Coupled modeling and heat and mass transfer analysis of white radish slices dried by infrared radiation combined hot air drying

This study aims to clarify the heat transfer inside the material during the combined infrared and hot air drying. A numerical model was also established for the moisture and temperature distribution during the drying process of white radish slices. The heat and mass transfer was compared in the process of single hot air drying and the infrared combined hot air drying under two shrinkage-related diffusion coefficients. The dynamic change of material diffusion coefficient during drying was considered to interfere with the realization of accurate simulation. A COMSOL Multiphysics 5.2a was used to establish a two-dimensional heat and mass transfer model, where the material and physical field were two-dimensional axisymmetric, particularly for the less solving time, computational cost, and memory. The results showed that the effective diffusivity of shrinkage was enabled to accurately describe the heat and mass transfer of radish slices in the process of single hot air drying and infrared combined hot air drying, where the coefficient of determination (R2) between simulated and experimental values were 0.893 and 0.911, respectively. The infrared radiation was the dominant factor affecting the rate of heat and mass transfer. Optimal performance was achieved under the condition of constant drying temperature of 60 ℃, where the infrared combined hot air drying effectively shortened the time (90 min) by 21.4%, compared with the single hot air drying material to the safe moisture content (10%), whereas, the infrared combined hot air drying sample reached the set temperature (60 °C) when shortening the time by 36.0%, compared with the single. The temperature change of the material center was more sensitive to the heat transfer coefficient, and then the heat transfer coefficient led to the rapid change of white radish slices center temperature. The mass transfer coefficient posed a strong influence on the material moisture content, but there was a weak sensitivity of the material center temperature to the mass transfer coefficient. A comprehensive analysis showed that the infrared radiation presented the high heat flux density and high heat transfer efficiency, where the heat transfer can be maximized systematically. However, the high temperature of the material can easily lead to quality deterioration. It was recommended to introduce the different combination sequences to overcome this challenge, such as sequential combined infrared and hot air drying (IR+HAD; HAD+IR) and intermittent combined infrared and hot air drying (IIRI+IHAD). The model can also provide a useful reference to simulate the other materials in the combined drying technology.