Microwave Output Power Research Articles

Microwave drying of mango ginger (Curcuma amada Roxb): prediction of drying kinetics by mathematical modelling and artificial neural network

SummaryCurcuma amada (Mango ginger) was dried at four different power levels ranging 315–800 W to determine the effect of microwave power on moisture content, moisture ratio, drying rate, drying time and effective diffusivity. Among the fifteen thin layer drying models considered for evaluating the drying behaviour, the semi‐empirical Midilli et al., model described the drying kinetics very well with R2 > 0.999. Drying rate and effective diffusivity increased as the microwave power output increased. Activation energy was estimated by a modified Arrhenius type equation and found to be 21.6 kW kg−1. A feed‐forward artificial neural network using back‐propagation algorithm was also employed to predict the moisture content during MW drying and found adequate to predict the drying kinetics with R2 of 0.985.

DRYING CHARACTERISTICS OF OKRA PODS DURING DRYING PROCESS IN A HOME MICROWAVE OVEN

Okra is an important vegetable for different people all over the world. Because of its high moisture content, okra is susceptible to rapid deterioration. The aim of the present work was to determine the effect of a home microwave output powers on drying of okra pods. Okra was dried from initial moisture content of about 89.12% (w. b) to a safe level of about 9.62% (w. b) under different microwave output power levels (75, 150, 300, 500, 700 and 900 W) using the process 1 min ON and switch OFF the microwave oven for 5 min. The obtained data show that, there by increasing the used microwave output powers for drying sample of 100 g, the drying time leading to decrease from 19 to 13 min. To determine the kinetic drying model parameters, the drying data were fitted to four drying models based on the ratio of the final and initial moisture contents versus drying time. Among of the models proposed, the Midilli-Kucuk model gave a better fit for all drying conditions applied. It is expected that this study would be very beneficial to those dealing with the microwave drying systems of the future.

磁绝缘线振荡器内气体碰撞电离数值模拟

Monte-Carlo collision (MMC) method was introduced for description of the gas collision ionization course. The MCC gas ionization module based on the method was developed for 3D fully electromagnetic and PIC code NEPTUNE. The magnetically insulated transmission line oscillator(MILO) model with He gas was simulated by the developed NEPTUNE code. The simulated results indicate that, in the condition of lower He gas density, He+ ions produced by ionization collision can not move freely because of their weight, and form channels which can neutralize space-charge field perfectly and then improve the bunching and beam-wave interaction efficiency obviously, so the output power of microwave increases, and the oscillation forming time shortens markedly. When the density of He gas increases to a higher level, the numbers of electrons and He+ ions increase quickly because of stronger ionization collision, the energy of electrons is separated due to collision courses, which goes against the bunching and beam-wave interaction improvement, and then the output power of microwave is decreased or cut off. The shortening of oscillation forming time is induced by the initial ionization stage with lower electron and ion densities. With the increasing gas density, the output power pulse will be shortened, and the diode of MILO will be shut down. In addition, the MILO models with air, vapor and carbon dioxide gas were simulated, respectively. The simulated results indicate that, in these conditions, the shortening of output microwave pulse is more severe compared with that for the MILO filled with He gas at the same pressure.

Net thrust measurement of propellantless microwave thruster

According to the classic theory of electromagnetic (EM) fields, we develop a propellantless microwave thruster system that can convert microwave power directly into thrust without the need of propellant. It is expected to be useful for spacecraft. Different from conventional space plasma propulsion, the system can obviate a large propellant storage tank and the issues related to plasma plume interference with the spacecraft surface. Different from huge solar sails and microwave-propelled sails, the system uses a cylindrical tapered resonance cavity as a thruster and uses an integrated microwave source to generate continuous EM wave so that the EM wave is radiated into and then reflected from the thruster to form a pure standing wave with amplified wave amplitude. The pure standing wave produces a non-uniform EM pressure distribution on the inner surface of the thruster. Consequently, a non-zero net EM thrust exerting on the symmetric axis and directing to the minor end plate of the thruster appears. In experiments a magnetron is used as a microwave source with an output microwave power of 2.45 GHz frequency. The generated net EM thrust is measured using a force-feedback test stand. The developed thruster system is experimentally demonstrated to produce thrust from 70 to 720 mN when the microwave output power is from 80 to 2500 W.

