Determination of parameters of GEM formula for dielectric properties of small size granular agricultural products

The dielectric properties are the response characteristics of bound charge in molecules of matter to external electric field, and the fundamental researches on the dielectric properties of agricultural products can provide a basis for microwave processing, nondestructive sensing, etc. In order to extend the application of general effective medium (GEM) in agricultural engineering and explore the novel calculation method for dielectric properties of agricultural products, granular agricultural products were taken as the study objects in this paper. The simulation model and numerical method for analyzing the effective dielectric properties of the accumulative granular agricultural products were established by using discrete element method, finite element method and average energy method. In addition, a great deal of simulation and experimental measurement was carried out. Based on the comparative analysis of numerical calculation data and experimental measurements data, the best dimensionless parameter of GEM formula for calculating dielectric properties of granular agricultural products at microwave band was proposed as A=5 and β=0.5 ( called Modified General Effective Medium of Agriculture, MGEMA). The feasibility, validity and accuracy of the numerical model and MGEMA formula were verified through experimentally measured data of some kinds of granular agricultural products, including grain, wheat, millet, maize pulp, and rapeseed, etc. The results showed that the repose angle error between the real and simulated of grain particle materials was 0.45%. Under the conditions of dielectric constant (2.0-10.0), dielectric loss factor (0.1-0.9), microwave frequency (2.0-12.2 GHz), moisture content (2.0%-19.7%, wet basis) and volume fraction (18.2 %-88.0 %) of agricultural product particles，the maximum error of dielectric constant and dielectric loss factor that calculated by MGEMA formula were 0.40% and 1.20%, respectively. The accuracy was better than some traditional theoretical formulas. The method can be used for simulation analysis of effective dielectric properties of other kinds accumulation granular agricultural products in three dimension, and the MGEMA provides a theoretical formula with high accuracy for the dielectric properties study of granular agricultural products at room temperature (24 ℃).