| The safe storage of grain is most important thing to our human being.According to relevant statistics,the loss of grain in the storage process is shocking,so there must be a scientific method to store grain effectively and safely.In this regard,scholars at home and abroad have proposed mechanical ventilation method to regulate grain by manual intervention.However,the method of mechanical ventilation is affected by the temperature,humidity of the outside atmosphere and ventilation time,so we must continue to study these factors which affect food security.Based on the theory of heat and mass transfer in porous media,the mathematical models of temperature,moisture,dew point temperature,fungus and other physical fields in grain and grain bulk were established.Then,a cubic-sized experimental silo was built for natural storage for 198 h.The rationality and applicability of the mathematical model were verified by numerical simulation and experiments.Secondly,the paddy in a grain depot in Hangzhou was selected as the research object.The storage period consisted of 75 days of non-manual intervention,144 hours of manual intervention and the second non-manual intervention.The multi-physical parameters(temperature,moisture,dew point temperature,pest growth rate,fungi)that affected the grain security storage were analyzed from the grain depot scale and grain size.The simulation results of grain stack show that the natural convection makes the micro-air flow in grain stack form a circulation,accelerates the heat and mass transfer process between grain and micro-air flow,and finally forms the situation of high temperature in the center of grain stack and low temperature near both sides and walls.Because of the influence of solar radiation,the temperature near the south wall is higher than that near the North wall.It can be seen from the figure of moisture that the grain moisture below 0.5m and near the north wall is on the high side,which is the key prevention and control area.The grain is in the "hot core grain" state,and the air temperature outside will not be lower than the dew point temperature of the grain during the 75-day storage period,so there will be no dew problem.The change of temperature and moisture resulted in a relatively high number of insect and mould in the central area of the silo and the grain near the grain surface,and a relatively small number in the area near the wall of the silo,but no outbreak of insect and mould problems occurred during the 75-day storage period.In ventilation stage,the moisture move towards the North wall,which results in a higher moisture content near the north wall and grain surface.Therefore,the area with the largest number of mould appears near the north wall and grain surface,which is the main prevention and control area.Finally,the coupling process between grain bulk and grain kernel is studied.The data of temperature and humidity of 9 representative points in grain bulk are selected as boundary conditions for grain simulation,and the temperature and moisture of grain kernel are researched and analyzed.The simulation analysis of grain temperature and humidity shows that grain itself is a multi-layer structure of active substance.It takes a certain time for heat and moisture to transfer from surface to interior,that is to say,there is a delay effect.In this way,the measured data of temperature and humidity probe in the actual silo will have some errors. |
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