Study On Numerical Analysis And Simulation Accuracy Of Microwave Heating Characteristics Of Food

Many simulations employed in microwave heating simplify the geometric model of the microwave cavity to reduce the complexity of the model.Some detailed design features of the cavity,such as magnetron and cavity depression,are usually ignored.This study built detailed geometric models(DGM)and simplified geometric models(SGM)to investigate the changes in the electric field in a microwave cavity.A three-dimensional finite element model was developed to simulate rotating and stationary food(mashed potatoes)during microwave heating,and the simulation parameters and strategy during rotation were optimized.The simulations using DGM and SGM were compared by comparing the simulated transient temperature profiles and spatial temperature patterns with physical experiments.The results indicate that the temperature patterns of the DGM differed significantly from the SGM,and the simulation results of the DGM were closer to the experimental results.The presence of the magnetron,plastic bearing,and detailed glass turntable had little influence on the electric field,but the dents had the great influence.The influence of dents on the heating uniformity was discussed in detail.The simulation model of microwave drying was studied based on electromagnetics,multiphase transport and solid mechanical deformation model with minced Antarctic krill as media.Microwave drying is carried out in a household microwave oven with a power of 100W(rated power of 1000W).The results of the simulation are obtained by solving electromagnetic equation,energy and momentum conservation and deformation equation by COMSOL Multiphysics.The model includes multiphysics of Maxwell's electromagnetic heating,energy conservation,Darcy's velocity,solid mechanics,mass conservations of water and gas,and phase change of melting and evaporation of water.Infrared thermal imager was used to photograph temperature distribution on the surface of the sample,and optical fiber sensor was used to measure the instantaneous temperature in the point of the sample.The spatial temperature distribution,transient temperature curve(RMSE=2.11 ?),moisture content(dry basis,RMSE=0.03)and volume ratio are in good agreement with the experimental values during intermittent microwave drying for 180 s,and the RMSE of temperature and moisture content without considering shrinkage model are 9.42 ? and 0.08,respectively.Microwave simulation of microwave drying is feasible.In addition,the temperature and moisture content of minced shrimp treated as deformable material were significantly different from that of rigid material during microwave drying simulation,and the former was closer to the experimental value.Water loss is the main factor of material deformation.Intermittent microwave drying allows the distribution of moisture,pressure andtemperature within the material uniform.The sensitivity analysis of the input parameters including intrinsic permeability of water and gas(±50%)and water absorption expansion coefficient(±50%)shows that the water content is more sensitive to the intrinsic permeability of liquid water(RMSE=0.089),less sensitive to the intrinsic permeability of gas(RMSE=0.023),and the volume ratio is very sensitive to the water absorption expansion coefficient(RMSE=0.053).The frequency of the magnetron in a domestic microwave oven is often not fixed and difficult to measure.In order to obtain more accurate simulation results,the effect of microwave electromagnetic frequency fluctuation on temperature distribution was investigated.This study assumes that the frequency of the domestic microwave oven fluctuates around 2.45 GHz and compares the simulated based on the finite element model and experimental results of different fluctuation ranges(2.44-2.46 GHz,2.43-2.47 GHz,2.41-2.49 GHz)and single frequency(2.45 GHz).The subjects were mashed potatoes heated in microwave oven for 60 s.The electromagnetic power density is calculated every 0.005 GHz for different frequency range distributions and weighted averaged according to the cosine distribution,and finally the electromagnetic power is used as a heat source to heat the mashed potatoes.The simulated results were compared with the experimental values of the surface temperature and instantaneous temperature of the mashed potatoes measured by thermal imager and fiber optic thermocouple.The predicted temperature field in the frequency fluctuation range of 2.44-2.46 GHz is in good agreement with the experimental values.In the range of 2.41-2.49 GHz,the uniformity of temperature distribution is the best.The model can provide theoretical basis for the development of microwave food.