Microwave Heating Uniformity Research Articles

Effect of load on the heating efficiency and temperature uniformity in multi-mode cavity applicators

ABSTRACTIn this paper, we investigate the effect of load size, geometry and properties on microwave heating efficiency and heating uniformity during microwave processing of materials using multi-mode cavity applicators. For this purpose, a numerical technique based on finite element method and implemented using COMSOL Multiphysics package is deployed to compute heating efficiency expressed in terms of power absorbed by the material divided by the input power. Both field variations and resulting temperature evolutions are computed for each size and shape studied. Results show that heating efficiency and temperature uniformity are dependent on the size, the geometry and the properties of the material to be treated and can be controlled to a large extent by just changing these material characteristics without the need of external impedance matching components. The model is experimentally validated and good agreement between the theoretical and experimental results is obtained.

Continuous tubular microwave heating of homogeneous foods: evaluation of heating uniformity

ABSTRACTA pilot-scale process for continuous in-flow microwave processing of foods, designed and implemented at SP Food and Bioscience, was evaluated for heat treatment of a homogeneous model food for high-temperature short-time (HTST) conditions, at constant total input microwave power, at 2450 MHz. The microwave system has three consecutive cavities, one excited by the TM020 microwave mode that heats primarily in the tube centre, and two TM120 mode cavities that heat primarily in the tube periphery. The temperature uniformity of the homogeneous model food after microwave heating is here evaluated in terms of spatial distribution, for different set-ups of input microwave power in each cavity and for different order of the placement of the cavities, while maintaining the total input microwave power. The microwave heating uniformity is evaluated, based on measured and calculated radial temperature profiles. Combined TM020 and TM120 heating was found to result in more uniform heating by means of spatial temperature uniformity over the tube cross section. Furthermore, appropriately selected microwave power distribution between the centre and periphery heating cavities results in a stable heating profile in the studied food, that differs only about 10 °C or less between highest and lowest average values directly after microwave heating.

Combining sliding mode neural network with Cuckoo Search to make a uniform microwave heating process

Combining sliding mode neural network with Cuckoo Search to make a uniform microwave heating process

Research of uniformity evaluation model based on entropy clustering in the microwave heating processes

This paper proposes a uniformity evaluation method based on Spectral Clustering and Maximum Information Entropy (ECUEM) for clustering the simulation results in the microwave heating system. The proposed method can effectively evaluate the dataset of the electric field E, the magnetic field H, the temperature field T, and analyze the non-uniformity phenomenon in the microwave heating processes. Compared with other clustering algorithms, the ECUEM can get better clustering results for the dataset in simulation of microwave heating. In particular, in the resonant cavity, the experimental results show that the minimum the evaluation results, the better the materials heating uniformity. In addition, when the ECUEM method is used to analyze the experiment of waveguide moving, the best position (0, 11/20⁎do, 3/14⁎ho) of waveguide can be obtained; at the same time, the uniformity or efficiency of materials microwave heating is the best. Moreover, other rules have been obtained in the microwave heating processes. Thus, the proposed method would provide a new method to guide the researchers who are working in the area of dataset clustering in the microwave heating.

Numerical and Experimental Validation of Optimization Results in Microwave Enhanced Infrared Landmines’ Detection

Landmines still pose a serious threat mostly to civilians. Therefore, there is no doubt that working on the methods of landmines’ detection and liquidation should be considered extremely important. In this paper, we introduce the active infrared thermography with microwave excitation used in nonmetallic landmines’ detection. The effectiveness of the method is strongly dependent on the manner in which microwaves illuminate the ground. In this paper, we propose a new waveguide flange’s construction, which guarantees the uniform field distribution, and through this, a relatively uniform microwave heating on a certain area. The flange’s shape is based on box-horn. Its dimensions will be determined using a hybrid optimization algorithm. Results obtained in an optimization process will be validated using the numerical models. The experiments showing flange’s efficiency, concerning heating of sand itself and sand with buried landmines, will also be presented.

