Indirect mode solar drying of stevia (Stevia rebaudiana) leaves: experimental and numerical investigation of fluid flow pattern, moisture, and temperature distribution profile

Present research deals with analyzing the drying characteristics, temperature profile, fluid flow pattern, and moisture profile during solar drying of stevia (Stevia rebaudiana) leaves. Utilizing an indirect mode solar dryer (ISD), the study explores both natural convection (NC) and forced convection (FC) conditions. In the FC mode, a substantial decrease in stevia moisture content was observed, leading to a notable 21.43% reduction in drying time compared to the NC mode. The average moisture diffusivity in FC readily accelerated with time, reaching an average value of 2.89 × \({10}^{-10}\) ${10}^{-10}$ m²/s, which was found to be 28.44% higher than NC. In the analysis of experimental data, nine thin-layer drying models were employed, and the Midilli and Kucuk model emerged as the best fit for both NC and FC modes. Akaike weights (w_i) played a key role in model selection, facilitating the comparison and assessment of models through likelihood principles. Employing COMSOL Multiphysics^® software, a 3D finite element model was developed to analyze temperature, moisture distribution profiles, and fluid flow patterns. NC mode revealed the presence of air vortices in the dryer, resulting in localized temperatures and causing significant microstructural and color degradation compared to the FC mode.