| Due to the unique advantages,the microwave heating techniques allowed widely application in industrial and agricultural productions and other fields.Its fundamental disciplines involve the electromagnetic theory,the heat and mass transfer theory,and electromagnetic calculation.Moreover,the dielectric properties of materials are essential characteristic parameter indices to study the process of microwave heating of materials.Efficiency dielectric properties are important parameters for characterizing the microwave energy absorbed by the mixing materials.Therefore,the study on the efficiency dielectric properties of particulate mixing materials has theoretical significance and actual application values.In this study,latest research status of the applications of microwave in agriculture and the main bottlenecks against multimode microwave heating was summarized.And the status of theoretical,numerical simulation,and experimental measurement studies progressing concerning the efficiency dielectric property of particulate mixing materials,are reviewed and analyzed comprehensively.On this basis,this paper deeper research on the the fundamental problems involved in microwave hot-processing of agricultural products and implements the following research contents:(1)Based on the electromagnetic theory,The dielectric polarization mechanisms is analyzed,and the principle of microwave energy transformation is elaborated from the perspective of the electromagnetic theory.A depth theoretical analysis on the action principle of dielectric properties of materials on microwave hot processing materials is performed.Moreover,the action principle and specific contributions of electromagnetic characteristic parameters ?,??,???,??,and ??? to the microwave hot processing of materials are discussed thoroughly.Subsequently,the microwave-absorbing characteristics of the ideal media,ideal conductor,ordinary conductor,and polar media are analyzed.(2)The route and method of improving the microwave heating efficiency and uniformity by optimizing the inner wall structure of microwave reaction cavity are proposed.Moreover,the microwave heating efficiency and uniformity for cavity inner wall with semi-cylinder convex groove,concave spherical surface,convex spherical surface and ridged concave groove are analyzed and calculated.Subsequently,the relevant basic law,which can increase the heating efficiency and uniformity,is obtained.The simulation results demonstrate that optimizing the inner structure of the microwave reaction cavity can increase microwave heating efficiency and also improve heating uniformity and inhibit the formation of high electric-field-concentrating region in the cavity.Compared with the conventional reaction cavity with smooth walls,setting the semi-cylinder convex and ridged concave groove can increase the heating efficiency and uniformity to the large extent(the maximum of heating efficiency is 98.75%,the maximum increase of heating uniformity reached 57.54%).The distribution area of the optimal structural parameters is relatively extensive.The results provide theoretical and technological support to the “hot point” alarming and optimal design of the reactor during microwave hot-processing of materials.(3)The efficiency dielectric properties of mixing materials with particle random filling are analyzed and calculated using the Monte Carlo(MC)stochastic simulation method and COMSOL Multiphysics finite element software(MC-FEM).The correctness of MC-FEM method is verified by a large number of numerical experiments.The novel Modified General Effective Medium(MGEM)modifier formula for calculating the efficiency dielectric properties(i.e.efficiency dielectric constant and loss factor)of particulate binary mixing material is proposed,and the MGEM calculation results are compared with those from the experiment,MC-FEM computation,and classical theoretical calculation,and verified the validity and accuracy of the MGEM formula.A highly accurate and easy-to-operate calculation formula of efficiency dielectric properties is provided for particulate agricultural products.(4)The influences of spatial position,volume fraction,conductivity,and dielectric properties of different components in the two-component,three-component,and core-shell structure mixing materials on their efficiency dielectric properties,microwave-absorption characteristics,and local electric field distribution are studied via MC-FEM.The study results demonstrate that the shape of spherical and cube particle composite structure can be used to simulate the efficiency dielectric properties of ellipsoidal particle(agricultural product)material,and the physical properties and dielectric properties of different components significantly influence the efficiency dielectric properties and microwave absorption characteristics of mixing materials and could enhance the local field in mixing fields.(5)The measurement scheme of dielectric properties for agricultural products based on uncalibrated coaxial transmission/reflection method is designed.Its implementation scheme is proposed based on the MGEM formula.In the experiment,the dielectric properties of powder agricultural(sideline)products(e.g.,potato,Panax notoginseng,Gastrodia elata,and Dendrobium),rapeseeds(e.g.,Bacterial leaf amaranth,cabbage mustard,red edible amaranth,coriander seeds,white rapeseed,bitter cress,and rapeseed),cereal particles(e.g.,Semen sesami nigrum,Maize grain,black rice,and millet),and grass seed particles(e.g.,Canada bluegrass,Trifolium repens,corn poppy,Jianguyin,Cynodon dactylon,and Rudbeckia hirta)are measured.The mathematical relationships between dielectric properties and loss factor,and the microwave frequency and moisture content of different agricultural(sideline)products are fitted.Additionally,the mathematical relationships between the dielectric properties of different agricultural products at 2.45 GHz and water(oil)contents are fitted.The experimental results show that the dielectric constant and loss factor of agricultural products measured are between 3.5–9 and 0.1–3.0 at room temperature(?)? F=2.45GHz),respectively.These research conclusions predict the dielectric properties of other agricultural products,and provide references for the microwave-assisted processing and device design of agricultural products.Concurrently,the proposed experimental measurement solved the difficulties of traditional measurement methods,such as clamp calibration,multi-sample measurement,and sample positioning.In this paper,the microwave hot-processing of agricultural products based on dielectric properties is discussed.Relevant conclusions could offer references for microwave heater design of agricultural products,improvement of microwave energy utilization,and promotion of microwave-assisted applications.Furthermore,relevant reports are applicable to microwave hot-processing of other particulate mixing materials. |
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