The moisture transfer mechanism and influencing factors in wood during radio-frequency/vacuum drying

Wood from Sugi trees (Cryptomeria japonica D. Don) was used and the driving force for moisture transfer, the contribution ratio (for evaporation from the cross section) of the drying rate along the longitudinal direction to the total drying rate, the moisture transfer rate along the longitudinal direction attributed to the driving force and the dynamic permeability during the radio-frequency/vacuum drying process were calculated. The change law for these parameters and the relationships between them were analyzed, and the moisture transfer mechanism was evaluated in detail. Results showed that for a lower ambient pressure, the contribution ratio is higher, and most of the moisture is transferred along the longitudinal direction to the surroundings in the form of vapor. With decreasing moisture content, the driving force would slightly decrease as well, successively leading to a lower drying rate. The permeability of the two types of the specimen is similar during the whole drying process. However, for a higher ambient pressure, the contribution ratio is lower, and the proportion of diffusion to the moisture transfer increases, lowering the influence of the driving force on the drying rate, and the relationship between the driving force and the drying rate becomes inexplicit. Independent of the drying conditions, the permeability decreases with decreasing moisture content.