Research Of Coupled Heat And Moisture Transfer In Hygroscopic Grain Bulk During The Warehousing Storage

The warehousing storage method is commonly used for the cereal grain after harvest. Cereal grain as an organism, it has moisture adsorption and desorption and respiration characteristics. Temperature and moisture are the two most important factors governing the quality of cereal grain during storage and air condensation. With changes of temperature and moisture in packed bed of stored cereal grain, it is mainly caused by the seasonal changes of the outside air temperature. After considering the grain harvest, during most of the time it is in the natural storage stage, that, non-ventilation drying process natural storage. Therefore, this paper considers the natural storage process of hygroscopic cereal grain bulks as the object of study, and then researches and analyzes the law of heat and mass transfer.By means of theoretical analysis, numerical simulation and experimental investigation, considering typical grain wheat as porous media matrix, focus on internal heat and moisture migration process with the sophisticated sources of thermology and humidity in hygroscopic porous media under local climate conditions, explore dynamic variation of temperature and moisture, and to provide a theoretical basis for the prevention of the stored grain fever mildew and condensation and take corresponding measures.Seasonal variations in ambient temperature can prompt safe moisture contents of warehousing storage grain bulks to fluctuate, due to the initial temperature gradient causing the natural convection of air in the porous medium and the migration of moisture and redistribution. From the perspective of fluid natural convection in porous media, it can analyze the interactive relationship among fluid flow in the static packed bed, temperature and moisture content gradient, and would obtain the coupling mechanism of hygroscopic porous media on the flow field, temperature field and humidity field. In this paper a mathematical model is developed by reflecting the change in air moisture on the grain moisture through the sorption isotherm. It is made of some coupled, unsteady and nonlinear partial differential equations. From the point of view of the mathematical relationship, it can clearly reveal the coupling effect of the flow and heat and mass transfer by means of convection and source terms. Moreover, the extended mathematical model is analyzed and conducted based on the finite element theory.Based on local heat and mass balance principle and macroscopic volume average method, the use of multi-physics numerical simulation techniques can simulate eight kinds of cases. The model is applied to a10m diameter,10m height cylindrical bin containing wheat, and is solved in a two-dimensional cylindrical coordinate system. Conditions approximating winter and summer extremes are independently simulted for a period of150days in each case. Two sets of constant wall and grain temperatures,273K and293K (0and20"C),263K and303K (-10and30℃) are evaluated. Similarly, two initial moisture contents,14%and18%(w.b.) are exmined. However, all boundaries for moisture flow are considered to be impermeable. The lower boundary of the bin is assumed to be adiabatic while the top boundary and outer walls are assumed to be isothermal at contant ambient temperature. These boundary conditions make the calculation domain axisymmetric, and hence calculations are performed on half of the bin profile. The approximate winter and summer conditions of temperature and moisture content trend is similar. Under winter conditions, migration of moisture occur toward the lateral and top boundaries, and is concentrated in relatively narrow bands near the edge. The maximum moisture contents occur in very narrow zones at the boundaries. Under summer conditions, moisture moves form the top and lateral edges inward creating a zone of increased moisture adjacent to a zone of decreased moisture nearer the interior. Temperature profiles show that in the lower half of the domain temperature varies only in the horizontal direction, while in the upper half of the domain, these changes can be seen in both the horizontal and vertical direction. For four kinds of cases, the initial temperature difference is increased from20"C to40"C, and moisture content is increased from14%to18%, and then it is contrasted to the changes of temperature and moisture contents. Finally, with a similar principle, the validity of the numerical model is ascertained by comparing the experimental results with the predicted results.