【文章內(nèi)容簡介】 as based on the assumption that natural air convection is the prevalent phenomenon within grain mass. The mathematical model was then tested experimentally [13]. For the tests, Khankari used a cylindrical silo, 10 m high and 10 min diameter, in which he stored maize grain with an average moisture content of 14% at an average temperature of 25176。C for the period of one year, beginning from October, without ventilation. Values of thermal conductivity calculated by means of the model conformed to the results of the experiment. Khankari et al. [12] also gave the values of the other parameters of diffusion for maize grain. They found that water migration increases with increasing temperature. During the initial period of storage, ., during the autumn rainy period and early winter, water migration is limited to areas close to the silo walls. The effect of natural convection on water flow appears at the end of December and at the beginning of January, when temperatures reach the maximum levels. Therefore, water migration rate is the fastest in winter. The studies showed that the increased thermal conductivity of grain has a limiting effect on natural air convection, and that water migration takes place in silos of all sizes, though it begins earlier in smaller silos. Cooling the grain down to 0176。C in the autumn permits its moisture content to be kept stable throughout the year. Lo et al. [17] used Chen39。s and Clayton39。s equation for the simulation of radial changes in the moisture content of wheat grain stored in a concrete silo. The equation was based on the assumption that moisture content changes are only related to temperature. Thompson [20] and Fan et al. [3] were involved with modeling the process of ventilation. Thompson [20] developed a model representing temperature changes of grain in storage, its moisture content, and dry mass distribution. He arrived at the conclusion that a true balance between the air and the grain is possible to maintain when the grain is ventilated with ambient air at low flow rates. Fan et al. [3] studied water diffusion in various varieties of wheat. They found that the coefficient of water diffusion in wheat grain can be expressed in the form of an opposite to the exponential function of absolute temperature,and the coefficient does not change its value for hard wheat within the temperature range of 2654176。C. They determined the coefficients of diffusion for several wheat varieties within a temperature range from 26 to 98176。C. The values spanned a range from2x1012 to 245 x 1012 m s1, depending on the temperature and the wheat varieties. Chang et al. [2] maintain that the average moisture content of grain stored during time t + △t is: Wu=W0+(H0Hu)Mr (1)where: Wu average moisture content in the grain layer, final or subsequent simulation for △t period, kg kg1 (decimal, .)。 W0 moisture content, initial or prior to simulation for △t period, kg kg1 (decimal, .)。 H0 humidity ratio of ambient air, kg kg1。 Hu humidity ratio of air leaving the grain layer, kg kg1。 Mr mass ratio of inlet air to the dry grain during △t.Chang et al. [1] studied wheat grain with an initial moisture content of %, stored in silos m high and in diameter. On the basis of the studies, they concluded that the simulation values of the grain moisture content coincided with the gain moisture values measured during a period of 15 months and that the moisture content in the layer close to the surface decreased by 2 to % during the summer months, while in the central and bottom parts of the silos, the changes in grain moisture content were only slight. Modeling of temperature and the moisture content of rice stored in silos was the subject of interest for Freer [4], and Haugh et al.[8]. Haugh et al. [8] conclude that grain temperature is the most important parameter in grain storage and should be maintained at 1015176。C irrespective of the broad range of the grain moisture content levels. According to those authors, grain temperature is the most significant, though grain moisture content is also very important. According to Freer et al. [4], the air temperature around the silo should be known in order to calculate the temperature differences between the grain in the silo and the ambient temperature. They presented equations for the calculation of the mean diurnal temperature for the year, taking into account the latitude, and for the determination of the moisture content of unpolished rice, as well as of dry mass losses. The experimental part of their study was performed by mans of a twodimensional model which they used to analyze changes in temperature and moisture content, the level of dry mass losses, and the level of water condensation. In their study they used initial grain temperatures of 10, 20 and 30176。C, moisture content levels of 11, 13 and 15%, and three charging times. In the test program they assessed the initial temperature of grain, the initial grain moisture content, and the charging time (the time of filling the silo with grain). Observations were conducted for 12 months. The charging time was found to have had little effect on the parameters under study. Relatively high losses of dry mass were observed at grain temperature of 30176。C at 15% initial moisture content. High initial temperatures and moisture content levels had a significant effect on water migration towards the top of the silo, which means that the top area is more conducive to the grain turning bad and to increased microbial activity. Increased grain temperature causes an increase in the pressure exerted by grain on the walls and bottom of silos. The effect of the properties of the material stored (sand, shelled maize, wheat, and sorghum) on lateral pressures induced thermally were studied by Puri et al. [18]. The results of the experiments indicate that thermally induced stress in storage tanks depends on the bulk density of the material stored. To calculate t