HEAT TRANSFER BY NATURAL CONVECTION IN STORED GRAINS.

摘要

This study was conducted to investigate experimentally the mechanisms of heat transfer by natural convection in stored grain in vertical cylindrical bins with a sinusoidal outside temperature profile. Two approaches were used; (1) a laboratory model of stored grain in a cylindrical bin using spherical beads and water, and (2) a field test in cylindrical bins holding wheat.;Temperature gradient in bins of aspect ratio of 0.5 existed along the radial and axial directions, whereas in cylinders of aspect ratios of 1 to 2, temperature gradient was mainly along the radial path. This was particularly true when the cylinder had large-sized spherical packing. At high frequency of the sinusoidal bath temperature, heat transfer occurred only near the wall.;Outdoor storage tests with wheat in steel cylindrical bins indicated that the average grain temperature inside the storage bins followed the average outside air temperature profile. The time lag between a change in ambient air temperature and a change in grain temperature was approximately seven to ten days.;Temperature gradients in the radial direction were higher than in the axial direction, and were pronounced in bins with a high aspect ratio. Hence, heat flowed along the radial direction. Natural convection enhanced heat transfer in grain near the wall and at the top of the bin.;Laboratory results revealed evidence of heat transfer by natural convection in spherical packing immersed in water simulating stored grain in bins. The transport of heat in beds with 4.0 mm beads and larger was enhanced by natural convection while in 0.75 mm glass beads packing, conduction was the predominant mode of heat transfer.;Grain moisture distribution indicated moisture gradients in both the radial and axial directions. Grain moisture reduction was more rapid at the top and near the wall of the bin where the effects of natural convection was more pronounced. Thus, in all bins, grain at the top and near the wall had lower moisture than in other parts of the bin. Moisture loss in all stored grain was partly due to the lower equilibrium moisture content of wheat at the observed average ambient storage conditions during the study.;It is recommended that in the future, flow visualization of natural convective heat transfer in stored grain and field tests be done under controlled conditions simulating the yearly seasonal periodic cycle.