Pulsed Vacuum Research Articles

MPI Parallelization of Numerical Simulations for Pulsed Vacuum Arc Plasma Plumes Based on a Hybrid DSMC/PIC Algorithm

The transport characteristics of the unsteady flow field in rarefied plasma plumes is crucial for a pulsed vacuum arc in which the particle distribution varies from 1016 to 1022 m−3. The direct simulation Monte Carlo (DSMC) method and particle-in-cell (PIC) method are generally combined to study this kind of flow field. The DSMC method simulates the motion of neutral particles, while the PIC method simulates the motion of charged ions. A hybrid DSMC/PIC algorithm is investigated here to determine the unsteady axisymmetric flow characteristics of vacuum arc plasma plume expansion. Numerical simulations are found to be consistent with the experiments performed in the plasma mass and energy analyzer (EQP). The electric field is solved by Poisson’s equation, which is usually computationally expensive. The compressed sparse row (CSR) format is used to store the huge diluted matrix and PETSc library to solve Poisson’s equation through parallel calculations. Double weight factors and two timesteps under two grid sets are investigated using the hybrid DSMC/PIC algorithm. The fine PIC grid is nested in the coarse DSMC grid. Therefore, METIS is used to divide the much smaller coarse DSMC grid when dynamic load imbalances arise. Two parameters are employed to evaluate and distribute the computational load of each process. Due to the self-adaption of the dynamic-load-balancing parameters, millions of grids and more than 150 million particles are employed to predict the transport characteristics of the rarefied plasma plume. Atomic Ti and Ti2+ are injected into the small cylinders. The comparative analysis shows that the diffusion rate of Ti2+ is faster than that of atomic Ti under the electric field, especially in the z-direction. The fully diffuse reflection wall model is adopted, showing that neutral particles accumulate on the wall, while charged ions do not—due to their self-consistent electric field. The maximum acceleration ratio is about 17.94.

Enhanced mass transfer of pulsed vacuum pressure pickling and changes in quality of sour bamboo shoots

Sour bamboo shoot is a traditional Chinese fermented vegetable food. The traditional pickling method of sour bamboo shoots has the disadvantages of being time-consuming, inhomogeneous, and difficult to control. Pulsed vacuum pressure pickling (PVPP) technology uses pulsed vacuum pressure to enhance the pickling efficiency significantly. To demonstrate the effects of salt content and PVPP technical parameters on the fermentation of bamboo shoots, the sample salinity, pH value, color, crunchiness and chewiness, nitrite content, and lactic acid bacteria content during the pickling process were investigated. The salt content inside the bamboo shoots gradually increased to the equilibrium point during the pickling process. The pickling efficiency of bamboo shoots under PVPP technology increased by 34.1% compared to the traditional control groups. Meanwhile, the uniform salt distribution under PVPP technology also obtained better performance in comparison with the traditional groups. The pH value declined slowly from 5.96 to 3.70 with the extension of pickling time and sour flavor accumulated progressively. No significant differences were found in the color values (L*, a*, and b*) and the crunchiness of the bamboo shoot under different salt solution concentrations, vacuum pressure, and pulsation frequency ratio conditions. Colony-forming unit of lactic acid bacteria (CFU of LAB) decreased, to begin with, and then increased until the 6th day, followed by a declining trend in volatility. The nitrate content of bamboo shoots samples under PVPP treatments did not exceed the safety standard (<20 mg/kg) during the whole fermentation process, which proves the safety of PVPP technology. In conclusion, PVPP technology can safely replace the traditional method with better quality performance. The optimal PVPP processing conditions (vacuum pressure 60 kPa, 10 min vacuum pressure time vs. 4 min atmospheric pressure time, salt solution concentration 6%) have been recommended for pickling bamboo shoots with high product quality.