Application of Response Surface Methodology to Optimize Microwave-assisted Extraction of Polysaccharide from Tremella

Tremella is an excellent source of polysaccharides. In this study, microwave-assisted extraction was employed to extract polysaccharides from Tremella with water. By using response surface methodology, the effects of microwave output power, extraction time, and solid-liquid ratio on polysaccharide yield were investigated and the optimal conditions were determined as follows: extraction time 60s, microwave output power 750 w, liquid-solid ratio 20. The average experimental polysaccharide yield under the optimum conditions was found to be 65.07±0.99%, which agreed with the predicted value of 69.07%.

Experimental demonstration of a compact high efficient relativistic magnetron with directly axial radiation

A compact relativistic magnetron with axial microwave radiation is experimentally investigated. Under the modified magnetic field distribution, only connecting a special horn antenna in the axial direction, the relativistic magnetron can stably radiate high power and high efficiency microwaves. The total length of the device is ∼0.3 m, and the volume is ∼0.014m3. In such working condition that the applied voltage is 539 kV and the magnetic field is ∼0.38 T, the output microwave power is ∼1.24 GW. Correspondingly, the total efficiency is about 34.1%. The radiating frequency is 2.35GHz, which is in agreement with the π mode frequency of the theoretical expectation.

Field emission - based many-valued processing using carbon nanotube controlled switches - Part 2: Architecture effectuation

A novel field emission Carbon Nanotube (CNT) - based controlled switch is introduced in the second part of the article. For the architecture effectuation of the new CNT field emission - based switching device, four field emission tubes having single CNT as emitters were previously tested and compared to a tungsten-tip tube, and the corresponding Fowler-Nordheim analysis was performed. Measurements conducted with the CNT suggested that mixer current could be 30 times greater if either SWCNT or MWCNT were used in place of metal emitters, increasing the microwave output power by 30 dB. Laser radiation was utilized to increase field emission current from a cathode with a dense field of CNT by a factor of 18. The extension of the new device from the two-valued to the general mvalued case is introduced, and the implementation of many-valued Galois circuits and systems is also shown.

Experimental study on an X-band coaxial multi-beam relativistic klystron amplifier

The advantages of the tri-axial relativistic klystron amplifier and the multi-beam klystron amplifier are analyzed, and then an X-band coaxial multi-beam relativistic klystron amplifier is designed in order to increase the output microwave power and frequency. Based on the results from partial-in-cell simulation, the experiment is performed on the Sinus accelerator. In experiment, the transmission and bunching of electron beam are analyzed, and then the amplifier is driven by a beam of 5.3 kA at 670 kV, and the maximum output power is 420 MW when input power is 30 kW, and frequency is 9.375 GHz. The experiment proves that a ten kW-level input power can drive the X-band coaxial multi-beam relativistic klystron amplifier to generate more than one hundred MW-level output power.

Design of a dual-frequency high-power microwave generator

AbstractA new direction for high-power microwave (HPM) development is to investigate devices capable of producing HPMs with a complicated spectrum. In recent years, some HPM sources with two stable and separate frequencies have been investigated theoretically and experimentally. However, many short-comings still exist in these devices. Especially, the beam-wave interaction efficiency and the output microwave power are low in such devices. This paper proposes a novel dual-frequency HPM generator based on transition radiation. In the device, the electromagnetic fields are localized near the resonator cavities in the form of standing waves, and thus the interference between the different HPM components with different frequencies is weak. Compared with the existing dual-frequency devices, the new structure allows high beam-wave interaction efficiency and high output microwave power. As indicated in particle-in-cell simulation, with an electron beam of 500 kV voltage and 15.0 kA current guided by a magnetic field of 0.8 Tesla, an average power of 1.60 GW with a total power conversion efficiency of 21.3% is obtained, and the frequencies are 1.53 GHz and 3.29 GHz, respectively. Power level between two HPMs is comparable. The simulation results verify the feasibility of the dual-frequency HPM generator.

