Increasing Microwave Power Research Articles

MODELLING A MICROWAVE/CONVECTION DRYER FOR DRYING OF POTATO SLICES

The aim of the present study is modeling a developed microwave/convection dryer using similitude analysis for drying a thin layer of potato slices (Solanum Tuberosum), which can be helpful in design and operation criterion of this developed thin layer dryer type for drying other agricultural products. Drying time as affected by microwave power and drying air temperature, effective diffusivity Deff, compatibility of experimental data to Lewis Newton's thin layer drying model and rehydration and color as quality indicators were also studied. Four different levels of microwave power, namely: 85, 175, 250 and 320W, three different drying air temperatures of drying air are 40, 50 and 60oC with air velocity of 1.5 m/s were studied. The results show that the drying time was decreased by increasing microwave power and air temperature. The effective moisture diffusivity Deff increased with decreasing moisture content, maximum values of Deff were recorded at highest microwave power of 320 W. Lewis Newton's thin layer drying model was highly compatible to experimental data with R2 ranging between 0.754 to 1.00. The rehydration ratios ranged between 2.19 to 3.41 under various drying conditions. There is no significant change in color of the dried potato compared to fresh potato slices

Mode changes of surface wave on the interface between dielectric rod and microwave plasma with increasing plasma density

A surface-wave plasma source with a cylindrical dielectric waveguide has been developed in our laboratory. A Teflon rod was used as waveguide where the microwave could excite the plasma around it. The transmitting modes of the surface wave on the interface between a dielectric rod and plasma changed with increasing microwave power. It was observed that the cutoff frequency of the surface wave decreased with the increase of the plasma density. And the transmitting modes of the surface wave changed from HE11 mode under no plasma or low plasma density to the degenerate mode of TM01 and TE01 with plasma density 8.9 × 1011 cm−3.

ICEPIC Simulation of a Strapped Nonrelativistic High-Power UHF Magnetron With a Transparent Cathode Operating in the Explosive Electron Emission Mode

The operation of a high-power nonrelativistic strapped ultra-high-frequency (UHF) multicavity magnetron with transparent cathode is simulated using the Improved Concurrent Electromagnetic Particle-in-Cell (ICEPIC) code. Results of the simulations are compared with simulations of the same magnetron with a solid cylindrical cathode. Both cathodes are explosive-emission electron sources that operate in the space-charge-limited mode of the electron extraction from the cathode. Simulations are part of an effort searching for methods and technologies that enable an increase of output microwave power of strapped nonrelativistic high-power UHF magnetrons by replacing thermionic direct-heated cathodes with a non-thermionic-emission cathode. Results of the simulations show that the anode current Ia and the output power Pout of the magnetron with the transparent cathode are higher than Ia and Pout of the same magnetron with a solid cathode. At the same time, the electronic efficiency ηe of the magnetron with the transparent cathode is less, and the startup time ts is shorter than ηe and ts of the magnetron with the solid cathode.

Non-thermal effects of microwaves and kinetics on the transesterification of soybean oil

A kinetic study of the transesterification of soybean oil was conducted using microwaves under various temperatures, power densities, and reaction times. Results show that power density affects the kinetics and yield. The biodiesel yield increased with increasing microwave power density. The non-thermal effects of microwave irradiation on transesterification reactions were evaluated at a constant reaction temperature () and power density (0.204 ). Microwave irradiation was found to increase the reaction rates by 3.52-7.06 fold.

The effects of microwave power and dielectric properties on the microwave-assisted decomposition kinetics of AIBN in n-butanol

The effects of microwave power and solvent–reactant dielectric properties on the kinetics of the decomposition of AIBN in n-butanol were investigated. According to the kinetic data, k increases with increasing microwave power, the dependence becomes stronger at high temperature. The Arrhenius parameters (Ea,mw, k0,mw) were higher than thermal counterparts. To elucidate the nature of microwave effect on chemical reactions, various models to relate the Arrhenius parameters to temperature, microwave power and the dielectric properties of the reaction medium were developed, nonlinear regression analysis was performed to estimate the model parameters and statistical analysis was conducted for model evaluation.

