Convection currents in bulk stred grain: detection, causes and consequences.

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University of Dar es Salaam
The phenomenon of Iocalized moisture content build-up in bulk stored grain has been attributed to several processes including the existence of cyclic air convection currents which transfer moisture from warm to cooler parts of a bulk. Other moisture transfer mechanisms are not quantitatively considered in this study, although at the end of the second field trial period internal raining is explicitly considered to have contributed to the observed moisture and temperature distributions. A simple theoretical model has been derived to predict the airflow velocities for a given temperature difference predicted velocities ranged from 1.3x10-4MS-1 to 5.0x10-4MS-1 for temperature differences ranging from 7oC to 25oC, the higher velocity being induced by the larger temperature difference. The convection flow was first investigate experimentally in a laboratory apparatus which was set up using two cylindrical vertical columns of shelled maize interconnected at the top and bottom by two horizontal tubes containing air and maize respectively, Convection flow was detected by a tracer gas technique using sulphur directly proportional to the temperature difference in the range 17oC to 25oC and in good agreement with theoretical predictions. Moisture accumulation was detected in the top portion of the cool grain. Two field trials were conducted in a 250-tonne capacity steel silo, filled with maize, to confirm the existence of convection currents, measure their velocities and to field-test low air flow, remote moisture content and temperature measuring equipment were identified. Diurnal temperature changes were found to significantly penetrate the grain bulk to a depth of only 0.40m the surface. For the first time intergranular air convection in practical bulk storage due to such (virtual) temperature differences was detected and measured. Relatively strong downward (1.9x10-4 to 3.3x10-4MS-1 and weak upward (0.2x10-4 to 2.2x10-4MS-1) flows were measured near the silo walls and in the central bulk interior respectively, when the ambient was colder than the grain bulk. Big grain moisture content changes occurred in the whole bulk during the first field trial due to poor storage conditions. In the second field trial significant moisture content changes (from 13.5% to 26.0%) occurred in the top grain and along the silo walls near the top, initially due to the convective transport but later in the period also due to internal raining and dripping. Little moisture content change occurred in the grain bulk. Some recommendations were formulated for safe and long term storage of maize under the studied conditions and the scope for future academic work has been indicated.
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Heat, Convection, Iocalized moisture
Uiso, C. B. S. (1989) Convection currents in bulk stred grain: detection, causes and consequences. Doctoral dissertation, University of Dar es Salaam. Available at (