Dynamic nuclear polarization at 9 T using a novel 250 GHz gyrotron microwave source

In this communication, we report enhancements of nuclear spin polarization by dynamic nuclear polarization (DNP) in static and spinning solids at a magnetic field strength of 9 T (250 GHz for g = 2 electrons, 380 MHz for 1H). In these experiments, 1H enhancements of up to 170 ± 50 have been observed in 1- 13C-glycine dispersed in a 60:40 glycerol/water matrix at temperatures of 20 K; in addition, we have observed significant enhancements in 15N spectra of unoriented pf1-bacteriophage. Finally, enhancements of ∼17 have been obtained in two-dimensional 13C– 13C chemical shift correlation spectra of the amino acid U– 13C, 15N-proline during magic angle spinning (MAS), demonstrating the stability of the DNP experiment for sustained acquisition and for quantitative experiments incorporating dipolar recoupling. In all cases, we have exploited the thermal mixing DNP mechanism with the nitroxide radical 4-amino-TEMPO as the paramagnetic dopant. These are the highest frequency DNP experiments performed to date and indicate that significant signal enhancements can be realized using the thermal mixing mechanism even at elevated magnetic fields. In large measure, this is due to the high microwave power output of the 250 GHz gyrotron oscillator used in these experiments.

Microwave Drying of Tilapia Fillets: Kinetics and Modeling

To study the effect of microwave output power on the drying kinetics of tilapia fillets, the drying experiments were carried out at 150W, 250W, 500W, 700W and 900W, respectively. And ten mathematical models were involved to fit experimental data. It was found that Midilli et al model gave a best fitness for this conditions applied. Besides, effective moisture diffusivity increased progressively from 1.6248×10-9 (m2/s) to 10.0735×10-9 (m2/s) as the power increased from 150W to 900W. In addition, to obtain more homogeneous samples, temperature distribution of tilapia fillets was analyzed when they were put in different layout forms.

EXPERIMENTAL ANALYSIS OF MICROWAVE DRYING KINETICS AND CHARACTERIZATION OF GINGER RHIZOMES

Ginger rhizomes of different thickness, microwave output power and sample load were dehydrated in a modified microwave oven, and effective moisture diffusivity was calculated. Among the various basic models considered, diffusion equation was most suitable to predict the drying behavior for the process conditions. Based on the interpretations, the constants and coefficients were rewritten in Arrhenius and logarithmic expressions for microwave power. The derived mathematical models were validated according to statistical parameters, viz. reduced chi-square, root mean square error, mean bias error and t-test values. The modified diffusion model explained the suitability of thin layer microwave drying behavior of ginger rhizome slices, yielding the highest R2 of 0.9983. The effect of microwave drying on extraction of oleoresins with hexane as the solvent was in the range of 2.53–4.55%. Scanning electron microscopy and Fourier transform infrared spectroscopy revealed enhanced integrity of texture and functional groups by microwave treatment. PRACTICAL APPLICATIONS Drying, a process of removal of moisture content, was carried out on ginger rhizomes. Besides several advantages during dehydration, some important changes take place, such as structural and physicochemical modifications, which affects the final product quality. Microwave heating at specific experimental conditions was aimed at the restoration of nutritional and medicinal properties of the raw material. Mathematical models applied in this research will be helpful to analyze, simulate as well as to design and scale up the microwave drying process for industrial and commercial applications. Furthermore, the product may have an increased shelf life, finished product quality; may reduced energy cost in comparison with the conventionally dried rhizomes; and may serve as ingredient for medicinal, pharmaceutical and cosmetics industry for the extraction of essential oils such as oleoresins.

