【作者】 王振华；

【导师】 刘相东；

【作者基本信息】 中国农业大学 ， 农产品加工及贮藏工程， 2014， 博士

【摘要】 为了实现粮食的安全储藏,需要监控仓内温度、水分的变化。结合数值模拟的方法预测仓内水分与温度的未来变化,可制定出更具前瞻性的方案。因此,本文建立了描述仓储粮堆温度和水分变化的三维六参数的湿热传递数学模型。主要工作如下：(1)针对粮堆的特点,以小麦为研究对象,确定了描述小麦粮堆多孔介质的结构参数,包括孔隙率、渗透率、当量直径、比表面积等,并给出了这些参数的试验测定方法。通过对粮堆中水分的存在形式和传递机理进行分析,确定了通风干燥模型和静态储藏模型应该考虑的水分传递途径和热量传递途径。(2)采用虚拟连续介质的方法,在粮堆中选取代表性单元体,采用局部非平衡法,依据质量守恒、能量守恒和动量守恒定律,建立了由气体的质量和热量控制方程、谷物相的质量和热量控制方程、连续性方程和达西定律组成的三维六参数通风干燥模型,模型中的热量与质量是相互耦合的；采用局部热平衡法,对通风干燥模型简化,引入气体流动的体积力,建立了粮堆的静态储藏模型。(3)利用小型试验仓进行了小麦的固定床干燥试验,测定了小麦和空气在干燥过程中不同位置、不同高度处的水分含量和温度,测定了小麦密度、孔隙率、颗粒尺寸等参数；利用彩钢板储粮仓进行了小麦的短期储藏试验,通过温度传感器采集了不同位置、不同高度处的粮食温度。(4)利用有限元软件COMSOL Multiphysics求解数学模型,只要输入建立的数学模型,设定初始条件和边界条件,便可实现快速求解,为复杂系统数值模拟的工程应用提供了可能。利用建立的模型分别对小麦静态储藏和通风干燥过程进行了模拟,并结合试验数据进行了分析。结果表明,模拟结果与试验结果较吻合,静态储藏模型能够较好地描述储粮仓内的温度分布,并可模拟仓内空气的自然对流情况：通风干燥模型可以较好地模拟通风干燥过程中的各个参数的变化。(5)利用湿热传递模型对静态储藏和通风干燥过程进行了分析,还分析了通风方式、孔隙率、谷物比热容等对干燥过程的影响。模拟结果表明：1)加入垂直隔膜,可减弱静态储藏期间外温变化对仓内温度的影响。2)固定床干燥有明显的干燥前沿,干燥不均匀度较大,且干燥前沿附近的温度梯度最大。3)环型通风方式由于通风死角面积小、压力损失小,其干燥速率比U型通风大。4)粮堆孔隙率越大,气体的流动阻力越小,孔隙中的空气湿度越低,干燥速率越大。5)谷物比热容随温度和水分变化较大,采用非恒定的比热容建立模型,模拟值更接近试验值。更多还原

【Abstract】 For safe storage of grain, the temperature and moisture content of grain must be monitored frequently. Based on the data, the prospective solutions can be made by numerical simulation of the future change of heat and moist transfer in the grain pile. Therefore, a three dimensional heat and moist transfer model with six parameters is built. The main works are as follows:(1) Considering the character of grain pile and taking wheat as the research object, the parameters describing the grain pile are confirmed, including porosity, permeability, equivalent diameter and specific surface area. Also, the methods are given to measure these parameters. The form of moisture existence is demonstrated, and the heat and moist transfer path is determined for the heat and mass transfer model and the static storage model.(2) Based on the hypothesis of virtual continuous continuum, the three dimensional heat and moist transfer model is built with the application of laws for the conservation of energy, mass and momentum. The model employs the method of local non-equilibrium and contains six equations including mass conservation in the air, mass conservation in the grain, energy conservation in air, energy conservation in grain, equation of continuity and Darcy law. The mass and heat transfer in the model is coupled. Also, a heat and moist transfer model for static storage is built by employing local thermal equilibrium, and it is simplified by the non-equilibrium model. The static storage model could simulate the natural convection of air in the bin due to the consideration of volume force.(3) The fixed bed drying experiment of wheat is operated in the test bin. Four parameters, in different height and position, is determined during the drying process, including the temperature and humidity of air and grain. Also, the porosity, permeability, equivalent radius and density are determined, respectively. Besides, the storage test is carried out in the steel silo, and the grain temperature at different height and position is monitored by the temperature sensor.(4) The models are solved by COMSOL Multiphysics with given initial conditions and boundary conditions, which are the same as the drying experiments. Drying process and wheat storage process are simulated with the above models. The results of numerical simulation are in good agreement with the experiments. The equilibrium model can simulate the temperature distribution and natural convection, and the non-equilibrium model can simulate the changes of heat and moist in the drying process.(5) Based on the above two models, the drying and static storage process are simulated and analyzed in various conditions. The results indicated that:1) By adding vertical membrane in the steel silo, the grain temperature is less affected by the temperature changes of environment.2) There is an obvious drying front in fixed bed drying. Moisture non-uniformity is serious and temperature gradient is big near the drying front.3) The drying rate of loop duct method is bigger than the U type duct method for the reason of less ventilation dead zone and pressure loss.4) Bigger porosity makes less air resistance and higher drying rate.5) The specific heat, which is changed with the grain temperature and moisture content, makes the simulated results closer to the experimental results.更多还原