储料竖向压力对粮仓中小麦粮堆湿热传递的影响

粮仓中存在压力场、温度场和湿度场等多物理场,为了得出各物理因子共同影响下的粮堆内湿热传递规律,该研究利用自行研制的粮堆多场耦合试验装置,针对仓内小麦粮堆单元体,研究在高温边界38.5℃、低温边界5.2℃,初始粮温25.8℃,竖向压力分别为50、100、150 kPa条件下小麦粮堆湿热传递情况。试验结果表明:竖向压力增加,粮堆孔隙率减小,热量通过粮食籽粒间传导增加,传递速率加快,竖向压力从50 kPa增大至150 k Pa,粮温较入仓时下降约0.5~1.3℃,温度梯度变化率达8.7%,不同压力下粮堆高温区面积随储藏时间呈幂函数减小。粮堆内湿空气在边界处累积至峰值时会有部分湿空气向粮堆内迁移。粮堆中部与靠近低温边界温差大于6.3℃时,粮堆内湿空气扩散加快,粮堆中部平均相对湿度下降速率随竖向压力增加而加快。研究结果可为散装粮堆多场耦合研究提供理论支持。

Effects of vertical pressure on moisture and heat transfer of wheat bulk in a granary

Multiple physical fields, such as pressure, temperature and humidity, can pose grains deterioration in the wheat bulk during storage. It is necessary to explore coupled moisture and heat transfer in wheat bulk under multi-field conditions for the safety of grain storage. In most previous studies, the significant findings are: 1) the porosity decreases with the increase of grain depth;2) the contact area between grains increases with increasing vertical pressure;3) the thermal conductivity of wheat bulk is 49 times that of air;and 4) the thermal conductivity between grains decreases with the decrease of porosity. However, an accurate prediction of grain moisture and temperature during storage is still lacking, in order to develop efficient strategies of ventilation conditions. This paper aims to find the influence of vertical pressure on the moisture and heat transfer in the wheat bulk, taking a wheat bulk unit in the silo as the research object, and thereby an experimental study was performed on a multi-field coupling test device. Temperature and humidity of the wheat bulk were measured under three vertical pressure conditions of 50, 100 and 150 kPa, according to the pressure range in large-scale wheat bulk. The temperature was set as high temperature boundary of 38.5℃, low temperature boundary of 5.2℃, and initial grain temperature of 25.8℃. The experimental results show that the grain temperature reduced by 0.5℃ in the vertical pressure of 50 k Pa, compared with the initial grain temperature, whereas, it reduced by 1.3℃ when the vertical pressure was 150 kPa. In a constant temperature field, the temperature gradient of wheat bulk decreased from 34.6℃/m to 31.6℃/m, while the rate of change reached 8.7% when the vertical pressure increased from 50 to 150 kPa. The area of high temperature decreased as a power function with storage period of wheat bulk under different vertical pressures, whereas, the power function index increased with the increase of vertical pressures. The temperature difference can cause the wet air in grain bulk to migrate from the higher temperature region to the lower temperature region. In the middle and upper grain bulk with the height greater than 0.3 m, under the action of a large temperature difference between the grain bulk and the high temperature boundary, the wet air in the grain bulk uniformly migrated from the high temperature boundary to the low temperature boundary with the micro airflow. In the middle and lower grain bulk with the height less than 0.3 m, the temperature difference between the two temperature control boundaries was small, and the micro-airflow effect was weak, due to the bottom of the unit cannot be absolutely insulated. The wet air near the high-temperature boundary moved to the low temperature, while the relative humidity was high below near high temperature boundary. A small amount of wet air in the grain bulk can migrate from the low temperature boundary to the wheat bulk after the relative humidity near the low temperature boundary region reached its peak value. When the temperature difference between the middle part of wheat bulk and the boundary near the low temperature was greater than 6.3℃, the migration rate of wet air in grain bulk increased with micro airflow, and the decrease rate of average relative humidity in the middle of wheat bulk increased with the increase of vertical pressure. The findings can provide theoretical support for the research on the evolution of grain temperature and moisture in the multi-field coupling of stored grain bulk under different ventilation conditions.