Simulation and experiment of heat and moisture transfer during ventilated drying process in maize grain pile

This study aims to accurately predict the distribution of heat and moisture in maize piles during ventilated drying. Taking the maize pile within the silo as the subject, the heat and moisture transfer model was established to simulate the ventilated drying of the corn. A systematic investigation was also implemented to clarify the suitable ventilation conditions. The heat was then released by maize respiration, according to the local thermal non-equilibrium equation. The experiment was carried out on the integrated silo of grain drying and storage using COMSOL Multiphysics. An experiment was conducted on the maize pile in the silo, with specific conditions, including air velocity, air temperature and relative humidity. The simulation was focused on the moisture content, temperature distribution of maize and temperature, humidity distribution of air in the maize pile. The results showed that the heat and moisture transfer model effectively simulated the ventilated drying of maize pile in the silo. The relative errors between the simulated and experimental values of maize temperature and moisture content at four points within the silo ranged from 1.4%-12.1% and 0.3%-14.5%, and the average relative errors were 4.8% and 6.5%, respectively. Similarly, the relative errors between the simulated and experimental values of air temperature and air relative humidity within the silo were 0.7%-15.1% and1.3%-15.4%, respectively, and the average relative errors were 5.5% and 8.9%, respectively. Notably, there was the unevenness of maize in the silo at the initial stage of maize ventilation. The inner layer of maize was experienced the higher rates of heating and drying. The higher air velocity resulted in the increased airflow per unit time. For example, the maximum difference in the moisture content reached 0.03 g/g along the ventilation direction. However, the heating and drying rates of maize were gradually reduced, as the ventilation progressed, leading to a decrease in the unevenness of maize pile. The equilibrium temperature of maize was then reached after 25 h, while the approximate equilibrium moisture content was reached after 60 h. The air temperature rose rapidly within the first hour of ventilation, followed by a slow increase to nearly 25 °C. The humidity initially increased and then decreased, where the rate of decline was gradually slowed down until equilibrium was reached. The cloud map analysis revealed that there was a significant difference in the area near the silo wall and the overall, indicating the need to optimize the silo and ventilation structure. The air velocity had also enhanced the heating and drying rate. But after reaching 0.16 m/s, there was a further increase in the air velocity without considering the change rate of corn. The relative humidity shared little effect on the trend and value of maize temperature. But the relative humidity decreased the drying rate, whereas increased the equilibrium moisture content of corn. The higher air temperatures resulted in a higher drying and heating rate with a lower content of equilibrium moisture. According to the maize temperature and moisture content under different conditions, the combination of ventilation parameters was achieved in the air relative humidity less than 75%, air velocity of 0.09-0.23 m/s, and air temperature with atmospheric conditions. The relative errors between the simulated and experimental values of the moisture content of maize in the pilot test of ventilated drying ranged from 1.3% to 16.7%, with an average value of 4.4%, indicating the further practicality of the constructed model. Then the moisture contents of the maize in the innermost, middle, and outermost layers approached the safe moisture content around 130, 180, and 250 h, respectively. The unit energy consumption of ventilated drying of maize was 890.2 kJ/kg, which was remarkable for energy saving. The quality indexes also showed the feasibility of ventilated drying and storage of high-moisture corn. Therefore, change-over ventilation and grain circulation were recommended in practical production, in order to improve the unevenness of temperature and humidity of the maize pile. These findings can also provide valuable theoretical support to optimize the corn-ventilated drying.