Numerical simulation and experiment on heat and mass transfer in different drying modes of red jujube slices

This study aims to reveal and compare the heat and mass transfer and the drying kinetic parameters of red jujube slices during hot air drying, infrared combined hot air drying, and infrared vacuum pulsation drying. A three-dimensional coupled model was established for the heat and mass transfer of jujube slices under the three drying modes, according to the control equations of Fick's law of diffusion, Antonin's equation and Beer Lambert's law. The reliability of the model was verified using experimental data. Subsequently, the utility of the model was tested by varying parameters, such as hot air temperatures, infrared radiation intensities, and vacuum pulsation ratios. Each drying was further analyzed to combine with the measured quality indexes, textural properties and microstructure. The actual geometry of jujube slices was simulated and solved using COMSOL Multiphysics 6.0. The result shows that: 1) The infrared hot air and infrared vacuum drying saved 46.43% and 41.07% of the drying time, compared with the hot air drying. The simulation was in better agreement with the measured values (the coefficients of determination R² for dry basis moisture content and temperature were 0.964, 0.959, 0.917 and 0.947, 0.922, 0.951, respectively). 2) The temperature field diagram showed that the effective heat of the inside of the material, and the center temperature of the material during infrared hot air and infrared vacuum drying increased by 11.33% and 5.59%, respectively, compared with the hot air drying, when drying for 20 min. The high energy density and penetrability of infrared radiation significantly improved the drying efficiency. 3) The simulation data showed that there was a significant effect of pressure variation on the drying kinetics in the infrared vacuum pulsation drying. The increasing vacuum time was favorable to the decrease of moisture. The water content and drying rate showed a step and peak distribution with the pressure pulsation. The sensitivity of the drying rate to the pressure change decreased with the decrease in the water content of the material. Further explanation was provided for the formation of porous structure in the red jujube slices during vacuum pulsation drying. 4) A comparison was made on the measured quality and textural properties of jujube slices with the simulated data. Segmented drying was proposed for the future direction in the numerical model of heat and mass transfer for fruit and vegetable drying. For example, the nutritional content and microstructure of products were coupled with the numerical model in the future. The nutritional, structural and rheological properties of materials can be expected to be quantified and visualized during drying. The numerical models of jujube slices were established and verified under three drying modes. The characteristics of each drying were obtained after simulation. It is of great significance to establish numerical models for fruit drying.