Numerical analysis of heat and mass transfer in kiwifruit slices during combined radio frequency and vacuum drying

Kiwifruit slices subjected to combined radio frequency (RF) and vacuum drying undergo a complicated process, including electromagnetic heating, heat and mass transfer along with a phase change of evaporation and shrinkage under a low pressure. The aim of this paper was to obtain an in-depth understanding of this complicated drying process. A 3D multiphase porous model was established to describe heat and mass transfer within the kiwifruit slices in a 3 kW, 27.12 MHz RF-vacuum drying system using COMSOL Multiphysics software. The validation results showed that the established model could be applied to describe the RF-vacuum drying process of kiwifruit slices since the maximum relative errors of temperature, moisture content, and drying rate between simulation and experiment were less than 10%. The temperature distribution of kiwifruit slices both from simulation and experiment indicated that the sample temperature at corners and edges were higher than that at the center of the container, and the sample temperature at the center was the lowest for a single kiwifruit slice. But the distribution of moisture content was opposite to that of temperature. Evaporation rate constant was found to be sensitive to moisture content of treated samples compared with temperature. The influences of sample thickness, electrode gap, and vacuum pressure of the RF-vacuum drying system on temperature and moisture content-time histories were determined during the RF-vacuum drying process. The findings of this research may help to gain a comprehensive understanding of RF-vacuum drying and optimize treatment parameters for drying processes. (C) 2020 Elsevier Ltd. All rights reserved.