Effect of osmotic dehydration on dielectric properties, microwave vacuum drying kinetics and quality of mangosteen

SummaryMangosteen (Garcinia mangostana Linn.) with and without osmotic dehydration (OD) in sucrose solution was dried by microwave vacuum drying at 1200, 1440 and 1680 W. Because of water loss (49.12–49.98 g 100 g−1) and solid gain (9.31–11.62 g 100 g−1) during OD, dielectric constant, loss factor and loss tangent of mangosteen were significantly increased (P ≤ 0.05) to 24.82–25.12, 11.52–14.18 and 0.47–0.50, respectively. With the decreased initial moisture content and the modified dielectric properties, drying time of osmotically dehydrated mangosteen was shorter than that of mangosteen without OD. Moreover, an increase in microwave power enhanced drying kinetics. With OD, Tg of dried mangosteen was increased from −7.01, −3.00 to 11.11–25.96 °C. Hardness and lightness (seedless part) were significantly increased (P ≤ 0.05). Structure of dried seedless mangosteen was well protected, resulting in the improved rehydration ability (P ≤ 0.05). Nonetheless, rehydration ability of the mangosteen containing seed was not improved (P > 0.05).

Spectroscopic Investigation of a Microwave‐Generated Atmospheric Pressure Plasma Torch

AbstractThe investigated new microwave plasma torch is based on an axially symmetric resonator. Microwaves of a frequency of 2.45 GHz are resonantly fed into this cavity resulting in a sufficiently high electric field to ignite plasma without any additional igniters as well as to maintain stable plasma operation. Optical emission spectroscopy was carried out to characterize a humid air plasma. OH‐bands were used to determine the gas rotational temperature Trot while the electron temperature was estimated by a Boltzmann plot of oxygen lines. Maximum temperatures of Trot of about 3600 K and electron temperatures of 5800 K could be measured. The electron density ne was estimated to ne ≈ 3 · 1020m–3 by using Saha's equation. Parametric studies in dependence of the gas flow and the supplied microwave power revealed that the maximum temperatures are independent of these parameters. However, the volume of the plasma increases with increasing microwave power and with a decrease of the gas flow. Considerations using collision frequencies, energy transfer times and power coupling provide an explanation of the observed phenomena: The optimal microwave heating is reached for electron‐neutral collision frequencies νen being near to the angular frequency of the wave ω (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Large-Volume Open-Air Microwave Plasma Based on Parallel Multislot Rectangular Waveguides

An open-air microwave plasma is generated in the resonant cavity based on parallel multislot rectangular waveguides by using an argon pre-ionization plasma jet device. The side photos show that the plasma separates from the argon jet and gradually shifts to the locations with high electric-field strength when the microwave power increases. In the absence of the plasma jet, the early plasma structure is relatively similar to the calculated electric field distribution. As the time goes on, the plasma connects the two waveguides. Small portions of plasma always move up and down between the waveguides. At the microwave power of 5.6 kW, the plasma size is evaluated to be (110-130) mm × (407-50) mm × 87 mm, which is significantly large. As the microwave power increases from 5.6 kW to 8.0 kW, the plasma electron temperature increases from 4400 K to 7600 K. The average electron density along its length is measured by using the Mach-Zehnder interferometer. The electron density is relatively high in the center and two ends near the slots of the plasma. The electron density is as high as 1017 cm-3, and it gradually decreases from 3.03 ×1017 cm-3 in its center to zero at its edge across the width of the plasma. The open-air plasma excited at 6.5 kW is close to local thermodynamic equilibrium.

Ultrafast synthesis and characterization of carbonated hydroxyapatite nanopowders via sonochemistry-assisted microwave process

Herein, carbonated hydroxyapatite (CHAp) nanopowders were synthesized via sonochemistry-assisted microwave process. The influences of microwave and ultrasonic irradiation on the crystallinity, morphology, yield, Ca/P molar ratio, specific surface area and dispersibility were investigated and compared with the conventional precipitation method. The results showed that sonochemistry-assisted microwave process significantly increased the synthetic efficiency. The well-crystallized nanopowders could be obtained at high yield of 98.8% in ultra-short-period of 5min. In addition, the crystallization process was promoted with the increase of ultrasonic and microwave power and the reaction time during the sonochemistry-assisted microwave process. The sonochemistry assistance also remarkably increased the specific surface area and dispersibility of the as-obtained products. These results suggest that the sonochemistry-assisted microwave process is an effective approach to synthesize CHAp with high efficiency.

Dehydration of crude protein from Ginkgo biloba L. by microwave freeze drying

The paper optimized the parameters of microwave freeze drying (MFD) of crude Ginkgo biloba protein (CPG) using response surface methodology (RSM) based on the analysis of its proximate composition. The results showed that coefficients of determination, R2 values for drying time and protein solubility were greater than 0.9500. The drying time and protein solubility of CPG varied curvilinearly with increase of microwave power and pre-freeze temperature, and drying time varied linearly with material thickness. The optimum MFD condition was microwave power of 408–421W, material thickness of 15mm and pre-freeze temperature of −20°C to −21°C, respectively.

