基于射频加热数值模型的介电系数混合方程研究

Study on Mixture Equations of dielectric properties using numerical model of radio frequency heating

针对农产品和食品粉状物料如何选择两相介质介电系数混合方程的问题,从介电系数应用的角度,采用射频加热数值模型分析方法,对介电系数混合方程——CRIME方程、LLLE方程、BE方程和LE方程进行研究。以脱脂奶粉为研究对象,将奶粉压缩为块状,排出颗粒间隙中空气后测得介电系数;在此基础上,分别应用4个介电系数混合方程计算得到未压缩(自然状态)的脱脂奶粉介电系数,将其应用到射频波电磁-热场耦合分析数值模型进行射频加热仿真,同时采用6kW,27.12 MHz的射频加热系统对脱脂奶粉进行射频加热试验,从仿真和试验的加热速率、表面温度大小及加热模式的对比中,得到适合计算脱脂奶粉介电系数的混合方程。研究结果表明:介电系数采用LE方程的仿真结果与试验结果最吻合。该研究方法可应用于其他农产品和食品粉状物料介电系数的研究。该数值分析模型可进一步用于射频系统加热机理及加热均匀性的研究。

Knowledge of dielectric properties（DPs）is essential for better understanding the heating behavior of argifood materials when subjected to radio frequency（RF）or microwave heating.DPs are also important parameters for nondestructive quality inspection and classification of argi-food products.But it＇s difficult to measure them in particulate materials due to the air mixed in the powder.In order to obtain accurate DPs,nonfat dry milk with 3.34% moisture content and bulk density 537.0±7.9kg/m3 was compressed into solid block with bulk density 1 243.2±18.4kg/m3 before measurement.Then the DPs of nonfat dry milk were calculated from the value of compressed samples using the Mixture Equations,which were Complex Refractive Index Mixture Equation（CRIME Equation）,Landau and Lifshitz,Looyenga Equation（LLLE Equation）,Bottcher Equation（BE Equation）and Lichtenecker Equation（LE Equation）.Those DPs from four Mixture Equations were finally verified with a computer simulation model respectively.The simulation model had been validated with a 6kW 27.12MHz RF system.The result shows that the LE Equation is the most suitable for nonfat dry milk.Other argi-food particulate materials can also be conducted using this method.The developed computer model can further be used to study the effect of some important parameters such as sample size,position,shape,and DPs on RF heating of particulate materials.