Experimental investigation and mathematical modelling of a natural convection Solar Tunnel Fruit Dryer
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Post-harvest Losses (PHLs) in fruits are high, in some cases, reaches 50 %. To reduce these PHLs, several technologies have been developed; among them is solar drying technology (SDT). Several SDTs are available including solar tunnel dryers. Solar tunnel dryers are described as relatively simple, and easy to construct, and this formed the basis for choice of a solar tunnel dryer in this study. To achieve a good quality product and to identify any areas for improvement in a solar dryer, experimentation is necessary. The purpose of this study therefore was to investigate the performance of a natural convection solar tunnel fruit dryer and to determine an appropriate thin layer model to predict the drying. A solar tunnel dryer comprising three major units: a black painted solar collector unit, a drying unit and a black painted vertical bare flat-plate chimney was constructed. The overall dimensions of the collector unit and drying units were 2 m×0.75 m (L×W) while the chimney was square, measuring 0.75 m×0.75 m (H×W) with 0.1 m air exit channel. To investigate its performance, 5 mm thin layer drying experiments with mango, a major fruit in Zambia, were carried out at the University of Zambia, Department of Agricultural Engineering under natural conditions. To find an appropriate thin layer model, the moisture content results were transformed to Moisture Ratio (MR) and fitted into 12 common thin layer models in literature using MatlabR2011b curve fitting tool and the best model determined based on three statistical parameters: coefficient of determination (r2), Sum of Square Error (SSE) and Root Mean Square Error (RMSE). The results showed that under solar insolation of between 470 and 1070 W m-2, air temperature of up to 70 oC was attained at the collector unit. Ambient relative humidity varied between 11.95 and 29.67 % and was lowered by over 50 % at the collector unit. Under these conditions, mango with an initial moisture content of 80 % (w.b.) was dried to between 13 and 14 % (w.b.) in 11 hours. The performance of the dryer was analyzed by calculating the collector, drying, and pick-up efficiencies. They were found to be 24.7 %, 11.3 % and 35 % respectively which compared well with values for natural convection dryers in literature. The buoyancy pressure ranged from 0.1530 to 0.3016 N m-2 and was sufficient to drive the flow of air in all the experiments. Among the 12 thin layer models tested, Midilli-Kucuk model was the best with r2, SSE and RMSE of 0.9959, 0.004634, and iv 0.01758 respectively. Based on the achieved drying time for the 5 mm mango slices, the collector and pick efficiencies, the dryer performed well. Further investigations using different fruits such as pineapple, banana and different slice thicknesses are recommended.
University of Zambia
- Engineering