Effects of pulsed vacuum drying temperature on drying kinetics, physicochemical properties and microstructure of bee pollen

In order to clearly analyze the effects of different drying temperatures on the nutritional components, toxic substances and other physicochemical properties of bee pollen, this work comprehensively investigated the effect of pulsed vacuum drying (PVD) temperature on the drying kinetics, amino acids, toxic products of Maillard reaction (α-Dicarbonyl compounds, α-DCs and hydroxymethylfurfural, HMF), antioxidant capacity, lipid oxidation, and microstructure of bee pollen. The results showed that when the drying temperature increased (35–75 °C), the drying time was shortened by 233 min, accompanied by the increase of antioxidant and lipid oxidation. When the temperature exceeded 55 °C, HMF and α-DCs accumulated in large quantities, and the amino acid content decreased significantly, among which the content of seven essential amino acids decreased by more than 32.2%. The changes in the microstructure support the accelerated water migration rate and the dramatic Maillard reaction at high temperature. This study explored the law of bee pollen quality changes under PVD drying technology, preliminarily analyzed the transformation and synthesis pathways of HMF and α-DCs, proposed 3,4-DGE could be used as a representative indicator to judge the safety of bee pollen, and recommended that the drying temperature of pollen should not exceed 55 °C.

Design and construction of the magnetic mirror magnets of the space plasma environment research facility (SPERF)

The Space Plasma Environment Research Facility aims to research the space plasma physics in the laboratory. Two elliptical magnets are used to form a magnetic mirror field simulating the interplanetary magnetic field to investigate the magnetic reconnection at the magnetopause and magnetotail of the earth on the ground. To perform the research, the issues facing the magnets in terms of high pulsed current, vacuum, plasma, and mechanical support must be resolved. The paper presents their design and construction process to settle these issues. A series of measures are adopted to enhance insulation, reduce the gas leakage, and avoid tip discharge, ranging from the dielectric layers to the Inconel spraying. A transition structure considering the effect of the strong pulsed current is designed for connecting the magnet's leads to the cables of its pulsed power supply. In addition, a supporting and motion mechanism is proposed to hold the magnets and drive them to move. The strength of the mechanism is assessed by the static mechanical analysis for the extreme magnetic force load. Meanwhile, a connection scheme compatible with the supporting and motion mechanism is proposed to guide the cables through the vacuum vessel. The great mechanical stress occurring at the interface between the cables and the stainless-steel blocks due to the sheared magnetic force is reduced considerably by a fixing plate. The results in the paper are valuable for designing the magnet conducting a large pulsed current in the vacuum environment for plasma applications.

Effect of the temperature of an electrode microprotrusion on the microcrater formation on the electrode surface upon pulsed and radiofrequency vacuum breakdowns

The formation of a microcrater on the surface of an electrode as a result of the explosion of a microprotrusion caused by pulsed and radiofrequency vacuum breakdowns was numerically simulated to elucidate how the microprotrusion temperature affects the process. It has been shown that when the microprotrusion temperature is close to the melting point, a zone with an elevated temperature occurs in the center of the microcrater, where the material is in a liquid or a mixed (liquid plus gas) state. Within a certain time after the explosion of the microprotrusion, this zone propagates with high velocity into the bulk electrode. The result is the formation of a micrometer-sized crater with a cylindrical cavity in its central part. Microcraters of this type, usually formed on electrodes subjected to vacuum breakdowns, are also observed on electrodes exposed to laser, beam, and plasma pulsed energy deposition.

Osmotic dehydration of fruits using unconventional natural sweeteners and non‐thermal‐assisted technologies: A review