Extraction of negative hydrogen ions from a compact 14 GHz microwave ion source

A pair of permanent magnets has formed enough intensity to realize electron cyclotron resonance condition for a 14 GHz microwave in a 2 cm diameter 9 cm long alumina discharge chamber. A three-electrode extraction system assembled in a magnetic shielding has formed a stable beam of negative hydrogen ions (H(-)) in a direction perpendicular to the magnetic field. The measured H(-) current density was about 1 mA∕cm(2) with only 50 W of discharge power, but the beam intensity had shown saturation against further increase in microwave power. The beam current decreased monotonically against increasing pressure.

Electron cyclotron resonance ion source plasma chamber studies using a network analyzer as a loaded cavity probe

A method and first results utilizing a network analyzer as a loaded cavity probe to study the resonance properties of a plasma filled electron cyclotron resonance ion source (ECRIS) plasma chamber are presented. The loaded cavity measurements have been performed using a dual port technique, in which two separate waveguides were used simultaneously. One port was used to ignite and sustain the plasma with a microwave source operating around 11 GHz and the other was used to probe the cavity properties with the network analyzer using a frequency range around 14 GHz. The first results obtained with the JYFL 14 GHz ECRIS demonstrate that the presence of plasma has significant effects on the resonance properties of the cavity. With plasma the frequency dependent behavior is strongly damped and this trend strengthens with increasing microwave power.

Effects of pressure and incident power on self-organization pattern structure during microwave breakdown in high pressure air

Pressure and microwave power are the most important parameters during microwave breakdown in air and affect the self-organization plasma pattern structure and its propagation directly. In order to study the effects of pressure and microwave power, an effective-diffusion fluid plasma equation is solved together with Maxwell's equations, and the double grid method is also used to meet the different grid size requirement of plasma equation and finite-difference-time-domain for Maxwell's equations. The numerical results show that with lower pressure the plasma behaves as a more diffuse plasmoid instead of a well defined plasma pattern structure under higher pressure, and the increase of incident microwave power will lead to a rapid growth of the front propagation velocity and a well separated and sharp pattern structure, and the higher incident power also results in jump-like front propagation.

Coherent control of two individual electron spins and influence of hyperfine coupling in a double quantum dot

Electric dipole spin resonance of two individual electrons and the influence of hyperfine coupling on the spin resonance are studied for a double quantum dot equipped with a micro-magnet. The spin resonance occurs by oscillating the electron in each dot at microwave (MW) frequencies in the presence of a micro-magnet induced stray field. The observed continuous wave (CW) and time-resolved spin resonances are consistent with calculations in which the MW induced AC electric field and micro-magnet induced stray field are taken into account. The influence of hyperfine coupling causes an increase and broadening of the respective CW spin resonance peaks through dynamical nuclear polarization when sweeping up the magnetic field. This behaviour appears stronger for the larger of the two spin resonance peaks and in general becomes more pronounced as the MW power increases, both reflecting that the electron-nuclei interaction is more efficient for the stronger spin resonance. In addition the hyperfine coupling effect only becomes pronounced when the MW induced AC magnetic field exceeds the fluctuating nuclear field.

Microwave vacuum drying of osmotically dehydrated mandarin cv. (Sai-Namphaung)

Mandarin [mandarin cv. (Sai-Namphaung)] was subjected to osmotic dehydration prior to microwave vacuum drying. Osmotic solutions were varied using different ratios of sucrose to glycerol (9:1, 7:3 and 5:5 w/w). Because of the decreased moisture content and solid gain during osmotic dehydration, dielectric properties of mandarin were changed significantly (P ≤ 0.05). The osmotically dehydrated mandarin was then dried further using microwave vacuum drying at 4.8 and 6.4 W g−1. Among thin layer models, page model was the best to describe the drying of osmotically dehydrated mandarin. An increase in the microwave power could increase drying rate without significant effect on hardness of dried samples. Nonetheless, the hardness was significantly (P ≤ 0.05) reduced by an increase in the glycerol ratio in the osmotic solution. The increase in microwave power and glycerol ratio significantly (P ≤ 0.05) decreased β-carotene content and thereby affected colour of the dried mandarin.

Optimization of microwave-assisted hot air drying conditions of okra using response surface methodology

Okra (Abelmoschus esculentus) was dried to a moisture level of 0.1g water/g dry matter using a microwave-assisted hot air dryer. Response surface methodology was used to optimize the drying conditions based on specific energy consumption and quality of dried okra. The drying experiments were performed using a central composite rotatable design for three variables: air temperature (40-70°C), air velocity (1-2m/s) and microwave power level (0.5-2.5W/g). The quality of dried okra was determined in terms of color change, rehydration ratio and hardness of texture. A second-order polynomial model was well fitted to all responses and high R(2) values (>0.8) were observed in all cases. The color change of dried okra was found higher at high microwave power and air temperatures. Rehydration properties were better for okra samples dried at higher microwave power levels. Specific energy consumption decreased with increase in microwave power due to decrease in drying time. The drying conditions of 1.51m/s air velocity, 52.09°C air temperature and 2.41W/g microwave power were found optimum for product quality and minimum energy consumption for microwave-convective drying of okra.

