Indirect and Mixed Mode Solar Drying Mathematical Models for Sultana Grape

Many research studies have been performed on forced convection (active) solar dryers for fruit and vegetables. A short survey of these showed that applying the forced convection solar dryer not only significantly reduced the drying time but also resulted in many improvements in the quality of the dried products. Active indirect and mixed-mode thin layer solar drying experiments were conducted on Sultana grapes. A combination of a solar air heater and a cabinet dryer was designed, constructed and tested for this product in the Agricultural Engineering Department at Shiraz University, Iran. Three air flow rates (0.085, 0.126 and 0.171 m3 s-1) and two types of drying systems (indirect and mixed-mode) were adopted. The south wall of the drying chamber was covered by a sheet of glass for mixed-mode and the glass sheet was covered with a thick sheet of cloth for an indirect solar drying system. Seven well-known thin layer drying models were used separately to fit the mixed-mode and indirect type experimental data for Sultana grapes. For experimental indirect data, the Modified Page model (r= 0.998, χ2= 0.000241) and for experimental mixed-mode data, the Page model (r= 0.999, χ2= 0.000169) showed the best curve fitting results with highest r (correlation coefficient) and lowest χ2 (reduced chisquare) values. The constants in these models explain the effects of drying parameters, air velocity and temperature. To take account of these effects, the best correlation equations between the constants and drying parameters were also introduced using multiple regression analysis.