AbstractIn the food preservation process, drying or dehydration reduces the moisture content that is needed for the growth and multiplicity of microorganisms that cause food spoilage. Among the most effective and appropriate methods for extending the shelf life of fruits and vegetables is osmotic dehydration. This process encourages the movement of water molecules from the food to the osmo‐active solution and a small amount of solute from the solution into the food, resulting in an organoleptic and functional product. Sugar, which is the most popular osmotic agent, presents many health risks. Therefore, this review discusses the mechanisms of osmotic dehydration and the use of unconventional natural sweeteners like honey, jaggery, coconut sugar, stevia, and sugar beet molasses as an osmotic agent. Furthermore, certain non‐thermal methods to help osmotic dehydration have been reviewed that may improve the drying and quality parameters of different fruits.Practical applicationsThe osmotic dehydration process in fruits reduces postharvest losses, as well as maintaining the initial fruit characteristics such as color, aroma, texture, and nutritional content. The uses of natural sweeteners viz. jaggery and honey as osmotic agents have numerous health benefits. Due to these benefits, it can be used at small scales, home‐scale industries, and large‐scale industries. Various methods can be used to enhance osmotic dehydration, such as pulsed electric fields, high pressure, vacuum, microwave, centrifugal force, or ultrasound, which can be applied to large‐scale industries.

Development of a pulsed vacuum ultraviolet light source with adjustable intensity.

This paper describes the development of a pulsed light source using the discharge from an electrode in a medium of various noble gases. This source can be used to aid in the characterization and testing of new vacuum-ultraviolet sensitive light detection devices. The source includes a novel spark driver circuit, a spark chamber into which different noble gases can be introduced, and an optical attenuation cell capable of being filled with different gases to allow for the attenuation of the pulsed light down to single photon levels. We describe the construction, calibration, and characterization of this device deployed at a dedicated light detection test stand at Oak Ridge National Laboratory.

Peanut drying: Effects of various drying methods on drying kinetic models, physicochemical properties, germination characteristics, and microstructure

The current work aims to explore the suitable drying technique for peanut pods which can be used for seeds or edible peanuts. Four drying methods, namely naturally-open sun drying as the control check (CK), hot air drying (HAD), pulsed vacuum drying (PVD), and radio frequency combined hot air drying (RF-HAD), were employed to dry peanut pods, and their effects on the nutritional quality attributes in terms of protein, fat, fatty acid contents, etc., germination characteristics, microstructure, color, texture, acid value and peroxide value of peanuts were explored. Mathematical models of peanuts drying with four drying methods were also established. According to the statistical parameters including the determination coefficient (R2)、root mean square error (RMSE) and chi-square value (χ2), the Weibull model was best for predicting the moisture ratio change kinetics of peanuts during its four drying processes. There were significant differences in physicochemical indexes of peanut by different drying methods (p < 0.05). Fat and oleic acid contents under RF-HAD were significantly higher than those by the other three drying methods. Compared with the naturally-open sun drying, RF–HAD reduced drying time by 76.70% and the microstructure of RF-HAD peanuts produced larger and more cavities. The RF-HAD kept better comprehensive nutritional quality, but the germination rate was reduced by 27.80%. PVD could maintain good nutritional quality and germination rate among these mechanical drying technologies. However, PVD had a longer drying time of 9.5 h than RF-HAD and HAD, and the microstructure of pulsed vacuum dried peanuts showed dense structure and less cavity. Hot air-dried peanut kernel held the highest protein (28.75%), fatty acids contents (26.11%) and germination rate (88.00%). However, peanut kernel dried by HAD showed poor qualities, such as high acid value, peroxide value and large color changes. These findings indicated RF-HAD was a promising drying technique for edible peanuts regarding the higher drying rate and better–quality preservation, while HAD was suitable for peanut seeds drying as it could well protect the germination rate.

Excellent Vacuum Pulsed Flashover Characteristics Achieved in Dielectric Insulators Functionalized by Electronegative Halogen–Phenyl and Naphthyl Groups

