Experimental study and modelling of the thermosiphon-conservation chamber coupling for the optimisation of onion storage conditions

Postharvest onion storage remains a major challenge in hot-climate regions, where limited access to refrigeration results in substantial storage losses. However, the thermo-aerodynamic performance of passive storage systems has not yet been fully characterised. This study addresses this gap through an integrated experimental and numerical investigation of the thermo-aerodynamic behaviour of a passive onion storage system ventilated by natural buoyancy-driven airflow. A multiphysics model incorporating heat transfer in solid, fluid, and porous media, together with laminar airflow, was developed and solved using COMSOL Multiphysics. The results demonstrate a significant attenuation of external thermal loads. Despite external wall surface temperatures reaching 56.3°C, the temperature fluctuation inside the storage chamber was limited to 1.5°C experimentally and 2.3°C numerically. Storage rack temperatures ranged from 28.45 to 32.5°C, with exceedances of the recommended 25–30°C range remaining mainly localised and transient. Relative humidity remained within 60–75% for most of the monitoring period, while no conditions conducive to condensation were observed. Solar heating enhanced the buoyancy-driven airflow, increasing the air velocity from 0.028 to 0.273 m/s and the volumetric airflow rate up to 0.410 m³/s demonstrating favourable airflow performance compared with most comparable configurations reported in the literature. Statistical validation yielded RMSE values below or close to 1°C, CV(RMSE) values ranging from 2.64 to 3.45%, and NMBE values between −3.52 and −1.65%, confirming the overall predictive capability of the model. These findings demonstrate that coupling the thermal inertia of the storage chamber with solar chimney-induced natural ventilation provides a stable storage environment suitable for onion preservation under hot-climate conditions.