Microwave-assisted synthesis of BSA-modified silver nanoparticles as a selective fluorescent probe for detection and cellular imaging of cadmium(II)

We have developed a microwave-assisted method for the synthesis of silver nanoparticles (AgNPs) whose surface is modified with bovine serum albumin (BSA). The reaction involves reduction of the BSA-Ag(I) complex by tyrosine in strongly alkaline solution to form BSA-AgNPs. The reaction takes a few minutes only owing to rapid and uniform microwave heating. The modified AgNPs were characterized by UV–vis and fluorescence spectroscopy, transmission electron microscopy and X- ray photoelectron spectroscopy. The BSA-AgNPs are yellow and display luminescence with a maximum at 521 nm if excited at 465 nm. They have a hydrodynamic diameter of 3–5 nm and possess good colloidal stability in the pH 4.6 to 12.0 range. The fluorescence of the BSA-AgNPs is enhanced by Cd(II) ion due to the formation of a stable hybrid conjugate referred to as Cd@BSA-AgNPs. The effect was exploited to quantify Cd(II) in spiked real water samples with a 4.7 nM detection limit, and also to fluorescently image Cd(II) in Hepatoma cells.

Numerical Investigation on Effect of Food Particle Mass on Spout Elevation of a Gas–Particle Spout Fluidized Bed in a Microwave–Vacuum Dryer

The effect of food particle mass on the instantaneous fluid dynamic characteristics of a gas–particle spout fluidized bed in a pulsed spouted microwave–vacuum drying (PSMVD) system was investigated by a numerical method. The spout fluidization process in a pseudo-2D spout fluidized bed was simulated by computational fluid dynamics (CFD) using the inviscid two-fluid theory method (TFM) based on the kinetic theory of granular flow. The effect of particle mass on the particle flow pattern, jet penetration depth, and pressure drop was evaluated. Under a specific particle mass, the spout velocity was an important factor controlling particle status in the spout fluidized bed and a critical velocity was identified for effective transition of the flow pattern. With an increase in particle mass, the spout velocity tended to increase to obtain the expected gas–particle flow pattern and the correlation between the particle mass and particle flow pattern was revealed. This research on the fluid dynamic characteristics of a gas–particle spout fluidized bed could be used to improve the uniformity of particle mixing and microwave heating.

Numerical study on spout elevation of a gas-particle spout fluidized bed in microwave-vacuum dryer

The dynamic characteristics of gas-particle spout fluidized bed in a pulsed spouted microwave-vacuum drying system (PSMVD) were investigated. The spout fluidization process in a pseudo-2-D spout fluidized bed was simulated by computational fluid dynamics (CFD) using the inviscid two-fluid theory method (TFM) based on the kinetic theory of granular flow. The dynamic characteristics of the spout fluidized bed and the effect of spout elevation on the particle movement were revealed, which could be used to improve the uniformity of particle mixing and microwave heating. The mathematical model demonstrated that the spout fluidization process includes isolated, merged and transitional jets and the fluidization at a specific spout gas velocity has a start-up stage and a quasi-steady fluidization stage. The spout velocity was an important factor controlling particle status in the spout fluidized bed and a critical velocity was identified for effect transition of the flow pattern. There was an approximately linear correlation between the jet penetration depth and the spout velocity. When the spout gas velocity increased up to the critical velocity region, the pressure drop tended to convert from negative pressure to positive pressure.

Microwave-Assisted Fabrication of Nanoparticulate TiO2Microspheres for Synergistic Photocatalytic Removal of Cr(VI) and Methyl Orange

High yield production of micro/nanostructured nanoparticulate TiO2 microspheres (NTMs) via a facile microwave-assisted hydrothermal approach was investigated. The rapid and uniform microwave heating could reduce the reaction time to 30 min, an order of magnitude shorter than that of conventional hydrothermal methods. The characterization data confirmed that the resultant NTMs were highly uniform in size, having an average diameter of ∼0.5 μm. The obtained NTMs were found to be constructed by well-crystallized anatase phase nanoparticles ranging from 5 to 10 nm that can be readily controlled by the microwave radiation temperature. Nitrogen sorption isotherm analysis revealed that the obtained NTMs possessed abundant mesoporous structures with a high specific surface area of 124 m(2) g(-1). An in situ self-aggregation formation process under controllable pH in presence of urea was proposed. The results obtained from the application of NTMs for simultaneous photocatalytic decontamination of Cr(VI) and methyl orange (MO) demonstrated a strong synergistic effect that dramatically enhanced both Cr(VI) reduction and MO oxidation removal efficiencies. This work not only enriched the synthesis methods of the micro/nanostructured TiO2, but also provided a new means to improve the photocatalytic efficiency via structural-induced synergistic effect, applicable to the other catalysis systems.

