Study On Heat And Humidity Migration And Granary Aeration Management Based On Absolute Water Potential

Grain is one of the important resources related to the national economy and people livelihood. Some factors turn grain bulk into a complex large system such as grain temperature, grain humidity, atmosphere temperature, atmosphere humidity and micro-airflow, etc. Local temperature and humidity anomalies can bring about the mildew during the heat and humidity transfer in the grain bulk.In order to explore heat and humidity migration, corn static storage and deep bed drying experiments were studied. Temperature and humidity integration monitor model was established in order to resolve the shortage of the humidity measure. It showed that the theoretical value by temperature and humidity integration monitor model agreed with practical values, therefore this calculating method was in validity. In the process of deep bed drying experiment, heat and humidity migration were analyzed by researching drying front and wet front migration. It was showed that hot wind temperature and speed influenced drying front boundary temperature and movement speed which reflecting thermal migration,and hot wind speed had less effect. Grain layer will also adsorb other grain layer moisture in the drying process. Hot wind speed and grain thickness influenced wet front movement speed which reflecting moisture migration step by step.The equilibrium moisture content and equilibrium relative humidity models of grain were established under different temperatures. The equilibrium moisture content and equilibrium relative humidity of wheat, corn, rough rice and rice were fitted according to the model under different temperatures. The fitting results indicated that the model had high precision to describe the isotherm equations of grain and could calculate the equilibrium moisture content directly. The model predicated the safe moistures under different temperatures and analyzed the thermodynamic properties of wheat, corn and rice, which indicated that the isosteric of sorption heat decreased with the increase of grain equilibrium moisture and gradually approached the latent heat of vaporization when the equilibrium moisture content reached 25%(w.b.) The isosteric of sorption heat regression equations were fitting based on the data analysis. It provides an available basis for the granary energy jobmanagement and the efficient reasonable energy-saving ventilation.The energy migration between grain and air was analyzed based on the Gibbs free energy and the first and second law of thermodynamics. The absolute water potential models of grain and air were built by the energy field analysis. According to the grain aeration management standards, the absolute water potential diagram was adopted to improve the aeration management by the analysis of aeration condition based on the energy field, which provided theoretical foundation for the energy analysis of grain heat and humidity control.The cooling window, the precipitation window and the tempering window were built in the absolute water potential diagram when corn temperature was 20℃and moisture was 14.5%.Single point rule was built by analyzing aeration condition based on the energy field.Aeration system controlled the aeration operation according to single point rule and multipoint grain situation monitoring in the absolute water potential diagram. Combining grain absolute water analysis, according to the corn mildew initial growth model, corn mildew growth forecast curve was drawn in the multipoint grain situation monitoring diagram when air temperature was 18.84 ℃ and air absolute humidity was 11 mmHg. It ensured provide a favorable basis for safe storage.Based on the absolute water potential aeration managemeny study, absolute water potential, moisture diffusion coefficient and activation energy of the corn ear were analyzed in the natural ventilation process. Results indicated that corn moisture drying to safe water at low temperature needed three or four months in the natural ventilation, absolute water potential of air and corn ear increased with the increase of temperature, corn ear were in the desorption state when corn absolute water potential was greater than air absolute water potential. When the absolute water potential of corn and air approached gradually, corn moisture remained unchanged because water molecules lacked sufficient thermal energy to overcome the resistance which prevented water molecules evaporating from the grain surface to air. Moisture migrated from west to east affected by water potential gradient in four granaries, corn moisture declined faster in windward side than others, and lower drying rate could be received during the initial period of natural ventilation. Corn moisture diffusion coefficient was in the range of 2.563? 10-12 to 5.34? 10-12m2/s, the change of moisture diffusion overall tended to increase with the increase of temperature and it exhibited a downward trend in the late stage affected by the absolute water potential.Different averages in different width of granaries indicated moisture diffusion coefficient affected by grain thickness. Arrhenius equation could describe the relationship betweenmoisture diffusion coefficient and temperature which obtained the average of activation energy was 35.76kJ/mol. It could provide theoretical basis for grain moisture control and early warning.