Browsing by Author "Uiso, Christian Basil Sawaki"

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    Convection currents in bulk stred grain: detection, causes and consequences.
    (University of Dar es Salaam, 1989) Uiso, Christian Basil Sawaki
    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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    A physical approach to piche evaporimeter behavior
    (University of Dar es Salaam, 1979) Uiso, Christian Basil Sawaki
    In this study a physical approach to Piche evaporimeter behaviour is presented in an attempt to interpret the indications of the instrument. Equations that are normally used in boundary layer calculations of mass and heat transfer from horizontal flat surfaces are outlined and assessed for their validity of application to Piche evaporimeter conditions. Where possible modifications are made. The problems of pure forced and pure free simultaneous convective hat and mass transfer and of mixed convection are discussed and solutions are suggested for the case of application to the Piche instrument. Measurements are reported of evaporation rates from the Piche which were made both indoors (controlled condition) and in a Stevenson screen outdoors (natural conditions as in use in East Africa), indoors for both forced and free convection conditions. A normal table fan was used as a source of wind in the indoor measurements to give a controlled horizontal air stream of speed ranging from 0 (fan off) to 5.0ms-l. Air temperature and humidity were recorded and the temperature distribution over the surface of the evaporating paper was carefully measured by thermocouples. For the outdoor measurements air movement in the screen was also continuously measured using a thermal anemometer. No clear cut correlation appeared to exist between air movement rates inside and outside the screen. A complete description of the instrumentation and experimental procedures is given. The determination of the characteristic dimension for the paper of the Piche., the average surface temperature and the environmental parameters used as input data in the calculation is outlined. Calculated evaporation rates are compared with those obtained experimentally. Calculated indoor evaporation rates were lower than the measured rates by 4% on the average for the forced convection conditions and by 26% for the free convection conditions. These results show a remarkable success in the application of the forced convection flat plate boundary layer equations to the paper of the Piche instrument under pure forced convection conditions (air stream speed between 0.80 and 5.0ms-l) and the need for further research of the free convection heat and mass transfer processes. The outdoor evaporation results for the same horizontal air movement as for the forced convection conditions indoors are comparable with the indoors forced convection results. For lower air movement rates (between 0.55 and 0.25 ms-l) a mixed convection regime appeared to prevail. No turbulence intensity influence appears to exist to enhance the mass transfer in horizontal air flow and apparently an influence of the particular surface temperature distribution over the paper on the transfer is also absent. The place of these results in ongoing literature debates is outlined. A discrepancy between our fan measurements and those of Parlange, et al. Has been attributed to bluff body effects in their experimental set up. A combination of pure and free convection contributions appears to improve the results in mixed convection cases. Finally, consequences regarding the use of the screened Piche evaporimeter and problem in extrapolation of Piche data to those of other evaporating bodies are briefly discussed. Our main conclusion must be that the physical approach to this instrument definitely has disqualified its within screen use for any useful purpose. Suggestions are made on possible applications of the instrument outside a screen.