Experimental investigation of convective drying behavior of in-shell hazelnuts: moisture diffusivity, transport phenomena, and pressure drop

Abstract

This study investigates the convective hot-air drying behavior of in-shell hazelnuts under controlled operating conditions, with emphasis on drying kinetics, thin-layer modelling, heat and mass transfer characteristics, and pressure drop behavior within the hazelnut bed. The experiment was performed in a laboratory dryer set to 50–60 °C and 1.5–2.1 m s⁻¹ airflow. The temporal evolution of moisture content, drying rate, and moisture ratio was experimentally determined, and the effective moisture diffusivity was evaluated using Fick’s second law of diffusion. The results showed that internal moisture diffusion governed the drying process. Higher drying air temperature and air velocity reduced the drying time and enhanced the drying rate. Effective moisture diffusivity increased with temperature. The two-term model provided the best agreement with the experimental data, efficient R² values between 0.9770 and 0.9953, RMSE values between 0.0178 and 0.0384, and X² values between 5.26 × 10− 4 and 2.90 × 10− 3. The coefficient of heat transfer was between 131 and 180 W m⁻² K⁻¹, governed by air velocity rather than temperature. The pressure drop values predicted by the Ergun equation showed good agreement with the experimental measurements, with deviations generally remaining below approximately 8% under the investigated airflow conditions.