Thin-Layer Convective Solar Drying and Mathematical Modelling of the Drying Kinetics of <i>Marrubium vulgare</i> Leaves

This study explores the thin-layer convective solar drying of Marrubium vulgare L. leaves under conditions typical of sun-rich semi-arid climates. Drying experiments were conducted at three inlet-air temperatures (40°C, 50°C, 60°C) and two air velocities (1.5 and 2.5 m·s−1) using an indirect solar dryer with auxiliary temperature control. Moisture-ratio data were fitted with eight widely used thin-layer models and evaluated using correlation coefficient (r), root-mean-square error (RMSE), and Akaike information criterion (AIC). A complementary heat-transfer analysis based on Reynolds and Prandtl numbers with appropriate Nusselt correlations was used to relate flow regime to drying performance, and an energy balance quantified the relative contributions of solar and auxiliary heat. The logarithmic model consistently achieved the lowest RMSE/AIC with r > 0.99 across all conditions. Higher temperature and air velocity significantly reduced drying time during the decreasing-rate period, with no constant-rate stage observed. On average, solar input supplied the large majority of the thermal demand, while the auxiliary heater compensated short irradiance drops to maintain setpoints. These findings provide a reproducible dataset and a modelling benchmark for M. vulgare leaves, and they support energy-aware design of hybrid solar dryers for medicinal plants in sun-rich regions.