Continuous-Flow Microwave Milk Sterilisation System Based on a Coaxial Slot Radiator.

Microwave continuous-flow liquid food sterilisation, in which the liquid is mainly heated by microwaves, has the advantages of fast sterilisation speed, energy saving, comprehensive elimination, and less nutrient loss. Circular pipes are commonly used in microwave continuous-flow liquid heating processing. However, with circular pipes, which are widely used in the industry, the heating is uneven owing to the phenomenon of tube focusing when adopting external radiation. In this study, a novel microwave continuous-flow milk sterilisation system based on a coaxial slot radiator is proposed. First, the coaxial slot radiator was designed to realise efficient radiation through the establishment of multi-physics model. The structure of the system was then optimised by comparing the heating efficiency and uniformity of simulation results. The effect of microwave coaxial slot radiator rotation on heating uniformity was simulated and the results show that the heating uniformity is improved obviously. Experimental equipment was set up to verify the results of the simulation. The experimental results are consistent with the simulation results. Finally, the sensitivity analysis of the system is performed to confirm that, when the dielectric properties and types of liquid food change, the heating of the proposed microwave continuous-flow system remains efficient and uniform.

Three-dimensional modeling of microwave discharges in a waveguide-based plasma source with experimental comparison.

This paper proposes a transient three-dimensional model to simulate microwave-induced discharges in a waveguide-based plasma source under intermediate pressures. A plane-symmetric simplification method is applied to simplify half of the microwave plasma source in the calculation domain, dramatically reducing the demand for computational resources and calculation time. Meanwhile, the numerical simulations remain in three dimensions without dimensionality reduction, which allows us to directly calculate the efficiency of power coupling from the incident microwave to the plasma. Besides, the computation decrease improves the convergence performance of the mathematical model, making it possible to model the entire discharge process from 1×10^{-9} to 1×10^{4}s. This period covers the instantaneous microwave breakdown to the formation of a stable plasma column near steady state. The results have revealed the electromagnetic waveguide structure change of the microwave plasma source during the discharge process. Several microwave power-coupling efficiencies of the waveguide-based plasma source with different thicknesses and permittivities of the glass tube are calculated and compared with the experimentally measured data. Furthermore, the effects of the glass tube on the electromagnetic modes of the traveling microwave propagating along the plasma column and the discharge properties are also investigated. The numerically obtained results generally agree with the theoretical analysis and the experimental data in our previous studies, demonstrating the validity of the proposed mathematical model and the plane-symmetric simplification method.

Radio Frequency Drying Behavior in Porous Media: A Case Study of Potato Cube with Computer Modeling.

To study the mechanism of heat and mass transfer in porous food material and explore its coupling effect in radio frequency (RF) drying processes, experiments were conducted with potato cubes subjected to RF drying. COMSOL Multiphysics® package was used to establish a numerical model to simulate the heat and mass transfer process in the potato cube and solved with finite element method. Temperature history at the sample center and the heating pattern after drying was validated with experiment in a 27.12 MHz RF heating system. Results showed the simulation results were in agreement with experiments. Furthermore, the temperature distribution and water vapor concentration distribution were correspondent with water distribution in the sample after RF drying. The water concentration within the food volume was non-uniform with a higher water concentration than the corner, the maximum difference of which was 0.03 g·cm-3. The distribution of water vapor concentration in the sample was similar to that of water content distribution since a pressure gradient from center to corner allowed the mass transfer from the sample to the surrounding in the drying process. In general, the moisture distribution in the sample affected the temperature and water vapor concentration distribution since the dielectric properties of the sample were mainly dependent on its moisture content during a drying process. This study reveals the mechanism of RF drying of porous media and provides an effective approach for analyzing and optimizing the RF drying process.

A self-boosting microwave plasma strategy tuned by air pressure for the highly efficient and controllable surface modification of carbon.

Surface modification is required to improve the activity and compositing ability of carbonaceous materials for their application in numerous areas such as energy storage, aerospace applications, and construction reinforcement. However, current strategies are facing problems such as the involvement of expensive and corrosive chemicals, poor controllability, and breakage of the carbon skeleton, thus sacrificing the mechanical and electrical properties. In this study, a green and controllable self-boosting microwave technology is proposed for the high-efficient surface modification of carbon. Air was used as the only oxidant. A carbon fiber cloth (CFC) is exposed to microwave irradiation in air for 90 s, yielding CFC with a surface oxygen content of 25.73%, 54.41%, and 52.56% at 1 atm, 8000 Pa, and 80 Pa, respectively, as determined via X-ray photoelectron spectroscopy. Notably, the content of each oxygen-containing functional group (e.g., -C-OH and -C[double bond, length as m-dash]O) is controllable by tuning the air pressure. Besides, CFC has enhanced mechanical and electrical properties. In comparison, CFC treated with a strong acid for 2 h only has a surface oxygen content of 21.4%, exhibiting greatly impaired electrical and mechanical properties. Numerical simulations at different pressures suggest that air plasma is triggered and boosted by the existence of CFC at 8000 Pa and 80 Pa, generating different electron number densities and electron temperature distributions, thus resulting in high-efficient and controllable modification.

Design of High Temperature Complex Dielectric Properties Measuring System Based on XGBoost Algorithm.

This paper aims to propose an online relative complex permittivity measurement system at high temperature based on microwave interferometer. A ridge waveguide with a TE10 mode was used in which the sample was heated and measured simultaneously at a frequency of 2450 MHz, and the microwave interferometer is used to collect the amplitude and phase difference of the incident signal. The Extreme Gradient Boosting (XGBoost) algorithm trained by the corresponding simulation data is used to construct the inversion model to calculate the complex dielectric coefficient of the tested material. Besides, this paper uses linear regression algorithm (LR) to calibrate the measurement system in order to improve the measurement accuracy. The entire system was tested using different materials at room temperature, and the maximum error of the measurement accuracy is less than 8% compared to the theoretical data. The robustness of the entire system was also tested by measuring Macor materials up to 800 °C. This proposed method provides an effective way to understand the mechanism between microwaves and matter at high temperatures.

