| As a functional food with high market value,fresh germinated brown rice(GBR)is prone to mildew and bacterial reproduction owing to high moisture content.Microwave drying can quickly reduce GBR to safe moisture content.However,the unclear mechanism of energy absorption and utilization limited the industrial application and promotion of microwave drying.To give the solutions of the above problems,the electromagnetic-thermal-mass coupling model was developed by using COMSOL Multiphysics software based on the experimental results,and the regularities of energy absorption and utilization were studied considering microwave energy,heat and mass transfer and distribution characteristics.Numerical simulation and bench experiment were employed to optimize the structure of microwave equipment with highest energy absorption rate.The effects of drying conditions in terms of microwave intensity,ventilation velocity,cycle time,layer thickness on the energy utilization efficiency and drying quality were investigated to propose a microwave drying mode with high energy efficiency and high drying quality.The research results and conclusions were developed as follows:(1)Based on the basic theory of electromagnetics and heat and mass transfer,a microwave drying process model of GBR with multi-physical field coupling characteristics was established.The simulation results showed that there was loss and reflection between magnetron feeders in continuous microwave dryer,which reduced the uniformity of electric field inside material layer,and the absorption capacity of microwave energy was weakened.When the absorbed microwave energy was converted into heat energy,the increase of temperature rise and precipitation in the material layer was lower than the increase of microwave input power.The microwave drying condition determined the changes of temperature and moisture characteristics,which affected the conversion efficiency of microwave energy to effective energy.(2)Optimization of the key structure parameters of continuous microwave dryer under optimum energy absorption efficiency.The result showed that the magnetron waveguides with triangle arrangement weakened the interference phenomenon between the excitation ports,and the microwave absorption rate reached the highest value in the continuous microwave dryer.The right angle transition mode of the adjacent walls of the drying chamber avoided the focus phenomenon of electric field,which balanced microwave energy absorption capacity.The change of the installation position of the conveyor belt(material layer location)affected the microwave transmission mode,the feeding port and the load were optimally matched under the distance from the bottom to surface of 100 mm.The uniformity of electric field and microwave energy absorption rate was higher 53.33% and 35.94% than that of existing machine,respectively.The verification result showed that the simulated results matched with the experimental ones,which proved that the optimization of structure simulation can be used as an effective means to solve the energy efficiency problem of the microwave dryer.(3)Investigation of the influence of various microwave drying conditions on the drying quality and energy utilization efficiency.The microwave intensity had significant effect on the yellowness value(b* value)of dried GBR,the microwave energy utilization efficiency increased and then decreased with the increase of microwave intensity.The introduction of ventilation may improve the distribution of the temperature and moisture inside material layer to promote the utilization efficiency of microwave energy.However,the degree of bursting increased when the ventilation velocity was higher than that of 2.0 m/s.The utilization efficiency of microwave energy tended to increasing trends followed by the decreasing trend with the per cycle time.The severe browning appeared for GBR kernel when cycle time was higher than that of 8 min.The heat transfer and moisture diffusion within the layer were reduced if the thickness value was higher than the microwave penetration depth to reduce the microwave energy utilization efficiency.Considering the quality of GBR and energy utilization efficiency,the reasonable microwave drying conditions were developed as microwave intensity of 2 W/g,ventilation velocity of 1.0-1.5 m/s,per cycle time of 6 min,thickness of material layer of 8 mm.The microwave energy utilization efficiency reached the highest value of 51.33% under these microwave drying condition.The research results are helpful to understand the microwave absorption and utilization process of GBR microwave drying,which provide a theoretical reference for the industrial application of GBR microwave drying. |
