Thermal and Airflow Modelling of a Natural Convection Solar Dryer for Okra Drying Under Central African Climatic Conditions

Post-harvest losses of perishable agricultural products remain a significant challenge in Central Africa, particularly in regions such as N'Djamena, where high ambient temperatures and limited access to preservation technologies accelerate food spoilage. Okra (Abelmoschus esculentus), widely consumed in the region, is especially susceptible to deterioration due to its high moisture content. Solar drying offers a sustainable and energy-efficient solution to extend shelf life while maintaining product quality. This study presents a comprehensive thermal and airflow modeling of an indirect natural convection solar dryer for okra drying under typcal Central African climatic conditions. The proposed model integrates the fundamental mechanisms of buoyancy-driven airflow, convective heat transfer, and moisture diffusion within a coupled framework based on the conservation of mass, momentum, and energy. Airflow within the system is induced by the stack effect resulting from temperature differences between the inlet and outlet, leading to continuous natural circulation of air through the dryer. The thermal performance of the system is analyzed through energy balance equations applied to both the solar collector and the drying chamber. The drying process is further characterized using a thin-layer drying approach, allowing the prediction of moisture removal over time. Climatic conditions representative of the region, including high solar radiation and low relative humidity, are incorporated into the model. The results indicate that increased solar radiation enhances air temperature and airflow rate, thereby improving drying efficiency. The drying process occurs predominantly in the falling-rate period, suggesting that internal moisture diffusion governs the kinetics. Overall, the study demonstrates that natural convection solar dryers are well adapted to semi-arid environments and provide an effective, low-cost solution for reducing post-harvest losses and improving food preservation.