Microwave Energy Distribution Research Articles

Sterilization of oil palm fruit utilizing continuous microwave sterilizer

This study evaluates the performance of a continuous microwave device in the sterilization of oil palm fruits spikelet. The device comprises four microwave generators (magnetron), a tunnel cavity driven by a belt conveyor, and a tray as a container for palm fruit. This study aims to evaluate the distribution and penetration of microwave energy into palm fruit and to examine the quality of palm oil concerning power density and residence time adjusted in this study. Continuous microwave sterilizer was operated at various masses of oil palm fruit (0.5, 1, 1.5 kg), residence time (6, 8, 10, 12, 14 min), and microwave power (180, 300, 450, 600, 800 W). The resulting temperature in this study ranged from 39 to 97 °C. Observation of the temperature gradient inside-out of a single fruit indicates the interior temperature at the kernel nut is always higher as compared to exterior temperature (abscission layer), with temperature differences ranging from 3 to 23 °C. A minimum power density of 300 W/kg, a residence time of 6 min, and a temperature of 50 °C were required to produce palm oil with FFA less than 5%. Performance indicators of the prototype used in this study are a number of moisture loss dan oil quality. The moisture loss during the sterilization process was around 0.5 to 10.6% which indicates some water content in the fruit, while the content of water, FFA, β-carotene and vitamin E ranging between 1.9 and 17.6%, 0.32-1.74%, 186.14-608.59 ppm and 165.47-610.17 ppm, respectively. © 2001 Elsevier Science. All rights reserved.

Irreversible thermochromic ink in the identification of over- and under-processed product segments in microwave-assisted freeze drying

The formation of over- and under-processed product regions is an inherent drawback of microwave-assisted freeze drying (MWFD) of food attributed to the inhomogeneity of microwave energy distribution. To overcome limitations in the investigation of inhomogeneity effects, the scope of this study was the development and validation of a new method using an irreversible thermochromic colour-component (ITC) with a particular colour intensity profile as a function of temperature (40–70 °C). For validation, thermal images and spatially-resolved biomarker inactivation was compared to the colour intensity pattern resulting from the temperature-induced colour reaction of ITC in MWFD. The ITC method provides advantages over existing methods, as it visualises the spatial distribution of maximum processing temperature independently of post-treatment effects and existing dryer design. This easy-to-use method has the potential to eliminate or reduce preconceptions due to poorly known inhomogeneity of microwave applications and pave the way for energy-efficient and fast MWFD of food.

Design of continuous-flow microwave reactor based on a leaky waveguide

Using microwave heating for biodiesel production has several advantages, including an accelerated reaction rate and improved product quality. However, large-scale microwave-assisted biodiesel production still suffers from low efficiency and uneven heating, which limit its industrial applicability. To address these issues, in this study, a reactor based on leaky waveguide is proposed for continuous microwave-assisted biodiesel production. Firstly, a radiator based on a leaky waveguide is designed to realize a uniform distribution of microwave energy along the axial and longitudinal directions. Then, a multi-physics model based on electromagnetism, heat transfer in fluids, chemical engineering, and laminar flow is developed to simulate the continuous microwave-assisted biodiesel production process. Compared to the traditional bottom-fed and cylinder-fed reactors, the leaky-waveguide reactor has the potential to improve the heating uniformity and energy efficiency, and the microwave energy utilization efficiency can more than 95 % and the value of the outlet COV can be reduced to 1.51 × 10−3. Experiments were carried out to validate the proposed model, and the experimental results are in good agreement with the simulated results. Finally, the effects of the pitch circle radius, radius, and number of turns (screw pitch) of the helical tube used for the microwave-assisted biodiesel production process were analyzed. The radius and number of turns (screw pitch) have a greater effect on the system than pitch circle radius. This continuous microwave-assisted biodiesel production reactor provides a possibility for large-scale industrial production and can also be widely applied to the chemical engineering with pressure continuous flow reactor and food industry.

A new reactor for process intensification involving the simultaneous application of adjustable ultrasound and microwave radiation

Ultrasound (US) and Microwaves (MW) are effective methods for processes intensification. Their combined use in the same reactor can lead to remarkable results. Recently there has been a resurgence of interest in this field for new synthetic applications using reactors based upon existing technologies. We describe here a new type of apparatus in which the thermal energy is continuously removed from the system making possible the use of high power and adjustable ultrasonic and microwave densities throughout the process. The installation consists of a glass reactor located in a monomode applicator which is immersed at the same time in an ultrasonic device which can be operated at different frequencies and powers. A liquid, transparent to microwaves, was used to couple ultrasonic energy to the reactor and to remove the heat generated. Comsol software was used to get information about the distribution of ultrasonic and microwave energy between the reactor liquid and the coupling fluid. The performance was assessed using the conversion of p-nitrophenol into 4-nitrocatechol as a chemical dosimeter and a transesterification.

