Study On Microwave Drying Process Of Carrot Based On Heat And Mass Transfer Analysis

Microwave drying technology has the advantages of high efficiency,low energy consumption,good drying quality after drying,and is widely applied in fruit and vegetable drying process,but microwave drying technology has problems such as uneven heating and rapid temperature rise in the late drying period.Considering that microwave drying has a complex heat and mass transfer process,a multi-physical field model of microwave drying was established,and a microwave drying experiment system was built.The effect of material rotation and different microwave intermittent time on the change of heat and mass transfer,drying characteristics and quality of carrots were studied.The drying process was segmented and different intermittent time was set for different drying stages to improve the drying quality.（1）A microwave drying multi-physics simulation model with carrot as a porous medium was built,and the accuracy of the model was verified.Coupled with"electromagnetic waves,frequency domain","heat transfer in solids","transport of diluted species",and combined with Maxwell’s equations,mass conservation equations,energy conservation equations,phase change equations,the calculation of the heat and mass transfer in the process of carrot microwave drying was simulated.An experimental system of carrot microwave intermittent drying was built:using an infrared thermal imaging camera to detect the surface temperature of sample online.Comparing the simulation results with the experimental results,a good consistency was achieved with the accurate of simulation.（2）The effect of static and rotating conditions on the drying process of carrots were studied.The results show that the distribution of the electric field in the microwave cavity was uneven.With the rotation during drying process,the sample alternately appeared in the strong and weak area of the electric field,which improved the uniformity of the electric field distribution on the sample,ameliorated the uniformity of the temperature distribution between the samples and individual sample at different locations,and the uniformity of the moisture ratio was enhanced.The drying degree of the rotated samples tended to be uniform,and the total drying time of the rotated samples was shortened.The quality and uniformity of the rotated sample after drying were better.（3）Based on the rotating material,the effect of microwave drying processes at different intermittent times（heating 20 s,intermittent 20 s,40 s,60 s,80 s,i.e.R1/2,R1/3,R1/4,R1/5）was studied.The RMSE of moisture ratio simulation results and experimental results were1.32%,2.87%,3.95%,3.76%,respectively.The shorter the intermittent time,the higher the samples temperature and drying rate,with the smaller the unit energy consumption.Drying at different intermittent times,the samples temperature was characterized by rapid increase in the early stage,keeping stable in the medium term,and increased rapidly in the later stage.The drying rate was also manifested as a rapid rise in the early stage,with a slow descend in the medium term,and a rapid decline in the later stage.According to the change trend of drying rate,the drying process was divided into three stages,namely acceleration period,constant speed period and deceleration period.The quality index results showed that too long or too short intermittent time that was not conducive to the quality.Through the analysis of drying characteristics and drying quality of carrots,R1/2 had the shortest drying time and the lowest unit energy consumption per unit,and the dried product quality was the best under R1/4conditions and the highest sensory evaluation was achieved.（4）Quality improvement of carrot based on thermal analysis of microwave drying.The quality improvement scheme（R-TS）was determined by setting different intermittent time in the three stages of drying process,and the intermittent cycle function of quality improvement scheme was obtained,and the Lab VIEW program was written to meet the requirements of the scheme.The results of comparison between simulated and experimental values of quality improvement scheme were as follows:RMSE_MR=2.7%of water ratio,RMSE_DR=0.024%/min of drying rate.Based on the evaluation of unit energy consumption and quality index,quality improvement scheme was compared with other drying conditions,and the results showed that the quality improvement scheme was feasible.