Microwave-Assisted Synthesis of NiCo2O4 Double-Shelled Hollow Spheres for High-Performance Sodium Ion Batteries.

The ternary transitional metal oxide NiCo2O4 is a promising anode material for sodium ion batteries due to its high theoretical capacity and superior electrical conductivity. However, its sodium storage capability is severely limited by the sluggish sodiation/desodiation reaction kinetics. Herein, NiCo2O4 double-shelled hollow spheres were synthesized via a microwave-assisted, fast solvothermal synthetic procedure in a mixture of isopropanol and glycerol, followed by annealing. Isopropanol played a vital role in the precipitation of nickel and cobalt, and the shrinkage of the glycerol quasi-emulsion under heat treatment was responsible for the formation of the double-shelled nanostructure. The as-synthesized product was tested as an anode material in a sodium ion battery, was found to exhibit a high reversible specific capacity of 511 mAh g-1 at 100 mA g-1, and deliver high capacity retention after 100 cycles.

A Microwave Thermostatic Reactor for Processing Liquid Materials Based on a Heat-Exchanger.

Microwaves have been widely used in the treatment of different materials. However, the existing adjustable power thermostatic reactors cannot be used to analyze materials characteristics under microwave effects. In this paper, a microwave thermostatic chemical reactor for processing liquid materials is proposed, by controlling the velocity of coolant based on PLC (programmable logic controller) in different liquid under different constant electric field intensity. A nonpolar coolant (Polydimethylsiloxane), which is completely microwave transparent, is employed to cool the liquid materials. Experiments are performed to measure the liquid temperature using optical fibers, the results show that the precision of temperature control is at the range of ±0.5 °C. Compared with the adjustable power thermostatic control system, the effect of electric field changes on material properties are avoided and it also can be used to detect the properties of liquid materials and special microwave effects.

Model Stirrer Based on a Multi-Material Turntable for Microwave Processing Materials.

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.

Characterizing Adsorption Performance of Granular Activated Carbon with Permittivity.

A number of studies have achieved the consensus that microwave thermal technology can regenerate the granular activated carbon (GAC) more efficiently and energy-conservatively than other technologies. In particular, in the microwave heating industry, permittivity is a crucial parameter. This paper developed two equivalent models to establish the relationship between effective complex permittivity and pore volume of the GAC. It is generally based on Maxwell-Garnett approximation (MGA) theory. With two different assumptions in the model, two quantificational expressions were derived, respectively. Permittivity measurements and Brunauer-Emmett-Teller (BET) testing had been introduced in the experiments. Results confirmed the two expressions, which were extremely similar. Theoretical and experimental graphs were matched. This paper set up a bridge which links effective complex permittivity and pore volume of the GAC. Furthermore, it provides a potential and convenient method for the rapid assisted characterization of the GAC in its adsorption performance.

Anomalous dielectric relaxation with linear reaction dynamics in space-dependent force fields.

The anomalous dielectric relaxation of disordered reaction with linear reaction dynamics is studied via the continuous time random walk model in the presence of space-dependent electric field. Two kinds of modified reaction-subdiffusion equations are derived for different linear reaction processes by the master equation, including the instantaneous annihilation reaction and the noninstantaneous annihilation reaction. If a constant proportion of walkers is added or removed instantaneously at the end of each step, there will be a modified reaction-subdiffusion equation with a fractional order temporal derivative operating on both the standard diffusion term and a linear reaction kinetics term. If the walkers are added or removed at a constant per capita rate during the waiting time between steps, there will be a standard linear reaction kinetics term but a fractional order temporal derivative operating on an anomalous diffusion term. The dielectric polarization is analyzed based on the Legendre polynomials and the dielectric properties of both reactions can be expressed by the effective rotational diffusion function and component concentration function, which is similar to the standard reaction-diffusion process. The results show that the effective permittivity can be used to describe the dielectric properties in these reactions if the chemical reaction time is much longer than the relaxation time.

A Phase-Shifting Method for Improving the Heating Uniformity of Microwave Processing 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.

Temperature cycle measurement for effective permittivity of biodiesel reaction.

The aim of this communication is firstly to describe the effective permittivity of the biodiesel reaction for multiphysics calculation of microwave heating. An innovatively method was proposed to get the effective permittivity of the biodiesel reaction solution through one measurement, which can be directly applied for the thermal analysis of microwave heating on biodiesel reaction based on the multiphysics calculation. The effective permittivity of biodiesel reaction at any temperature and concentration obtained in this paper. The maximum relative error of real part is 4.10%, and 5.65% for imaginary part. The calculated results also show good agreement with the measured results at constant temperature.

Thermal analysis on the process of microwave-assisted biodiesel production.

The aim of this work was firstly to do a precise thermal analysis of microwave assisted production of biodiesel. In this paper, the effective permittivity of biodiesel synthesis was updated with two methods: a traditional method and a bivariate function of temperature and concentration of one component, then the thermal analysis of the reaction process were accomplished with multi-physics calculation. The results show that there exists large distinction in temperature between these two simulation results calculated by the two methods. The two hot spots locate in the opposite side and their temperature's difference is up to 9°C when the reaction is just carried out for 18s. But the temperature risings and distributions calculated by the new method are closer to the measured results. The thermal analysis based on the new method will be helpful for the industrial design of biodiesel production.