On finite element analysis of material removal rate in microwave drilling of borosilicate glass

Abstract In the present work, microwave drilling of borosilicate glass has been investigated at 2.45 GHz using experimental and simulation tools. Microholes were produced on glass (Thickness = 1.3 mm) using a graphite concentrator (Diameter = 500 µm) at different dielectric velocity (0.2 ms−1, 1.0 ms−1, 1.8 ms−1, 2.6 ms−1). Effect of power and dielectric velocity has been studied on material removal rate. A model of the same was developed using the finite element method. Experimental results were found in reasonable agreement with the simulation results. Study on distribution of input microwave energy during microwave drilling indicated that maximum usable energy is utilized in heating the workpiece, whereas, rest of the usable energy is absorbed by the dielectric and concentrator. Material removal rate increases with increase in power but also attracts defects like crack around the hole. It was observed that increase in dielectric velocity up to 1 ms−1 improves material removal rate due to the efficient flushing of glass residue. However, further increase in dielectric velocity decreases the material removal rate due to the loss of excess heat flux from the machining zone.

Assessment of uniformity of microwave-based heating profiles generated by solid-state and magnetron systems using various shapes of test samples

In a previous paper, we established a test routine and found in cuboid gellan test samples containing Maillard reactive agents that the solid-state system yields a more uniform heating profile. This was attributed to a more uniform microwave energy distribution profile in the solid-state oven cavity. In this work, we aim at determining the heating profile in various test sample shapes exposed to classical magnetron and innovative solid-state microwave systems. Therefore, we assessed the uniformity of heat processing between both solid-state and magnetron microwave generators using other test sample shapes of spherical and cylindrical forms and compare the results with the test samples with cuboid shape. The comparison confirms that the solid-state microwave system also results in more uniform heating of the cylindrical test samples like in the cuboid test sample, while the spherically-shaped test sample this was not the case. The explanation most likely is that the focus of microwave energy in the center of the spherical sample is more dominating such that the external microwave energy distribution is less decisive for a uniform heating profile. In other words, the possibility of distinguishing the differences between the solid-state and magnetron microwave generators and comparing the uniformity of their thermal processing requires a certain sample shape. Spherical samples are less suitable, because of the focusing of energy towards the sample center and difference between the systems can be determined much less sensitively compared to other shapes. These results contribute to understand microwave heating systems better and provide guidance in product development in terms of shapes most suitable to achieve heating profiles with reduced issues related to hot/cold spots in the product.

Modeling the Distribution of the Electromagnetic Field in the Device for Adding the Power of Microwave Magnetrons

Substantiation of the physical nature of adding the power of magnetron generators in the device’s waveguide, assessment of the electromagnetic properties of objects without experimental verification, testing of scientific and technical hypotheses for adequacy without creating prototypes are possible by modeling the distribution of the electromagnetic field in the CST Studio program. (Research purpose) The research purpose is to simulate the distribution of the electromagnetic field in the device for adding the power of microwave magnetrons. (Materials and methods) CST Studio software was used to design models of the distribution of the electromagnetic field in the device for adding the power of microwave magnetrons. (Results and discussion) Models of the distribution of the electromagnetic field in the device for adding the power of microwave magnetrons was designed using the CST Studio program. It was found that at operating frequencies of 1, 3, 4 and 5 GHz, the distribution of the electromagnetic field of different densities is dispersed throughout the volume of the waveguide, which does not allow the use of a waveguide of the appropriate size to perform the basic functions of transmission and distribution of microwave energy, while at the operating frequency of 2 GHz the basic operating conditions of the waveguide are observed. It was found that, within the boundaries of the waveguide of the developed device, the force lines of the electromagnetic field propagate from the surface of the emitters into the space of the waveguide, are reflected from the walls, distributed over the volume of the waveguide, and rush to the output open end. (Conclusions) It was found that the generated pulses of the power flow of electromagnetic energy of an ultrahigh frequency of the flux of two magnetrons, corresponding to a value of 116 Volts·Amperes at square meter, propagating throughout the volume of the waveguide at different time intervals according to the operating frequency, take a one-way travel of their electromagnetic field lines from short-circuited end in the direction of the open part of the waveguide, are localized with the highest concentration in the output horn end of the waveguide, which is explained by the phenomena of power addition after providing traveling wave due to the coordination of the metric dimensions of the waveguide and the working wavelength.