Designing electrical insulation materials with excellent surface flashover strength in a vacuum environment is crucial for high-power equipment and aerospace devices. In the present paper, the effect of two types of electronegative groups, the halogen-phenyl groups and the aromatic π-conjugated naphthyl groups, is used to greatly improve the vacuum flashover characteristics of polystyrene (PS), a commonly used polymer dielectric material in high-power devices. By polymerization of the monomers containing these electronegative groups, the bulk insulation material as a whole is modified expediently. In comparison to the base polymer PS, the electron affinity of the structures containing strong electronegative groups is studied with first-principles calculations based on the density functional theory. The nanosecond pulsed vacuum flashover testing results show that the vacuum flashover strength is increased by 10% after replacing the PS pendant phenyl groups with fluorophenyl groups and increased by 44% when replaced with the naphthyl groups. Furthermore, the thermally stimulated current and secondary electron emission yield spectroscopies are measured, to study the influence of strong electronegative groups on the trapping characteristics and further the electron-emitting features of the polymer dielectrics, which are closely related to the charged particle multiplication process during the vacuum flashover. The results prove that introducing strong electronegative groups can inhibit the triggering of vacuum flashover, suppress the electron emission, delay the flashover process, and thus greatly increase the vacuum flashover voltage. The study of this paper not only puts forward two groups of easily processable polymers with excellent vacuum flashover strength but also paves ways for the future material design of special insulation polymers.

Pulsed vacuum drying of banana: Effects of ripeness on drying kinetics and physicochemical properties and related mechanism

In current work, pulsed vacuum drying was employed to dry the bananas and the effect of ripeness on drying kinetics and physicochemical properties was explored and related mechanism was understood through the analysis of sugar and the microstructural changes. Results showed that, with the increase in the ripeness of banana, the drying time decreased from 6.0 to 5.0 h, the rehydration ratio increased from 1.76 ± 0.09 to 2.98 ± 0.11. Fourier transform infrared spectroscopic analysis illustrated ripening and pulsed vacuum drying hardly affects the chemical composition of starch molecules. Regardless of fresh or dried samples, the reducing sugar and total sugar content both increased with increase in ripeness. Compared to the dried ripeness III samples, total color difference of samples with ripeness I and II decreased by 25.03% and 25.49%, respectively. As ripeness increased, the total phenolics and DPPH assay of fresh samples decreased, whereas in pulsed vacuum dried samples it significantly increased. The overall findings indicate that the decrease of starch and the increase of sugar with the increase in ripeness have a significant impact on the pulsed vacuum drying kinetics, microstructure and physicochemical properties of the banana samples.

Progress on research and development of goji berry drying: a review

ABSTRACT Goji berry, as a kind of typical and special agricultural material, has high water content, high sugar content, thin epidermis with waxy covering, and dense fleshy cell structure, which makes the drying process more complex than the dehydration of other bio-materials. To obtain high drying efficiency and the high quality products, most researchers have paid much attention on the different kinds of pretreatment and drying technology. This study aimed to examine all the corresponding published data in the literature and to compare the drying characteristics and quality of wolfberry dried by conventional and advanced wolfberry drying methods as well as different drying parameters. We conclude that heat pump drying with simultaneous control of temperature and humidity could replace traditional hot air drying in commercial production to increase the drying efficiency and quality of wolfberry. Infrared drying and microwave drying can be combined with other drying methods to improve drying efficiency; freeze drying, pulsed vacuum drying and electro-hydrodynamic drying are conducive to maintain the appearance and nutrients as well as higher rehydration ratio because of more porous microstructure compared with hot air drying. In addition, this study gives some suggestions and new horizons for directed wolfberry drying development both on the research and application in the future.

Improvement of drying efficiency and quality attributes of blueberries using innovative far-infrared radiation heating assisted pulsed vacuum drying (FIR-PVD)

