Study On Ventilation And Anti-condensation Of Underground Silo Lined With Plastic

Food security is related to the national economy and people’s livelihood.In order to ensure national food security,it is necessary not only to increase grain production,but also to strengthen grain reserves.In the process of grain storage,there are many factors that affect the safe storage of grain,and condensation is one of the important threats to the destruction of safe grain storage.Temperature and humidity are important influencing factors of condensation.Local heating,condensation and mildew in the grain piles in the silo due to uneven distribution of temperature and humidity occur from time to time.Most of the existing grain storage silo in our country are above-ground silo,which have problems such as low land utilization,high energy consumption,and easily affected by changes in the external environment.The use of underground silo can not only increase the utilization rate of land,but also use the ground temperature to maintain the environment outside the silo at a relatively constant level and reduce the impact of changes in the environment outside the silo on the grain piles in the silo.However,there is a risk of condensation in the process of underground storage of grain,and it is necessary to reveal the conditions for condensation of grain piles in underground storage to develop effective anti-condensation measures to achieve safe grain storage.This paper takes high-moisture grains as the research object.Firstly,the experimental methods are used to analyze the changes in the temperature field and humidity field in the underground silo during the static storage and mechanical ventilation phases,and then the effects of temperature,relative humidity and moisture content in the storehouse on the radial and axial condensation of the grain pile are studied.Then,using the multi-physics numerical simulation software COMSOL,a numerical model was established for the test silo.The reliability of the model was verified through verification with test data,and the occurrence of moisture transfer,heat generation and condensation caused by temperature differences was deeply explored.In the static storage stage,based on the actual pile height,the influence of grain respiration in the silo on the temperature field distribution in the grain pile is fully considered.In the mechanical ventilation stage,through simulation,the effect of different ventilation and the initial state of the grain pile on the silo is studied.The influence of the internal temperature field determines the influence of temperature changes on condensation under static storage and mechanical ventilation conditions.The results show:(1)In the static storage stage,the respiration of the grain is strong.As the storage time increases,the grain pile begins to heat up at the bottom and gradually forms a high-temperature heat core.Then,the position of the heat core gradually moves to the middle and upper part of the grain pile,and finally stabilized at 1m away from the grain loading line.On the 11 th day of storage,the heat core of the grain pile increased by 7℃compared with the initial grain temperature.The change of the temperature field in the silo causes the water migration between the grains.On the third day of storage,the local relative humidity rose to the highest value in the static storage stage,which is located near the grain loading line in the silo,which is also the area with high humidity in the grain pile.As a whole,500 mm below the grain loading line is an easy dew area,which is similar to the above-ground silo.The difference is that the risk of condensation in the center of the grain pile is higher than that at the silo wall.(2)In the mechanical ventilation stage,the grain pile is first cooled from the air outlet,and then gradually spread to the entire bin.The study found that the diffusion path is related to the setting of the air duct.When the supply air temperature difference of 11℃,after 2hours of mechanical ventilation,condensation occurred in the upper air area near the polyurethane thermal insulation board of the silo roof,and the relative humidity of the grain pile surface decreased.Therefore,the return air should be dehumidified,or porous moisture-absorbing materials should be used with top insulation.As the duration of mechanical ventilation increases,the relative humidity of the upper air area in the test chamber gradually decreases.After ventilating for 10 hours,the temperature of the grain pile in the test silo fell below 10℃ from the middle to the bottom of the silo,and only reached15℃ above the return air outlet near the grain loading line.After 14 hours of mechanical ventilation,the relative humidity in the test chamber has stabilized,but its distribution is not uniform.The areas with higher relative humidity are concentrated at 500 mm below the grain loading line,the relative humidity reaching 91.3%.Similar to static grain storage,this depth is still easy to knot.Exposed area,but its distribution is close to the silo wall.The increase in air supply speeds up the temperature drop of the grain pile;the difference in the initial state of the grain pile has little effect on the cooling effect of mechanical ventilation.In the experiment and simulation,the mechanical ventilation adopts a uniform air supply method,which has a significant effect on reducing the temperature of the grain pile in the silo and eliminating condensation on the upper part of the grain pile.During the ventilation process,when the air supply temperature and the temperature inside the silo are 9.9℃and 21.8℃,condensation occurs;when the air supply temperature and the temperature inside the warehouse are 11.3℃and 19.9℃,they are already within the safety threshold.