation can widely be expected to balance the temperature differences in the bulk rice, the natural current, and moisture migration during storage. The findings can also provide a strong reference and theoretical basis for grain storage.