Microwaves have been widely used in the treatment of materials, such as heating, drying, and sterilization. However, the heating in the commonly used microwave applicators is usually uneven. In this paper, a novel multi-material turntable structure is creatively proposed to improve the temperature uniformity in microwave ovens. Three customized turntables consisting of polyethylene (PE) and alumina, PE and aluminum, and alumina and aluminum are, respectively, utilized in a domestic microwave oven in simulation. During the heating process, the processed material is placed on a fixed Teflon bracket which covers the constantly rotating turntable. Experiments are conducted to measure the surface and point temperatures using an infrared thermal imaging camera and optical fibers. Simulated results are compared qualitatively with the measured ones, which verifies the simulated models. Compared with the turntables consisting of a single material, a 26%–47% increase in temperature uniformity from adapting the multi-material turntable can be observed for the microwave-processed materials.

Microwave processing of materials has been found to deliver enormous advantages over conventional processing methods in terms of mechanical and physical properties of the materials. However, the non-uniform temperature distribution is the key problem of microwave processing, which is related to the structure of the cavity, and the placement and physical parameters of the material. In this paper, a new microwave cavity structure with a sliding short based on phase-shifting heating is creatively proposed to improve the temperature uniformity. An electronic mathematical model based on the Finite Element Method (FEM) is built to predict the temperature distribution. Meanwhile, a new computational approach based on the theory of transformation optics is first provided to solve the problem of the moving boundary in the model simulation. At first, the experiment is carried out to validate the model, and heating results from the experiment show good agreement with the model’s prediction. Based on the verified model, materials selected among a wide range of dielectric constants are treated by stationary heating and phase-shifting heating. The coefficient of variation (COV) of the temperature and temperature difference has been compared in detail between stationary heating and phase-shifting heating. A significant improvement in heating uniformity can be seen from the temperature distribution for most of the materials. Furthermore, three other materials are also treated at high temperature and the heating uniformity is also improved. Briefly, the strategy of phase-shifting heating plays a significant role in solve the problem of non-uniform heating in microwave-based material processing. A 25%–58% increase in uniformity from adapting the phase-shifting method can be observed for the microwave-processed materials.

This article proposes a simple and cheap complex permittivity measurement system at the frequency of 2450 MHz based on a waveguide six-port reflectometer (SPR) and a ridge waveguide. The SPR composed of a waveguide-stripline-coaxial directional coupler and three SubMiniature version A (SMA) probes is designed. The general theoretical model, simulation, and implementation of the waveguide SPR with an optimal accuracy are given. Within the dynamic measurement range of the power meters, the designed SPR can accurately measure the reflection coefficient and have high measurement stability at different power levels. The ridge waveguide is used as the carrier of the material to obtain good monotonicity of the complex permittivity, and a backpropagation neural network (BPNN) is established for the relationship between the complex permittivity and the reflection coefficient. Compared with the reference values in the literature, it can be found that the error of the whole system in measuring the complex permittivity is also within the acceptable range. The SPR based on rectangular waveguide provides a basis for the measurement while heating with high-power microwave energy.

Conventional microwave heating has serious problems such as non-uniform heating and low efficiency. A novel magnetron microwave sweep frequency heating method is proposed to improve microwave heating uniformity. In this method, the frequency-sweeping signal is injected into the magnetron by the injection frequency-locking technique, and the microwave sweep frequency heating of the magnetron is realized. In this paper, a complicated injection frequency locking system is given and analyzed and a multiphysics calculation model based on the finite element method for electromagnetic waves and heat transfer is established. The calculation of microwave sweep frequency heating is realized by the combination of COMSOL and MATLAB. The results show that the sweep frequency heating has an obvious superiority. An experiment is carried out to verify the simulation results. The simulation results are in agreement with the experimental data. Moreover, the effect of sweep bandwidth and sweep interval on heating uniformity is discussed.

Microwave heating is well known for its efficiency and instantaneity. However, the non-uniformity results of the microwave heating have limited the development of its large-scale industrial application. In order to solve this problem, a metal patch placed on a rotating turntable is proposed as a mode stirrer in this paper. A commercial finite element software, COMSOL Multiphysics, is used to build the proposed model based on a BJ-22 rectangular waveguide and a microwave oven. Maxwell's equations, heat conduction equation, implicit function and level set methods, which are used to realize the rotation of metal patch, are coupled to describe the microwave heating process. The electric field intensity, the temperature, the coefficient of variation and the mean temperature rise in different cases are compared to optimize the parameters of mode stirrer. The point temperatures are measured by optical fibre to verify the simulated results. The results show that the rotation of metal patch has a huge influence on improving the heating uniformity and heating efficiency, but the rotating speed has little influence if the heating time is larger than a rotating period.

Microwave heating of solid stack materials is common but bothered by problems of uneven heating and electric discharge phenomena. In this paper, a method introducing fluid materials with different relative permittivity is proposed to improve the heating uniformity and safety of solid stack materials. Simulations have been computed based on the finite element method (FEM) and validated by experiments. Simulation results show that the introducing of fluid materials with proper relative permittivity does improve the heating uniformity and safety. Fluid materials with the larger real part of relative permittivity could obviously lower the maximum modulus value of the electric field for about 23 times, and will lower the coefficient of variation (COV) in general, although in small ranges that it has fluctuated. Fluid materials with the larger imaginary part of relative permittivity, in a range from 0 to 0.3, can make a more efficient heating and it could lower the maximum modulus value of the electric field by 34 to 55% on the whole studied range. However, the larger imaginary part of relative permittivity will cause worse heating uniformity as the COV rises by 246.9% in the same process. The computed results are discussed and methods to reach uniform and safe heating through introducing fluid materials with proper relative permittivity are proposed.