| Food is the first in politics and the primary need of the people.Food security is a major prerequisite for national security,and reducing losses during storage is an important way to ensure food security.The breakage and segregation of maize kernels happened easily in the process of grain warehousing,which is the main reason for the complex distribution of porosity inside the maize bulk.Because it can significantly change the heat and moisture transfer path in the grain bulk,the complex porosity distribution that exists in the natural packing maize bulk is the main reason for the local heating problem.And the broken kernels increase the risk of fungi erosion,stimulating the cycle of“fungal growth–temperature rise”,seriously affecting the quality and safety of stored grain.Given this situation,the study focused on the impact of kernel breakage,segregation and compression deformation affecting on heat generation and coupled heat and moisture transfer in grain storage.Taking the maize grain bulk in storage as the object,a combination of theoretical analysis,experiments,and numerical simulation was used to analyze the differences in porosity distribution caused by natural packing maize bulk,as well as the impact of the breakage and segregation of maize kernels on grain storage quality,and the method and conclusions were applied to grain storage and food security prediction.The main research content of this study are as follows:（1）Developed an anisotropic porosity distribution model for maize bulk considering the effects of the breakage,the segregation and compression deformation.Firstly,through the natural packing experiment of maize,the distribution patterns of defective kernels,inorganic impurities,foreign kernels,and organic impurities were summarized and analyzed based on segregation mechanisms such as trajectory,fluidization,sieving,and impact separation.Considering the effects of the breakage and segregation,the porosity distribution relationships along the radial and vertical directions of grain bulk were developed.The porosity in the center of the silo was the smallest,and the porosity gradually increased with the radius of the silo increases.The average porosity near the sidewall of the silo increased by 7.16%compared to the center.Then,the variation of porosity in the vertical direction was corrected based on?～σmodel.The compression deformation made the distribution difference of porosity in the vertical direction of the maize bulk more obvious,and the average porosity at the top of the silo increased by 2.8%compared to the average porosity at the bottom.Finally,an anisotropic porosity distribution model for maize bulk was developed,considering the combined effects of drop height,initial fines content,and compression deformation.The corresponding conclusions lay the research foundation for the analysis of coupled heat and moisture transfer inside grain bulk based on anisotropic porosity.（2）Developed a creep model of maize bulk considering the effect of the kernel breakage.Based on the uniaxial compression experiment of maize bulk unit,the effects of vertical pressure,broken kernel content,and representative broken kernel distribution on the compression deformation of the maize were analyzed.Kernel breakage and vertical pressure could lead to a large compression deformation of maize bulk.The compression deformation increased with the increase of the content of broken kernels.The distribution form of kernels and the initial density of maize affect the vertical compression of maize bulk.Under vertical load,the deformation of maize mainly undergoes three stages:instantaneous deformation,rapid deformation,and steady-state creep deformation.The deformation during the increase in load stage was the main reason for compression deformation.The creep stage curves of maize bulk with different broken kernel contents were basically parallel.The three-parameter creep model,which was connected in series with the Hook spring and Kelvin model,could accurately describe the creep process of maize bulk under uniaxial compression conditions.The creep deformation coefficient in the steady-state creep stage was-0.07.（3）A newly empirical method was developed to analyze the heat generated by fungi in grain bulk.A coupled heat and moisture transfer model considering compression deformation was developed to simulate the temperature changes inside the grain bulk caused by physical conditions such as heat conduction and convection at room temperature.To verify the effectiveness of the numerical model,physical heat transfer for 3 different materials,i.e.,quartz sand,wheat,and maize,were investigated with both testing and FE methods.A newly empirical method was developed to analyze the heat generated by fungi in grain bulk.Based on the uniaxial compression experiment results of maize,the effects of vertical pressure,broken kernel content,and representative broken kernel distribution on the generated by fungi in grain bulk at room temperature were comprehensively analyzed.The number of aerobic plate counting was positively correlated with the temperature of maize and the content of broken kernels,while negatively correlated with vertical pressure.The distribution of kernels affected the development of fungal growth in grain bulk.The aerobic plate counting,temperature,and calculated heat results of heat generated by fungi in grain bulk under different conditions all correspond well.The effectiveness of the method for the heat generated by fungi during the storage process of maize bulk was verified.（4）The law of coupled heat and moisture transfer in no-moldy and locally heated maize bulk under gradient temperature field environment was summarized.The left wall of the multi-field coupling experimental device of grain bulk was set as a cold wall and the right wall was set as a hot wall.The effects of heat and moisture transfer and local heating on the temperature and humidity distribution in maize bulk under the condition of 30°C/m constant gradient field were analyzed.The micro airflow in the no-moldy grain bulk flowed in an inverse temporal manner,and the heat transfer rate in the upper layer of the maize bulk was greater than that in the lower layer.The gas concentration inside the maize changed relatively little,and the respiratory function of the grain was relatively low.In a gradient temperature field environment,there would be a point（or area）in the grain bulk with 19.69%moisture content that was suitable for the germination and growth of fungi.After the formation of local hotspots,the cycle of“fungal growth-temperature rise”promoted the rapid development of fungi.The heat transfer caused by gradient temperature field and the heat change caused by fungi activity in grain bulk were considered separately,and the heat reached its peak after stored for 12 d.The result of heat calculation based on temperature sensor detection was higher than that based on CO2 concentration.As storage time increases,the physical factors required for fungal growth were gradually depleted,leading to a decline in fungal growth,manifested by a decrease in temperature in the hot spot area.The impact of kernel breakage effect on the growth of fungal growth in maize bulk was further considered.Under the condition of a moisture content of 16.3%and the same gradient temperature field,the highest temperature value and average respiratory rate caused by fungal heating in the 6.14%broken kernel content maize bulk were 3.05%and 3.9%higher than those in the 4.26%broken kernel content maize bulk,respectively.（5）A mathematical element model for heat and mass transfer during the ventilation process of maize bulk based on anisotropic porosity distribution was developed.Based on the multi field coupling simulation experimental platform of grain storage environment,the anisotropic porosity model of maize considering the breakage,the segregation and compression deformation established through natural packing experiments was introduced into the heat moisture coupling transfer model of grain bulk ventilation process.A pilot silo ventilation numerical model based on anisotropic porosity was developed,and the effectiveness of the numerical model was verified through vertical ventilation experiments.The differences in heat and moisture coupling transfer between homogeneous porosity distribution and anisotropic porosity distribution in grain bulk were compared and analyzed.According to the distribution law of broken kernels,the high moisture wet maize was buried in the area with high broken kernel content in the upper layer of the grain bulk to study the influence of hot spot formation on the temperature and humidity distribution of the maize bulk in the silo.The ventilation resistance,temperature and humidity distribution,and energy consumption of vertical and radial ventilation methods for grain bulk were compared and analyzed by using numerical simulation methods.The present method and corresponding conclusions are hoped to be helpful to provide necessary theoretical and key technical support for storage safety and precise control of grain storage. |