Microwave Drying of Food and Agricultural Materials: Basics and Heat and Mass Transfer Modeling

Microwave drying is based on a unique volumetric heating mode facilitated by electromagnetic radiation at 915 or 2,450 MHz. The responses of a lossy food product to dielectric heating result in rapid energy coupling into the moisture and lead to fast heating and drying. A significant reduction in drying time in microwave drying is often accompanied by an improvement in product quality, making it a promising food dehydration technology. The need for improvement in engineering design and process optimization for microwave drying has stimulated the development of computer simulation techniques to predict temperature and moisture history and distribution in the product to be dried. In this article, we present the basics of dielectric heating and drying, examine the heat and mass transfer models developed for the simulation of microwave drying processes, and discuss dielectric properties of selected food products as influenced by moisture, temperature, and porosity. In addition, we analyze nonuniform heating caused by the geometry and composition of the product, as well as by the nonuniform distribution of electromagnetic field in a microwave cavity, followed by a discussion on how to improve the microwave heating uniformity. We focus the discussion on heat and mass transfer models developed over the years to simulate microwave drying, including simplified ones, those based on diffusion theory, and coupled heat and mass transfer analysis with the Philip and de Vries theory, Luikov scheme, Whitaker method, and two-region model. In the end, the determination of the heat source term in the energy equation, numerical schemes used to solve the partial differential equations, and the model validation are also discussed.

Characterization of a combination oven prototype: Effects of microwave exposure and enhanced convection to local temperature rise in a moist substrate

The combination of microwaves exposure with the classical convection heating is seen as a practical solution to improve the uniformity and control of pure microwave heating of moist porous substrates. In the present paper, the complex coupling of electromagnetic exposure and enhanced fluid flow and heat transfer is explored, with an emphasis on the competition of such different mechanisms. To this end, some simple experimental techniques have been adopted to determine the thermal response of a common biological substrate in a microwave/jet-impingement oven prototype (1 kW of nominal power), allowing for the local characterization of its electromagnetic and fluid dynamic behavior. A rational approach is proposed by presenting a number of descriptors to help identify the interrelationships for all phenomena at stake, including the total process time (up to 1 min), the jet temperature (in the 60–100 °C range) and Reynolds number (in the 8000–15 000 range). Therefore the effects of microwave exposure and relaxation times, working air velocity and temperature on the substrate's local temperature rise, are reported and discussed. Even in the explored range of microwave and jet thermization potentials investigated herein, different substrate portions experienced different temperature rises. The proposed configuration and analysis can be used to exercise due control of material conditioning and treatment in a combined microwave/forced convection framework.

Combining microwave and jet-impingement in a oven prototype

The combination of microwaves exposure with the localized convection heating is seen as a practical solution to improve the uniformity and control of pure microwave heating of moist porous substrates. This effect is explored here, with an emphasis on the competition of such physical mechanisms. Simple experimental techniques have been adopted to determine the thermal response of a common biological substrate in a microwave (MW)/jet-impingement (JI) oven prototype (1kW of nominal power), allowing for the local characterization of its electromagnetic and fluid dynamic behaviour. A rational approach is proposed by presenting a number of descriptors to help identify the MW/JI interrelationships that drive the substrate's local temperature rise, including the total process time (up to 1min), the JI temperature (in the 60-100°C range) and Reynolds number (in the 8000-15000 range).

Uniform microwave heating of semifinished abrasive tools with thermoreactive organic binders

Uniform microwave heating of semifinished abrasive tools with thermoreactive organic binders

Hamburger Composition and Microwave Heating Uniformity

ABSTRACT: The importance of recipe modifications on the microwave heating uniformity and dielectric properties of a hamburger was investigated in a household oven. Factorial design was used in the experimental setup, and data were evaluated by analysis of variance. Temperature was monitored by a fiberoptic device, infrared imaging, and thermocouples. Dielectric data of the hamburger components were measured by a cavity perturbation method. An analytical model for calculating microwave energy balance was developed and compared with experimental data, and numerical modeling was also used in the analysis, and the results were compared. Uneven heating is difficult to improve by recipe modifications only. The differences between the analytical and numerical modeling indicate that the uniformity of heating in relatively small loads is difficult to achieve.

