Design, experimental characterization and numerical analysis of a compact nearly spherical solar dryer

A compact solar dryer with a nearly spherical geometry is investigated through a combined experimental and numerical approach to assess its thermal behaviour under controlled and real operating conditions. An extensive experimental campaign is carried out, including outdoor tests under natural solar irradiation and indoor experiments performed using a solar simulator, which allows reproducible testing conditions for the experimental characterization of the dryer and subsequent numerical validation. No-load thermal tests are conducted to characterize the temperature field inside the drying chamber, verifying the suitability of the achieved temperature levels for apple drying. Complementary drying tests with apple slices are then performed, confirming the actual drying capability of the prototype under both indoor and outdoor conditions. Based on the no-load experimental temperature measurements, a three-dimensional CFD model accounting for buoyancy-driven airflow, conjugate heat transfer, and radiative effects is developed and validated, with average deviations comparable to the measurement uncertainty. The validated CFD model is subsequently employed as a design tool to investigate the influence of the absorbent surface material. Replacing painted steel with aluminium improves vertical temperature uniformity due to the higher thermal conductivity, but reduces the intensity of buoyancy-driven ventilation. Overall, the combined experimental and numerical analysis provides quantitative insight into the thermal performance of the proposed dryer and supports informed material-based design choices for naturally ventilated solar drying systems.