Comparisons between conventional, microwave- and ultrasound-assisted methods for extraction of pectin from grapefruit

Microwave-assisted Extraction (MAE) and Ultrasound-assisted extraction (UAE) techniques have been employed as complementary methods to extract carbohydrates, polysaccharides and other functional compounds from vegetable sources. In this work, the effect of microwave power and heating time on the yield and quality of extracted pectin from grapefruit was investigated. The highest total amount of pectin yield was found to be 27.81% (w/w) for 6 min of extraction at 900 W. It was observed that yield, the galacturonic acid content (GalA), and degree of esterification (DE) increased with an increase in microwave power and heating time. Besides, the molecular weight decreased with an increase in heating time; however, the effects of power on the molecular weight were dramatically more than heating time. In addition, laboratory studies on the extraction of pectin treated with high-intensity ultrasound were carried out. The effects of temperature and time on quality and quantity of extracted pectin were investigated. The highest yield was for sonication time of 25 min (17.92%) in a constant bath temperature of 70 °C. Furthermore, a preliminary ultrasonic heating of grapefruit solution, as a pretreatment for MAE, was found to provide a higher yield. Intermittent sonication gave better results in comparison to the continuous sonication.

Microwave-Assisted Thin Layer Drying of Wheat

Drying characteristics of Canada Western Red Spring (CWRS) wheat were studied using a domestic microwave convective oven. The effects of microwave power level, grain bed thickness, and initial grain moisture on the drying kinetics were investigated. Wheat samples with initial moisture levels of 0.18 to 0.29 kg water/kg of dry matter were dried for different drying periods of 180 to 360 s. The moisture loss data were recorded at regular short intervals. Then moisture loss data were fitted to various models (Page equation, modified drying equation, and Midilli equation) to study the drying kinetics of wheat. The results showed that wheat moisture loss increased with increasing microwave power level. A mathematical model was developed by coupling mass and energy balances, resulting in a system of non-linear equations. The predicted moisture loss data from the developed model were compared by fitting to experimental microwave data that were in good agreement.

Methane coupling in microwave plasma under atmospheric pressure

Methane coupling in microwave plasma under atmospheric pressure has been investigated. The effects of molar ratio n(CH 4)/ n(H 2), flow rate and microwave power on the reaction have been studied. (1) With the decrease of n(CH 4)/ n(H 2) ratio, methane conversion, C 2 hydrocarbon yield, energy yield and space-time yield of acetylene increased, but the yield of carbon deposit decreased. (2) With the increase of microwave power, energy yield of acetylene decreased, but space-time yield of acetylene increased. (3) With the increase of flow rate, energy yield and space-time yield of acetylene increased first and then decreased. Finally, under the reaction conditions of CH4 flow rate of 700 mL/min, n(CH 4)/ n(H 2) ratio of 1/4 and microwave power of 400 W, the energy yield and space-time yield of acetylene could reach 0.337 mmol/kJ and 12.3 mol/(s·m 3), respectively. The reaction mechanism of methane coupling in microwave plasma has been investigated based on the thermodynamics of chemical reaction. Interestingly, the acetylene yield of methane coupling in microwave plasma was much higher than the maximum thermodynamic yield of acetylene. This phenomenon was tentatively explained from non-expansion work in the microwave plasma system.

Effects of microwave rendering on the yield and characteristics of chicken fat from broiler abdominal fat tissue

Microwave rendering process and effects of rendering conditions on yields and characteristics of chicken fat from broiler abdominal fat tissue were studied. Microwave rendering process could be divided into three phases: warming-up, constant temperature and heating-up phases. Increasing microwave power increased the rate of temperature rise and final temperature of chicken fat, shortened the duration of warming-up and constant temperature phase, and decreased the moisture content of chicken fat. Longer irradiation time resulted in higher final temperature, lower moisture content and darker, browner color of chicken fat. The chicken fat rendered at power level of 2.75W/g for 10min had the highest yield (70.55%) with the lowest peroxide value, acid value and thiobarbituric acid value (p < 0.05). The major fatty acids in chicken fat obtained at optimum conditions were oleic acid (45.73%), palmitic acid (26.13%), linoleic acid (15.12%), palmitoleic acid (6.07%), and stearic acid (5.81%).