Evolution of the driving forces during convective drying of carrot slices

Abstract

To explore the drying characteristics, a model combining the Fick’s second law of diffusion with a convective boundary condition on the material surface is solved numerically. And the evolutions of the moisture gradient within the sample (internal driving force) and the relative humidity difference between the bulk air and the material surface (external driving force) with drying time are obtained. These obtained results show that the relative humidity difference remains its maximum for some period at the beginning of drying, and then falls progressively to zero for samples with high initial moisture content. However, it falls from its maximum to zero without constant period for samples with low initial moisture content. And the moisture gradient in the outer is always greater than that in the inner within the sample. Correspondingly, the moisture migration flux in the outer is always greater than that in the inner within the sample. These observations indicate that both the external and the internal driving force simultaneously control the whole drying process, and the transferring rate of internal moisture cannot be equal to or higher than that of surface moisture. Thus, the magnitude of drying rate is directly dominated by the external driving force.