SIMULATION OF THE INDIRECT NATURAL-CONVECTION SOLAR DRYING OF ROUGH RICE

A mathematical model to simulate the indirect natural convection solar drying of rough rice is presented. The temperatures along the collector are described by a numerical solution. The drying of the grain is described by the introduction of a deep-bed solution procedure. The variation of the air temperature in the collector and across the air bed is incorporated in the prediction of the thermal buoyancy effect. The results show that, at the low air flow rates produced by natural convection, the drying wave moves slowly and the layer of grain cannot be considered uniform, with serious overdrying taking place in the bottom layer. The deep bed model is therefore essential in order to obtain a realistic simulation. Also, due to the temperature drop across the bed, the chimney height makes little contribution to buoyancy. Sensitivity analysis showed the system to be most sensitive to grain bed depth and collector length and least to chimney height and cover to absorber spacing. The simulation results have a number of practical implications. The grain bed should be turned regularly to avoid over-drying and give a tempering effect. The collector and plenum chamber upstream of the grain bed should be well sealed to contain the buoyancy pressure. The effect on performance of reducing or removing the chimney should be examined.