Mathematical modelling of selected agricultural crop dryers for the humid tropics.

摘要

Mathematical models for various types of crop dryers for use in rural communities in the humid tropics have been developed. The following dryer types have been considered: (i) open sundrying, (ii) direct solar dryer, (iii) high-temperature continuous flow dryer, (iv) natural convection flow pit dryer and (v) indirect solar dryer. The models have been developed to provide an understanding of the flow phenomena, temperature, heat and mass fluxes in the dryers and to provide a method for the systematic design, development and evaluation of such crop dryers.;Technical, economic and socio-cultural aspects of the drying systems considered have also been briefly discussed to provide guidelines for researchers in their future studies and for end users in solving their crop drying problems.;The results obtained from the models indicate that drying rates in open sundrying and in the direct solar dryer are affected by the amount of solar radiation received, the crop bed thickness and the type of crop being dried. Ambient conditions such as air temperature, relative humidity and wind speed do not significantly affect the drying rate. The optimum paddy thickness for the conditions considered is about 3 cm with open sundrying and about 4 cm with direct solar dryer. The model for the high-temperature continuous flow dryer was found to provide a good indicator of the optimum length of the drying cylinder. The grain outlet temperature predicted by the model is comparable to that observed experimentally. The grain drying model gave acceptable estimates of the drying rate. The natural convection flow pit dryer model indicated that the dominant factors affecting the drying rate are the heat exchanger temperature and the depth of the crop. The distance between the crop bed and the heat exchanger has a weak effect on the total heat flux to the crop and radiative heat transfer contributes 2/3 of the total heat flux to the crop. The study of indirect solar dryer indicated that the air flow through the bed and the temperature of the crop were uniform for the conditions considered.