Microwave heating and non-thermal effects of sodium chloride aqueous solution

ABSTRACTThe dielectric thermal and non-thermal properties of sodium chloride aqueous solution under the microwave region have been estimated. The dielectric properties, hydrogen bonding, transport properties, energy distribution and local structure have been evaluated by classical molecular dynamics method. In the process of microwave energy distribution, the direct coupling of rotational motion, vibration and redirection is revealed. Microwave energy is converted into kinetic energy and interaction energy between two molecules. A mechanism for exploring the effects of microwaves on the non-thermal effects of brine systems over a longer simulation time and a wider microwave range is proposed. The increase in field intensity is usually accompanied by local damage to the water structure near the hydrated ions. More specifically, above the field threshold, the residence time of water molecules near the ions significantly decreases.HighlightsMicrowave energy is transferred to the kinetic energy and the energy between the molecules.The increase in field intensity is usually accompanied by local damage to the water structure near the hydrated ions.The larger electric field strength amplifies the effect of frequency.The residence time of water molecules near the ions significantly decreases.

Energy transformation and dynamic properties of N,N-dimethylformamide aqueous solution under E/M field using NNVE MD simulations

Nonequilibrium NVE (NNVE) molecular dynamics (MD) simulations have been performed to investigated the N,N-dimethylformamide aqueous solution in both the absence and presence of E/M field (E=5×108V/m, the microwave frequency are from 0 to 100GHz). The main target of this work is to explore the energy transformation and dynamic properties of DMF aqueous solution during the microwave heating process. The temperature variation, microwave heating rate and energy distribution have been obtained. The direct coupling of rotational motion, vibration and reorientation have been revealed during the microwave energy distribution, and there are two-thirds microwave energy are stored as the intermolecular energy. The structural characteristics are further obtained from the radial distribution functions, tetrahedral order parameters and hydrogen bond. Compare to the conventional heating, the molecular conformations show significant difference and the structure of aqueous solution is intensely disturbed. The apparent increase of diffusion coefficient discloses that high microwave frequency accelerates the molecules motion.

Effects of Microwave Power on Extraction Kinetic of Anthocyanin from Blueberry Powder considering Absorption of Microwave Energy

Microwave power as directly controlled parameter determines the absorption of microwave energy inside extraction vial and the yield of objective component in microwave-assisted extraction (MAE). The aim of this study was to elucidate the effects of microwave powers on the yield of anthocyanin from blueberry (Vaccinium spp.) powder based on the absorption of microwave energy in extracts under MAE. Rising microwave powers have little effect on the distribution of microwave energy in extraction vial, but increase its temperature. The simulation results indicated that strength of electrical field tends to decay trend along with microwave irradiation; however, temperatures have the highest level in center location in an extraction vial. High microwave power strongly breaks cell wall of blueberry to open diffusion route of interior anthocyanin toward extraction solvent. A critical extraction temperature of 50.75±0.88°C is obtained with the highest anthocyanin yield under MAE. Three monomers of anthocyanin including pelargonidin, cyanidin, and delphinidin are, respectively, of the highest content of 1.02 μg/mL, 0.66 μg/mL, and 0.31 μg/mL. The research results contribute to the improvement of efficiency of microwave energy and yield of anthocyanin.

Open source ELMER software based FEM modeling of waveguides and resonant cavities for microwave heating and drying devices

Abstract Microwave devices are widely used in the industry and in the specialized laboratory analyses. Development of such devices requires the possibility of modeling of microwave energy distribution in the specific resonant chambers. Until now, such modeling was possible only with the use of commercial software or was limited to specific cases. The paper presents an open-source module for ELMER software for solving timeharmonic Maxwell’s equations, allowing modeling of microwave waveguide lines. Three test cases of different resonant chambers are investigated at 2.45 GHz frequency. Modeling results obtained from the open-source ELMER Vectorial Helmholtz module show that the application of this software can be effective in R&D works, enabling high-tech small and medium enterprises involvement in advanced microwave technology.