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

ABSTRACT The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.

MICROWAVE AND WATER BLANCHING OF CORN KERNELS: CONTROL OF UNIFORMITY OF HEATING DURING MICROWAVE BLANCHING

Corn kernels were blanched in water and by microwave. The blanching efficiency was measured by four parameters; the thermolability of soluble peroxidase (POD), of two insoluble isoforms (membrane-bound and cellulose-bound POD) and of soluble protein. Using a water-shield model to improve the uniformity of heating by microwave and to control the rate of heating, microwave blanching was significantly more efficient as measured by all four parameters when blanched at 85C. There was no significant difference in the POD isoform heat-stability when blanched at 70C by the two methods. POD insoluble isoforms were more heat-stable than the soluble isoform. POD isoforms, occupying three cellular environments were more efficiently inactivated by uniform microwave heating compared to water blanching, demonstrating the viability of microwave blanching.

Microwave Heating Uniformity of Ready Meals as Affected by Placement, Composition, and Geometry

ABSTRACTThe importance of chemical and physical modifications in a ready meal on microwave heating uniformity was investigated. Experimental materials were four‐component chilled ready meals which were heated in a domestic microwave oven. The experiment was set up by using 25 factor design and data were evaluated by analysis of variance. Temperatures during and after microwave heating were measured. Heating uniformity could be mainly modified by arrangement and geometry of components and type of tray. In contrast, chemical modifications had no effect or some combined effects with other factors.

MICROWAVE COOKING OF RESTRUCTURED PORK ROASTS

Restructured pork roasts were microwave cooked to an endpoint temperature of 73°C and evaluated for proximate composition, cooking loss and cooking time. Restructuring formulations were: (1) 0.75% salt and 0.2% phosphate (S/P), (2) S/P and 0.5, 1.0 or 2.0% carboxymethyl cellulose, (3) S/P and 0.5, 1.0 or 2.0% algin, and (4) 0.5, 1.0 and 2.0% algin with calcium carbonate and lactic acid added at a constant ratio of 6:1:1.5, respectively. The algin/calcium restructuring resulted in faster (P< 0.05) cooking time and less cooking loss. Decreased cooking time was attributable to improved penetration and/or dispersion of microwaves. Algin/calcium restructuring may interest companies utilizing microwave energy to precook meats and to consumers desiring quickly prepared meals. More uniform microwave heating of pork could reduce the concern of ingesting viable trichinae or other pathogens.

Fundamentals of Microwave-Material Interactions and Sintering

ABSTRACTBasic interaction mechanisms are shown to depend strongly on the dielectric and magnetic properties of a process material. This causes a strong dependence of power absorption on frequency, material particle size, shape, temperature, and density. Sintering dynamics cause the microstructure of the treated material to change resulting in a change in microwave (1W) heating uniformity and rate. The concept of dielectric mixtures is introduced to predict the dielectric and heating properties of a host material with its inclusions in the form of shells, ellipsoids, spheres, disks and needles. Simplified models are described to give a process designer some insight into the behavior of MW sintering. Microwave power, by its very nature, gives better heating control and efficiency and provides internal heat to aid the material transport during sintering. No inherent temperature limit exists for MW sintering although refractory materials used to contain the process material create an artificial upper limit. It is shown that very high (1500–2000°C) temperatures in small samples can be readily achieved using commercial microwave ovens if appropriate MW transparent sample holders are used.

Ridged Waveguide Applicators for Uniform Microwave Heating of Sheet Materials

AbstractThis paper contains an analysis of wave propagation in ridged rectangular waveguides used as single pass applicators for treating thin dielectric sheet materials with microwave power. Two waveguide configurations with double-tapered and double-inclined ridges symmetrically placed about the center, and loaded with lossy dielectric webs are investigated. The variation of the attenuation coefficient and specific microwave power absorption is illustrated as a function of angle of tapering or inclination. The computed results are given for the standard heating frequency of 2.45 GHz.