Heat Transfer Coefficient of the Constant Rate Period in the Drying of Agar Gel, Carrot and Sweetpotato

Abstract

The heat trader coefficient in the constant rate period of food drying by hot air has been studied in this work. The samples used were agar gel, carrot, and Sweetpotato. The samples were spherical, cubic, cylindric or slab-shaped. The heat transfer coefficient could be represented in a Nusselt type equation, Nu=2.0+αRe_p^1/2Pr^1/3, as for a single droplet of pure solution. But, α values obtained were slightly larger than the reported values of α, which mostly ranged from 0.55 to 0.65, under our experimental conditions. To the non-spherical samples, the above equation also could apply with the same α value as to the spherical samples if the equivalent diameters presented by Pasternak et al. were used. We also found that the heat transfer coefficient was influenced by the humidity of air and increased with the rise in relative humidity at any level of temperature.