Skull and cerebrospinal fluid effects on microwave radiation propagation in human brain

The determination of microwave absorption distribution in the human brain is necessary for the detection of brain tumors using thermo-acoustic imaging and for removing them using hyperthermia treatment. In contrast to ionizing radiation, hyperthermia treatment can be applied to remove tumors inside the brain without the concern of including secondary malignancies, which typically form from the neuronal cells of the septum pellucidum. The aim of this study is to determine the microwave absorption distribution in an adult human brain and to study the effects of skull and cerebrospinal fluid on the propagation of microwave radiation inside the brain. To this end, we simulate the microwave absorption distribution in a realistic adult brain model (Colin 27) using the mesh-based Monte Carlo (MMC) method. This is because in spite of there being other numerical methods, the MMC does not require a large memory, even for complicated geometries, and its algorithm is simple and easy to implement with low computational cost. The brain model is constructed using high-resolution (1 mm isotropic voxel) and low noise magnetic resonance imaging (MRI) scans and its volume contains voxels, covering the brain completely. Using the MMC method, the radiative transport equation is solved and the absorbed microwave energy distribution in different brain regions is obtained without any fracture or anomaly. The simulation results show that the skull and cerebrospinal fluid guide the microwave radiation and suppress its penetration through deep brain compartments as a shielding factor. These results reveal that the MMC can be used to predict the amount of required energy to increase the temperature inside the tumour during hyperthermia treatment. Our results also show why a deep tumour inside an adult human brain cannot be efficiently treated using hyperthermia treatment. Finally, the accuracy of the presented numerical method is verified using the signal flow graph technique.

Bubble formation in water with magnetite nanoparticles during microwave irradiation

In this study, bubble formation phenomena in the dispersion medium of magnetite nanoparticle in water during microwave irradiation were investigated. From the experimental results, it was found that under microwave treatment, the maximum bubble size was greatly influenced by the suspension density at a given temperature due to the distribution of microwave energy to the particles itself or the liquid-air interface of bubbles around the particle. Nanoparticles in the dispersion medium were also found to play a key role in achieving stable nucleation of bubbles on particles surface. Smaller bubble sizes were produced at higher suspension density. Microwave power also has a positive impact on the maximum bubble size especially at low suspension density. Moreover, particle material having dielectric constant property was also found to be an important factor for bubble size during microwave irradiation. From these results and findings, it is envisaged that bubble size can be controlled by the presence of nanoparticle and microwave irradiation.

Investigation on the microwave drying kinetics and pumping phenomenon of lignite spheres

The influence of microwave power levels (231–700W) on microwave drying behavior of compressed lignite spheres (10mm and 20mm in diameter) in a modified microwave drying setup was evaluated. The microwave energy distribution in the sample was obtained through numerical simulation based on the Maxwell equation. The temperature distribution in the samples was detected through an infrared camera. The average drying rates for the 10-mm and 20-mm samples increased by 73% and 56% as the microwave power level rose from 231 to 700W. The microwave energy mainly concentrated on the lower part of the geometric center of the samples and the uniformity index increased by 32% as the sample size rose from 10 to 20mm, while the uniformity index of temperature distribution increased by about 2.5%. The values of activation energy for the 10-mm and 20-mm samples were determined as 134 and 41.93Wg−1 based on a modified Arrhenius equation. The pumping phenomenon was only detected on the surface of 20-mm raw lignite sphere block in the warm-up period at power levels of 539 and 700W.

Non-thermal effects of microwave in sodium chloride aqueous solution: Insights from molecular dynamics simulations

Abstract We investigated the microwave non-thermal effects of the sodium chloride aqueous solution by molecular dynamics simulations, from water microcosmic structure insights. And a field-induced threshold effect is observed, which is consistent with experiment. The hydrogen bond dynamics dependant on microwave non-thermal effects is also discussed. We are presented a mode that microwave energy distribution is partly stored to internally rearrange hydrogen-bond network in the sodium chloride aqueous solution. A less intuitive pattern of the interaction between microwave and electrolytes is also observed through the second solvation shells fluctuation induced by the microwave.

Mechanism for the elimination of pollutants from aqueous solutions adopting NiR2O4 (R = Fe, Cr and Al) with microwave energy

