Experimental and fundamental critical analysis of diffusion model of airflow drying

ABSTRACT

Scientific literature of agromaterial drying present contradictory conclusions in terms of the kinetic effect of airflow velocity. Some authors confirmed that it does not trigger any modification of drying, while some articles tried to establish empirical models of the effective diffusivity D_eff versus the airflow velocity, what is fundamentally erroneous. By analyzing internal and external transfer phenomena, this research aimed at recognizing that once air velocity is higher than a critical airflow velocity (CAV), the internal transfers become the limiting phenomenon. CAV depends on the effective diffusivity and the product size. It was calculated in the cases of two studied raw materials (apple and carrot), differently textured by instant controlled pressure drop (DIC). Values of CAV greatly depend on diffusivity of water within the matrix. At temperature T = 40°C, they were 1 m/s for untreated carrot and 2.1 m/s for DIC-textured carrot, whose D_eff values were 1.31 and about 3 × 10⁻¹⁰ m²/s, respectively. Also, at temperature T = 40°C, they were 2.1 m/s for untreated apple and 3 m/s for DIC-textured apple, whose D_eff were 1.4 and about 10.4 × 10⁻¹⁰ m²/s, respectively.

KEYWORDS:

• Agro-material
• critical airflow velocity
• drying kinetic model
• heat transfer
• mass transfer