Multiphysics modeling of microwave heating of solid samples in rotary lifting motion in a rectangular multi-mode cavity

Heating performance of dual-source microwave heating using different frequency shifting strategies in a solid-state system

Solid-state microwave heating holds potential in mitigating the issue of nonuniform heating commonly observed in magnetron-based microwave ovens. Previous work on solid-state application primarily focused on single-source microwave heating and modeling of dual-source heating with fixed frequency combinations. This study experimentally investigated the port interactions and heating performance of four different frequency shifting approaches (in range of 2.4 and 2.5 GHz, at an interval of 0.01 GHz), i.e., Fixed-frequency without shifting, Synchronized-shifting, Inverse-shifting and Distinct-shifting, in a dual-source microwave system under both stationary and rotatory conditions. Results showed that port interactions were dependent on the microwave frequency and load position, both of which significantly affected the microwave power efficiency. All three shifting strategies significantly improved heating performance compared to the Fixed-frequency heating. Additionally, the use of a turntable in our dual-source microwave configuration did not improve the heating uniformity in the context of various frequency shifting processes, suggesting that the inclusion of a turntable might be dispensable in solid-state microwave systems if proper multi-port frequency shifting strategy could deliver more uniform heating.

Method for Solving the Microwave Heating Temperature Distribution of the TE10 Mode

Microwave heating is a process in which the electric, magnetic, and temperature fields are coupled with each other and are characterised by strong non-linearity, high time variability, and infinite dimensionality. This paper proposes a method for predicting the microwave heating temperature distribution of the TE10 mode, because the traditional numerical calculation method is not conducive to designing microwave controllers. First, the spatial distribution of the main electromagnetic mode TE10 waves in a rectangular waveguide was analysed using the principal mode analysis method. An expression for the transient dissipated power and a heat balance equation with infinite-dimensional characteristics were constructed. Then, the microwave heating model was decomposed into electromagnetic and temperature field submodels. A time discretization approach was used to approximate the transient constant dielectric constant. The heating medium was meshed to solve the electric field strength and transient dissipated power in discrete domains, and the temperature distribution was obtained by substituting this value into the finite-dimensional temperature field submodel. Finally, the validity of the proposed numerical model was verified by comparing the results with the numerical results obtained with the conventional finite element method. The methodology presented in this paper provides a solid basis for designing microwave heating controllers.

A Novel Bandstop Filter Based on Two-Port Coaxial Cavities for the Installation of Metallic Mode Stirrers in Microwave Ovens

Metallic stirrers are commonly used in mode-stirred reverberation chambers but are less common in microwave ovens, because the use of a metallic axis creates a coaxial port in the cavity wall where the axis is introduced, which may result in significant microwave leakage levels. However, due to electromagnetic, mechanical, temperature, or chemical requirements, metallic axes must be used in some cases. In this paper, we present a high power coaxial filter that enables the use of metallic stirrers in microwave applications. The filter employs two-port coaxial cavities that are analyzed to determine their design parameters. These cavities are coupled via coaxial lines to achieve proper attenuation levels. The cavities were designed using commercial electromagnetic (EM) software, and a filter prototype was validated using vector network analyzer (VNA) measurements. The simulated and measured results show that a three-stage configuration can achieve attenuations greater than 70 dB at the 2.45 GHz ISM band, allowing the use and external handling of the metallic axis inserted into microwave ovens with negligible microwave radiation leakage.

Effects of Metal Boundary Stretching and Sample Translational Motion on Microwave Heating

For the purpose of improving uniformity and efficiency of microwave heating, moving components are widely used. In this paper, a kind of stretching microwave oven with a conveyor belt is designed. The conveyor belt and the stretching motion of the upper surface of the microwave oven make the electric field in the microwave cavity continuously change during heating, so that the absorption pattern of materials does not remain constant. The transformation optics method is used to simulate the stretching motion of the upper wall of the microwave oven, and the implicit function method is used to simulate the translational motion of the sample on the conveyor belt. The correctness of the simulation model is validated by experiments. The heating effects for the cases of fixed heating, stretching and translational motion are compared. Finally, the heating effects for the proposed model with different heated materials are also discussed.

Improvement of Heating Uniformity by Limiting the Absorption of Hot Areas in Microwave Processing of CFRP Composites

Carbon fiber reinforced polymer (CFRP) composites are integral to today’s industries. Curing or consolidation are vital processes for manufacturing CFRP components. Microwave processing has many advantages compared with conventional processing technologies using ovens or autoclaves; however, the uneven temperature distribution caused by the non-uniform microwave field has a significant influence on the quality of the cured products. In this study, we propose a new idea to solve this problem, i.e., limiting the absorption of hot areas. Under such circumstances, cold ones can catch up with them more easily. To adjust the absorbing capability of the CFRP laminate, periodically arranged metallic resonance structures supported by a dielectric spacer are introduced on its surface. The dielectric spacer, made of epoxy matrix and strontium titanate particles, is designed to possess a dielectric constant positively related to temperatures. In this situation, the microwave absorption (2.45 GHz) of the metal-dielectric-CFRP configuration is changed from 97.6% at room temperature to 55.9% at 150 °C continuously. As a result, a reduction of 43.1% in maximum temperature difference and 89% in standard deviation has been achieved.

Microwave-assisted preparation of polyphosphoric acid in a continuous-flow reactor

Polyphosphoric acid (PPA) is widely used in inorganic salt production, petrochemical industry, electronic material preparation and other manufacturing industries. Conventional preparation methods of PPA has disadvantages of pollution, high energy consumption and long production time. To address this problem, microwave continuous-flow preparation may be a desirable way due to its advantages of environmentally-friendly, rapidity and high efficiency. Therefore, to explore the process of preparing PPA by microwave continuous-flow method, a continuous-flow microwave reactor was designed for the dehydration process of orthophosphoric acid to prepare PPA in this paper. The microwave-assisted dehydration process was studied in comparison with the conventional dehydration process and the “closed” microwave-assisted dehydration process in terms of energy efficiency, process times and treatment capacity. The effect of input microwave power, reduced pressure and inlet flow velocity of orthophosphoric acid on the performance of the dehydration process was studied. The results showed that the influence of the microwave power on the temperature rise process during dehydration is greater than that of the reduced pressure. Moreover, the inlet flow rate has a great impact on the treatment capacity and product quality of the dehydration process. Bedsides, the comparison with the other two methods showed that microwave heating can effectively shorten the dehydration time, and the continuous-flow treatment can effectively improve the treatment capacity of microwave heating. The perspectives of the process scale-up by continuous-flow microwave heating method is also discussed.