## 3 Results and Discussion  
### Investigation of Drying Kinetics in Different Strategies  
In this research, four different drying strategies were used for apple slices drying in the combined hot air-infrared dryer. All the results were presented separately for each drying strategy. Moreover, the optimal results obtained in the combined strategies were compared with the optimal results in the hot air method. In the first strategy, where the drying process was with hot air (HA), the results showed that, as the air temperature increased from 40 to 70 \({}^{\circ}\)C, the drying time decreased from 230 to 120 min. The reason was that using a higher temperature created a higher thermal gradient inside and outside the product and increased the molecular motion in the drying product and the water molecules moved with more energy. Finally, the moisture of the slices is then removed faster. The effect of increasing air temperature in reducing drying time was also reported in another study. In research [39], a result was reported that, by increasing the temperature from 40 to 60 \({}^{\circ}\)C, the drying time decreased from 175 min to 100 min. Moreover, in the results reported from other research [8], with an increase in air temperature from 40 to 60 \({}^{\circ}\)C at an air speed of 0.5 m/s, the drying time decreased by 22.2%. In the drying experiments conducted with infrared (IR) radiation, while increasing the intensity of infrared radiation from 250 to 750 W, the drying time decreased from 340 to 100 min. The reason is an increase in moisture output rate and rapid evaporation due to a higher thermal gradient inside and outside the apple slices. Similar results have been reported in other research on the effect of increasing the intensity of infrared radiation in reducing the drying time. In further research results [20], with the increase in radiation intensity from 0.22 to 0.49 W/cm\({}^{2}\), drying time decreased from 103 to 67 min. Moreover, in the results reported from [42], increasing the temperature from 30 to 60 \({}^{\circ}\)C and the radiation intensity from 0.21 to 0.41W/cm\({}^{2}\) resulted in the drying time decreasing by 42.38%. Figure 2a is related to the dimensionless moisture ratio (MR) chart versus drying time for the first drying strategy, which shows that MR decreases via time. As shown in the figure, the increase in air temperature and intensity of infrared radiation with the rapid removal of moisture reduced the drying time and increased the slope of the MR curve. According to the results, the drying time for high-intensity infrared radiation was not much different from the hot air-drying method. However, in the low radiation intensity of infrared, the time difference was significant, and it was not very suitable in terms of drying time. The reason for this may be the delay in increasing the internal temperature of the slices in the low intensity of infrared radiation, which causes the free moisture to leave slowly at the beginning of drying compared to other treatments. However, in hot air drying, a high percentage of moisture was removed at the beginning of the drying process. Figure 2b is related to the dimensionless moisture ratio (MR) chart versus drying time for the second drying strategy. In this strategy, two sources of hot air and infrared (HA-IR) are used simultaneously for drying. Similar to the first strategy, increasing the temperature and intensity of infrared radiation will decrease the drying time of the apple slices. The drying time maximum was in the HA40-IR250 method (260 min), and the minimum drying time was in HA70-IR750 (80 min). The results showed that the simultaneous use of two sources of hot air and infrared radiation reduces the drying time compared to the asynchronous use of these two methods with one energy source. The reason for this is the increase in temperature in different parts of the drying slices because the infrared waves penetrate the slices and result in an increase in the temperature inside.