Study On Ventilation Test And CFD Numerical Simulation Of Pilot Silo In Bulk Wheat Grain Bulk

It is very import to ensure food security and reduce the loss of grain storage under the current complex international situation,since food security is related to the national economy and people ’s livelihood.However,affected by the external environment,grain respiration and other factors,it is easier to form an environment with high temperature and high humidity which suitable for the growth of insects and molds in the grain bulk under the process of grain storage.Therefore,the temperature and water content in the grain bulk are two important indicators to judge the safety of grain storage.Mechanical ventilation cooling and drying is the main way to reduce the loss of grain storage and improve the quality of grain storage since it can change the ecological environment of grain in the warehouse and minimize the risk of pests and destroy the conditions suitable for mold growth by controlling the temperature and moisture content of grain during storage.In this paper,taking the bulk wheat grain pile in silo as the research object,and the numerical simulation and experiment were carried out it by considering the influence of porosity distribution in grain pile to better simulate the actual grain storage situation.The main research of this paper as follows:(1)Based on the thin-layer drying theory and the heat and mass transfer theory of porous media,a heat and moisture transfer model of wheat grain heap was established,and the specific heat capacity,thermal conductivity,density and other related physical parameters of wheat and air in the model were determined.Taking the thin layer drying test of wheat as the object,the numerical simulation of ventilation drying was carried out.The grid independence and time step independence of the numerical simulation method were discussed.The accuracy of the established heat and moisture transfer model of wheat grain bulk was verified by comparing with the experimental results in the literature.The variation of average temperature and water content of wheat thin layer during ventilation drying was analyzed.(2)Based on the multi-field coupling simulation test platform of grain storage environment,the cooling and water retention test and numerical simulation of wheat grain bulk in silo were carried out.The mathematical model of porosity distribution along the longitudinal direction of wheat grain bulk in silo was determined,and the influence of uniform and non-uniform distribution of porosity on the numerical simulation results was compared and analyzed.The results show that the heat and moisture transfer model of wheat grain bulk established in this paper is not only suitable for the numerical simulation of temperature change of wheat grain bulk in silo under ventilation,but also can accurately predict the change rule of water content of wheat grain bulk.Compared with the uniform porosity distribution model,the numerical simulation results based on the non-uniform porosity distribution model have a better agreement with the experimental results in predicting the temperature change of wheat grain bulk.(3)Using the numerical simulation method and parameters after the above verification,the influence of different ventilation cage heights on the cooling effect of wheat grain stacks on the inner wall of the silo was studied.By comparing the streamline distribution and velocity distribution in the silo under different ventilation cage heights,the reasons for the non-normal temperature distribution of cold skin and hot core during the ventilation test of wheat grain stacks in silos were analyzed.Finally,the temperature distribution and cooling effect of wheat grain stacks under different ventilation cage heights were compared and analyzed,and the ventilation cage height with better cooling effect was determined.(4)Based on the optimized silo ventilation system,the numerical simulation method in this paper is used to explore the influence of different ventilation temperature and ton ventilation rate on the change rule of temperature and water content of wheat grain pile in silo.The results show that under different ventilation temperature conditions,the disappearance speed of high temperature accumulation area inside wheat grain pile gradually accelerates with the decrease of ventilation temperature,and the water content of wheat grain pile is that the longer the ventilation time,the greater the rewetting area near the position of ventilation cage.With the increase of the ventilation rate per ton,the overall temperature reduction rate of the grain pile is correspondingly accelerated,and the temperature distribution is more uniform,but the area of the rewetting area near the ventilation cage becomes larger.