Modeling, characterization, and evaluation of efficiency and drying indices for microwave drying of Zingiber officianale and Curcuma mangga

ABSTRACTMethods for efficiently drying agricultural products are an ever‐increasing demand. Due to its rapid and thorough dehydrating ability, microwave techniques were used to evaluate the thin‐layer drying kinetics of organic ginger and mango ginger. Microwave drying characteristics, efficiency and indices determined with respect to sample thickness, microwave output power and sample load revealed higher power (300 W), lesser thickness (0.001 m) and load (25 g), increased drying rates and reduced drying time. Effective diffusivity values were found within the range reported in literatures (9.17 × 10−11 and 11.6 × 10−11 m2 s−1). Scanning electron microscope (SEM) and energy‐dispersive X‐ray (EDX) images revealed that the surface morphology and the elemental composition gradually degraded during drying. The ability of eight different thin‐layer mathematical models was evaluated for representing the experimental drying profiles. Among the eight new models derived from the basic diffusion model, statistical analysis inferred that the modified diffusion model no. 4 could predict most accurately changes in drying behavior of organic ginger and mango ginger rhizomes for all drying conditions applied. Copyright © 2010 Curtin University of Technology and John Wiley & Sons, Ltd.

COLOR CHANGE KINETICS OF CELERY LEAVES UNDERGOING MICROWAVE HEATING

The aim of this study was to investigate the effect of microwave output power and sample amount on color change kinetics of celery leaves (Apium graveolens L.) during microwave heating. The color parameters of the materials were quantified by the Hunter Lab system. These values were also used for calculation of the total color change, chroma, hue angle, and browning index. The microwave heating process changed color parameters of L, a, and b, causing a color shift towards darker region. The mathematical modeling study of color change kinetic showed that L, a, b, and chroma fitted to a first-order kinetic model, while total color change (ΔE), hue angle, and browning index (BI) followed a zero-order kinetic model. For calculation of the activation energy for color change kinetic parameters, the exponential expression based on the Arrhenius equation was used.

ChemInform Abstract: Synthesis of in situ Network‐Like Vapor‐Grown Carbon Fiber Improved LiFePO4 Cathode Materials by Microwave Pyrolysis Chemical Vapor Deposition.

AbstractThe title cathode materials are prepared from mixtures (ball milling for 6 h) of FeC2O4, NH4H2PO4, LiOH, carbon black, and EtOH by microwave pyrolysis chemical vapor deposition (Ar, 800 W microwave output power).

Development of compact high voltage switched mode power supply for microwave plasma sources supply for low pressure plasma

Although microwave induced plasmas are well known as high efficiency plasma sources, their uses in laboratories are limited since the microwave power systems are complicated and expensive. The output power of commercially available low-cost microwave ovens is fixed and discontinuous resulting from the high voltage doubler topology of the magnetron tube power supply. In this paper, a high voltage switched mode power supply of forward topology has been developed for continuous microwave power radiation. The forward converter can generate a no-load high voltage output maximum of 7 kV. When driving the magnetron tube, the microwave output power could be varied from 0 to 35 W while the high voltage output level was constantly regulated at -3.4 kV. A microwave induced plasma system was setup to investigate the plasma produced. A low pressure argon plasma was produced with only 2 W over a wide range of pressures.

Investigation on the drying kinetics of sliced and whole rosehips at different moisture contents under microwave treatment

Microwave drying kinetics of rosehips (Rosa Canina L.) harvested at different moisture contents was investigated. The experiments were conducted for both sliced and whole rosehips at different microwave power outputs (90 to 800 W) in a modified microwave drying set-up. Rosehips with initial moisture contents of 0.44 to 0.80 (g water/g dry matter) were dried to moisture content lower than 0.1 (g water/g dry matter). The effects of microwave power level, initial moisture content and slicing on moisture content and drying rate were studied. Drying took place in the falling rate period. To determine the kinetic parameters, drying data were fitted to ten mathematical thin layer drying models. Among the models proposed, Page model precisely represented the microwave drying behavior of rosehip with the coefficient of determination higher than 0.997 and mean square of deviation and root mean square error lower than 4.7×10-4 and 17 ×10-3, respectively for all output powers studied. Values of drying constant (k) were in the range of 0.0049 to 0.23 min-1. Key words: Drying kinetics, microwave drying, thin layer drying models, rosehip.