Abstract The microwave-induced catalytic oxidation (MICO) process has demonstrated promise for the treatment of dye contaminants in water but its mechanism has not been fully elucidated so far. In this study, NiFe2O4 catalysts were synthesized using sol-gel and chemical co-precipitation (cp) methods and applied to decolorizing a series of pollutants in a MICO process. NiFe2O4(cp) calcined at 100 °C or 300 °C decolorized over 60% of brilliant green (BG) in 10 min, but the decolorization efficiency decreased quickly when the catalyst was reused. NiFe2O4(sol-gel), however, did not achieve any significant removal of BG. Moreover, the solution pH rose to 8.6 when suspensions containing NiFe2O4(cp-100 °C) were irradiated with microwave (MW) energy. Control experiments show that addition of NaOH achieved efficient decolorization of aqueous BG solution. This indicates that MW irradiation may promote the release of OH− residues and enhance the removal of dyes. Furthermore, in suspensions containing organic polydimethyl siloxane fluid and NiFe2O4(100 °C), the MW energy distribution could be greatly changed, accompanying with quick increase of temperature. Variations of XRD patterns for NiFe2O4(100 °C), however, were not observed, even if suspensions contained carborundum (SiC). This indicates that so-called hot-spots and reactive oxygen species could be hardly formed in suspensions containing inorganic solvent H2O.

Investigation of Microwave Energy Distribution Character in a Resonator Type Plasmatron

An investigation of microwave energy distribution character in a plasma of microwave discharge inside a plasmatron based on a rectangular resonator has been carried out. The experiments were done applying the "active probe" method. Microwave discharge was excited in the air and oxygen. It has been found out that the readings of the "active probe" along the discharge chamber are of periodic character. The readings of the "active probe" and data on the local electric conductivity of plasma obtained using electrical probes have been compared.

Microwave curing of carbon–epoxy composites: Penetration depth and material characterisation

Microwave heating has several major advantages over conventional conductive heating when used to cure carbon–epoxy composites, especially in speed of processing. Despite this and many other well-known advantages, microwave heating of carbon–epoxy composites has not taken off in industry, or even academia, due to the problems associated with microwave energy distribution, arcing, tool design and (ultimately) part quality and consistency, thus leading to a large scepticism regarding the technique/technology for heating such type of materials. This paper presents some evidence which suggests that with the correct hardware and operating procedure/methodology, consistent and high quality carbon–epoxy laminates can be produced, with the possibility of scaling up the process, as demonstrated by the micro- and macro-scale mechanical test results. Additionally, the author proposes a methodology to practically measure the maximum microwave penetration depth of a carbon–epoxy composite material.

Thermoacoustic imaging over large field of view for three-dimensional breast tumor localization: a phantom study.

Previous studies demonstrated that thermoacoustic imaging (TAI) has great potential for breast tumor detection. However, large field of view (FOV) imaging remains a long-standing challenge for three-dimensional (3D) breast tumor localization. Here, the authors propose a practical TAI system for noninvasive 3D localization of breast tumors with large FOV through the use of ultrashort microwave pulse (USMP). A USMP generator was employed for TAI. The energy density required for quality imaging and the corresponding microwave-to-acoustic conversion efficiency were compared with that of conventional TAI. The microwave energy distribution, imaging depth, resolution, and 3D imaging capabilities were then investigated. Finally, a breast phantom embedded with a laboratory-grown tumor was imaged to evaluate the FOV performance of the USMP TAI system, under a simulated clinical situation. A radiation energy density equivalent to just 1.6%-2.2% of that for conventional submicrosecond microwave TAI was sufficient to obtain a thermoacoustic signal with the required signal-to-noise ratio. This result clearly demonstrated a significantly higher microwave-to-acoustic conversion efficiency of USMP TAI compared to that of conventional TAI. The USMP TAI system achieved 61 mm imaging depth and 12 × 12 cm(2) microwave radiation area. The volumetric image of a copper target measured at depth of 4-6 cm matched well with the actual shape and the resolution reaches 230 μm. The TAI of the breast phantom was precisely localized to an accuracy of 0.1 cm over an 8 × 8 cm(2) FOV. The experimental results demonstrated that the USMP TAI system offered significant potential for noninvasive clinical detection and 3D localization of deep breast tumors, with low microwave radiation dose and high spatial resolution over a sufficiently large FOV.

Microwave Setup Design for Continuous Fine‐Chemicals Synthesis

AbstractA feasibility study in state‐of‐the‐art multi‐ and monomode microwave cavities was conducted to outline important functionalities necessary in a microwave setup for continuous fine‐chemicals synthesis. For multimode operation, the heating efficiency depends on the position as well as on the volume distribution of the load. For monomode operation, the heating efficiency does not depend on the axial position and has a maximum with respect to the load diameter. Based on this study, a microwave setup consisting of four cavities was designed and constructed. The setup provided a defined microwave field pattern and allowed focusing of the microwave power. A near‐uniform distribution of microwave energy in all four cavities of the setup was achieved at reduced grid to applicator losses.