A new drying technology, far-infrared radiation heating assisted pulsed vacuum drying (FIR-PVD) was applied to process blueberries. Influences of drying temperature, vacuum pressure duration (VPD), and ambient pressure duration (APD) on drying characteristics and quality attributes of blueberries were investigated under FIR-PVD. Results showed that drying temperature, APD, and VPD had significant influence on drying time and quality attributes of dried blueberries expect for color and cohesiveness. Compared to hot air dried blueberries, FIR-PVD seriously damaged the cellular walls of blueberries during drying at 65 °C. The highest total phenolics (TP) and total monomeric anthocyanins (TMA) contents of dried blueberries were obtained at 65 °C with pulsed ratio of 15 min: 4 min and 15 min: 2 min, respectively. The FIR-PVD products obtained lower color difference, better mechanical properties and rehydration performance and maintained higher antioxidant capacity by shortening drying time, reducing exposure to oxygen, and modifying the microstructure compared to hot air drying. Industrial relevanceBlueberry is a popular function fruit around the world. With high moisture content and tender tissue, fresh blueberry is very susceptible to microbial infection even under low temperature storage conditions. Drying is a good alternative to extend the shelf life and increase the availability of fresh blueberries. However, the existence of waxy hydrophobic layer covered the surface of blueberries inhibits moisture diffusion during drying. Chemical dipping and thermal blanching pretreatments were usually used to reduce the influence of skin hydrophobicity and promote moisture diffusion. Nevertheless, the chemical additive residue leads to food safety risk and it's a big challenge to deal with the corrosive waste chemical solutions, while thermal blanching resulted a high loss of bioactive compounds. FIR-PVD is a recent developed emerging drying technology to process berries with waxy layer beyond pretreatments. Results indicated that FIR-PVD is a very promising technology for processing blueberries with the improvement of drying efficiency and quality attributes.

A study on surface-temperature measurement by means of a vacuum-arc cathode under different current conditions

Accurate detection and analysis of the cathode surface temperature delivers an important reference value as regards the prediction of electrode life and corrosion degree, a thorough study of the vacuum-breakdown phenomenon, design and application of the vacuum-arc ion source and vacuum switch. In this paper, multispectral true temperature measurement T of the surface temperature field of pulsed vacuum arc cathode materials under different discharge currents is carried out. Firstly, aiming at solving the problem of synchronous imaging measurement of the temperature field across the cathode of pulsed vacuum arc, a multi-wavelength monochromatic image-acquisition optical path is designed, based upon the image acquisition module of a high-speed monochrome camera. Also, multiple monochromatic spectral radiation images of the target are synchronously acquired on the focal plane of the same monochrome camera detector. As a result, the spectral-radiation image information of multiple wavelengths can be obtained in a single exposure time, which greatly shortens the temperature-field image acquisition time. Secondly, based upon the multispectral true-temperature calculation method, high-speed multispectral imaging measurement of the target temperature field is realized. Finally, the two-dimensional true temperature distribution of the cathode temperature field under different current conditions was studied by setting up an experimental platform for measuring the cathode temperature of the vacuum arc under different current conditions.

Pulsed Vacuum Drying of Pepper (Capsicum annuum L.): Effect of High-Humidity Hot Air Impingement Blanching Pretreatment on Drying Kinetics and Quality Attributes.

With a high moisture content, fresh peppers are perishable and rot easily. Drying is essential for shelf-life extension. The natural thin wax layer on the pepper surface hinders moisture transfer. Traditionally, chemical dipping or mechanical pricking is used to remove this wax layer. However, in chemical dipping, chemical residues can trigger food-safety issues, while the low efficiency of mechanical pricking hinders its industrial application. Feasible pretreatment methods are advantageous for industrial use. Here, an emerging pretreatment technique (high-humidity hot-air impingement blanching, HHAIB) was used for peppers before drying and its effects on drying characteristics, microstructure, and polyphenol oxidase (PPO) activity were explored. The impact of drying temperature on color parameters and red pigment content of pulsed-vacuum-dried peppers was also evaluated. PPO activity was reduced to less than 20% after blanching at 110 °C for 60 s. HHAIB reduced drying time and PPO activity and promoted chemical-substance release. Effective water diffusivity was highest (5.01 × 10−10 m2/s) after blanching at 110 °C for 90 s, and the brightness value and red pigment content were highest (9.94 g/kg) at 70 °C. HHAIB and pulsed vacuum drying are promising pretreatment and drying methods for enhancing the drying rate and quality of red peppers.