MICROWAVE‐ASSISTED EXTRACTION AND LIQUID CHROMATOGRAPHY/MASS SPECTROMETRY ANALYSIS OF FLAVONOIDS FROM GRAPEFRUIT PEEL

ABSTRACT The flavonoids of grapefruit peel were obtained by microwave‐assisted extraction. Five constituents have been identified by liquid chromatography/mass spectrometry analysis such as hesperitin‐7‐O‐glucoside, isonaringin, naringin, 2′,5′‐dihydroxy‐3,4′,5,6,7‐pentamethoxyflavone and neoeriocitrin. The last two compounds are reported first in grapefruit peel. The effects of solvent, time, ratio of solvent to material (liquid/solid) and output power on the extraction were investigated. The results show that the better extraction conditions are: 80% (v/v) ethanol as extraction solvent; 3 min of extraction time, 25:1 (mL/g) as the ratio of liquid to solid; and 560 W of microwave output power. Microwave‐assisted extraction is more efficient for getting flavonoids from grapefruit peel than reflux and ultrasonic extraction methods. The yield of optimized microwave‐assisted extraction reached 5.81%. PRACTICAL APPLICATIONSGrapefruit is one of the delicious fruits with abundant nutrition, such as vitamin C, carbohydrate and a small quantity of some special compounds. It may have some beneficial effects on human health, with anti‐oxidant, anti‐inflammatory and anti‐cancer properties, or may act as agents of other mechanisms contributing to cardio‐protective action. The main use of grapefruit in food industries is to make fresh juice or citrus‐based drinks. The peels are often used to feed animals or are thrown away directly. Very large amounts of wastes are formed every year, because almost half of the grapefruit is composed of peels. It is reported that grapefruit peels contain a high concentrationof phenolic compounds. It is significant to find a feasible way to obtain these useful compounds from peel wastes to increase their value. In our work, microwave‐assisted extraction of flavonoids from grapefruit peel is established and discussed. The extraction yield is higher than that of using traditional extraction methods. So, the results of this paper are very helpful in utilizing and making great use of natural resources.

Particle-in-cell simulations for virtual cathode oscillator including foil ablation effects

We have performed two- and three-dimensional, relativistic, electromagnetic, particle-in-cell simulations of an axially extracted virtual cathode oscillator (vircator). The simulations include, for the first time, self-consistent dynamics of the anode foil under the influence of the intense electron beam. This yields the variation of microwave output power as a function of time, including the role of anode ablation and anode–cathode gap closure. These simulations have been done using locally developed particle-in-cell (PIC) codes. The codes have been validated using two vircator designs available from the literature. The simulations reported in the present paper take account of foil ablation due to the intense electron flux, the resulting plasma expansion and shorting of the anode–cathode gap. The variation in anode transparency due to plasma formation is automatically taken into account. We find that damage is generally higher near the axis. Also, at all radial positions, there is little damage in the early stages, followed by a period of rapid erosion, followed in turn by low damage rates. A physical explanation has been given for these trends. As a result of gap closure due to plasma formation from the foil, the output microwave power initially increases, reaches a near-flat-top and then decreases steadily, reaching a minimum around 230 ns. This is consistent with a typical plasma expansion velocity of ∼2 cm/μs reported in the literature. We also find a significant variation in the dominant output frequency, from 6.3 to 7.6 GHz. This variation is small as long as the plasma density is small, up to ∼40 ns. As the AK gap starts filling with plasma, there is a steady increase in this frequency.

Effects of microwave drying on the contents of functional constituents of Eucommia ulmoides flower tea

In order to make male flower tea of Eucommia ulmoides, this paper systematically investigated the effects of microwave drying, a newly emerging drying method, on the contents of functional constituents contained in the flower (such as total flavonoids, chlorogenic acid, aucubin and geniposidic acid) from the aspects of microwave output power and time of treatment. A comparison was conducted in the view of tea quality between microwave drying and conventional drying methods used in tea manufacturing, such as pan baking drying and oven ventilation baking drying. The results indicated that microwave drying method could maximally maintain the functional constituents. Further comparison on sensory quality showed that by microwave drying mode, the shape of the stamens of the flower was intact, the color was greenish, the liquor of the flower tea was fresh with strong Eucommia male flower flavor.