Evaluation of Convective Heat and Mass Transfer Coefficients for Different Products in a New Natural Convection Solar Dryer

Abstract

The purpose of the present study was to evaluate the Nusselt and Sherwood numbers during the drying of different products in a new conical type natural convection solar dryer. A comparison with the box-type collector-mode solar dryers was done. So, three types of solar dryers were designed and fabricated at Faculty of Engineering-Shebin El-Kom, Minoufiya University, Egypt. The dryers were constructed from materials that are readily available in the local area. The conical solar dryers with cone height to dryer diameter ratios ranged from 0.5 to 2 were tested. The performance of solar dryers was evaluated at the month of July 2009. The range of climatic conditions during the test period varied from 23 °C to 37 °C for ambient air temperatures and from 30 % to 55 % for the relative humidity during this test period. The products used in these experiments (grapes, apricots, onions and potatoes) were obtained from the local markets and were cleaned well from dirties before the drying process. The Nusselt and Sherwood numbers were calculated with the aid of both moisture evaporation measurements and Chilton-Colburn analogy. The results indicated that the highest Nusselt and Sherwood numbers occur for the conical solar dryer with height to dryer diameter ratio of unity. It is also observed that, the values of Nusselt and Sherwood numbers depend upon the drying mode and the product to be dried. In additions, the highest values of the Nusselt and Sherwood numbers were obtained for potato slices compared with the other products. The analogy between heat and mass transfer coefficients was also investigated.