Numerical Simulation And Experimental Study On Heat And Mass Transfer Of Stored Grain Pile

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.