Heat transfer characteristics and drying kinetics of hematite thin layer during hot air convection

Highlights: • The thin layer drying behavior of the hematite during hot air drying was examined. • The sample in the second falling rate period presented higher moisture diffusivity. • Two isothermal temperature stages are noted during the hot air drying. • The heat transfer coefficients increased with the rising hot air temperature. • The Dimensionless heat transfer correlation of hematite was developed. -- Abstract: Heat transfer characteristics and drying kinetics of the hematite thin layer during hot air convection based on a lab-scale setup were examined at seven temperatures (100, 110, 120, 130, 140, 150, and 160 °C). Four periods during the drying process were detected, a warm-up period, a constant rate period, the first and second falling rate periods. Two isothermal temperature stages were captured during the hot air drying. The average heat transfer coefficients of the whole drying process increased by 28%, 40%, 49%, 56%, 61% and 66% as the hot air temperature rising from 100 to 110, 120, 130, 140, 150 and 160 °C. The effective moisture diffusivities of the hematite thin layer were from 8.37 × 10⁻⁹ to 2.05 × 10^-8 m² s⁻¹ for the first falling rate period, and from 1.24 × 10⁻⁸ to 4.06 × 10^-8 m² s⁻¹ for the second falling rate period. The isothermal temperature increased with the elevated hot air temperature, which increased by about 56% as the hot air temperature rose from 100 to 160 °C.