Thermal Radiation Characteristics of Pulse Vacuum Arc Discharge Cathode Surface

Thermal Radiation Characteristics of Pulse Vacuum Arc Discharge Cathode Surface

Effects of different pulsed vacuum drying strategies on drying kinetics, phenolic composition, and antioxidant capacity of chrysanthemum (Imperial chrysanthemum)

This work aimed to discuss effects of pulsed vacuum drying (PVD) at different temperatures (45°C, 50°C, 55°C and 60°C), vacuum durations (5 min, 10 min and 15 min) and multi-stage heating on drying kinetics, colour attributes, phenolic compounds and antioxidant capacity of chrysanthemum (Imperial chrysanthemum). Results indicated that successive temperature increase reduced the drying time and enhanced the drying rate and moisture diffusivity. Lower temperature (45°C) and the multi-stage (35°C-55°C-60°C) drying presented the superiority in the protection of color, preservation of phytochemical composition (chlorogenic acid, luteolin, total phenolic and total flavonoid content) and improvement of antioxidant capacity (DPPH and FRAP) of chrysanthemum, which was attributed to the low-oxygen drying environment and reduction of thermal degradation losses. Based on the results of drying efficiency and drying quality, the multi-stage heating (35°C-55°C-60°C) has the excellent potential to produce high-quality dried chrysanthemum on a commercial scale. Keywords: pulsed vacuum drying, drying kinetics, chlorogenic acid, luteolin, antioxidant capacity DOI: 10.25165/j.ijabe.20221504.7359 Citation: Xu H H, Wu M, Zhang T, Gao F, Zheng Z A, Li Y. Effects of different pulsed vacuum drying strategies on drying kinetics, phenolic composition, and antioxidant capacity of chrysanthemum (Imperial chrysanthemum). Int J Agric & Biol Eng, 2022; 15(4): 236–242.

Модификация материала катода вокруг центров взрывной электронной эмиссии в искровой стадии вакуумного пробоя

On the basis of numerical calculations estimates for the fields of mechanical stresses around the explosive electron emission centers of the cathode during the development of pulsed vacuum breakdown are given. Expansion of material due to conduction loss was taken into account. It is shown that the sizes of regions with broken crystalline structure is much larger than the corresponding of explosive emission centers.

Modification of the cathode material around the explosive electron emission centers in the spark stage of vacuum breakdown

On the basis of numerical calculations estimates for the fields of mechanical stresses around the explosive electron emission centers of the cathode during the development of pulsed vacuum breakdown are given. Expansion of material due to conduction loss was taken into account. It is shown that the sizes of regions with broken crystalline structure is much larger than the corresponding of explosive emission centers. Keywords: vacuum breakdown, explosive electron emission, mechanical stresses.

Effects of pulsation ratio on center temperature and drying characteristics of pineapple slices with pulsed vacuum drying

Effects of pulsation ratio on center temperature and drying characteristics of pineapple slices with pulsed vacuum drying

Advanced osmotic dehydration techniques combined with emerging drying methods for sustainable food production: Impact on bioactive components, texture, color, and sensory properties of food.

The food industries are looking for potential preservation methods for fruits and vegetables. The combination of osmosis and drying has proved the efficient method to improve the food quality. Osmotic dehydration is a mass transfer process in which water molecules from the food move to an osmo-active solution and the solutes from the solution migrate into the food. Advanced osmotic dehydration techniques such as electric field pulse treatment, ultrasonic and microwave-assisted dehydration, pulsed vacuum, and osmodehydrofreezing can improve the nutritional quality (bioactive) and sensory properties (color, texture, aroma, flavor) of fresh and cut-fruits without changing their reliability. Emerging osmotic dehydration technologies can preserve the structure of fruit tissue by forming microscopic channels and increasing effective water diffusivity. However, it is important to analyze the effect of advanced osmotic dehydration techniques on the quality of food products to understand the industrial scalability of these techniques. The present paper discusses the impact of recent osmotic dehydration techniques on bioactive, antioxidant capacity, color, and sensory profile of food.