Application of an Electric Field to Low-Frequency Oscillation Control in Hall Thrusters

In order to satisfy the national demands for developing a long-life satellite platform, lunar exploration, and deep-space exploitation, Hall thrusters are now considered the preferred candidate for spacecraft propulsion. A Hall thruster is a type of electric propulsion with an annular structure, in which a propellant, usually xenon, is ionized and then accelerated by electrostatic force to create a propulsive thrust. Low-frequency discharge current oscillations, also called breathing mode oscillations in some references, are among the major research topics of Hall thrusters. Low-frequency oscillations in the range of 10–100 kHz might affect the reliability of power processing unit and reduce the efficiency and specific impulse of Hall thrusters. The control of low-frequency oscillations is an essential challenge in the space application of Hall thrusters. It is proved that the electric field is a highly important influence factor for low-frequency oscillations; therefore, application of a dynamic electric field is a practical way to control low-frequency oscillation

Repetitive Nanosecond Volume Discharges under Airflows

Atmospheric pressure discharges are widely used in active airflow control, material synthesis, and air treatment. The key to an optimal application performance lies in how to generate stable and diffuse plasma especially in a large volume and in high-speed airflows. This chapter presents the study of repetitive nanosecond volume discharges under high-speed airflows. The volume discharge strongly depends on the airflows, and the corresponding discharge modes vary from filament to diffuse modes with addition of airflows. The role of airflows provides negative effects on discharge currents as well as discharge densities. Moreover, a type of discharge device with upstream and downstream structure is proposed to demonstrate that charged particles produced by the upstream discharge are transported to the downstream zone and play a pre-ionization and enhanced effect to the downstream discharges

Preparation and thermal analysis kinetics of the core–nanoshell composite materials doped with Sm

The core–nanoshell composite materials with magnetic fly-ash hollow cenosphere as core and nano SmFeO3 as shell were synthesized by high-energy ball milling method. The magnetic fly-ash hollow cenosphere, samarium nitrate, and iron nitrate were used as raw materials. The synthesis and growth kinetics of the composite materials were investigated using the thermogravimetry and differential thermal analysis (TG–DTA) at different heating rates. The results show that the precursor of the composite materials decomposes in three steps. The apparent activation energy of each stage was calculated using the Doyle–Ozawa and Kissinger methods. The reaction order, frequency factor, and rate equations were also determined. The activation energy of the nano crystallite growth is calculated to be 16.12 kJ mol−1 according to kinetics theory of nano crystallite growth. It can be inferred that the crystallite grows primarily by means of an interfacial reaction during the thermal treatment. The magnetic properties and microwave absorbing properties of samples were analyzed by the vibrating sample magnetometer analysis and vector network analyzer. The results indicated that the exchange coupling interaction happens between ferrite of magnetic fly-ash hollow cenosphere and nanosized ferrite coating, which cause outstanding magnetic properties. In the frequency between 1 MHz and 1 GHz, the absorbing effectiveness of the composite absorbers can achieve −32 dB. The magnetic properties of the composite material are better than those of single phase. So it is consistent with requirements of the microwave absorbing material at the low-frequency absorption.National Natural Science Foundation (China) (No. 20976018)Liaoning Sheng (China) (Science Research Plan Project, Higher Education Department (No. L2013176))China. Ministry of Education (Key Laboratory of Industrial Ecology and Environmental Engineering, KLIEEE-12-03)China. Ministry of Education (National Project of China ‘‘Innovation and Entrepreneurship Training Program of Undergraduate students’’ (201310150002)

Long-Life Technology for Space Flight Hall Thrusters

The vastly improved durability of spacecrafts, coupled with the simultaneous continuous development of thrusters for high power output, has created a strong demand for Hall thrusters (HT) with long service lives. However, erosion of the discharge channel walls by high-energy ions is the most impactful and visible process that limits the lifetime of the thruster. This process is very sensitive to the operation mode of the thruster and the corresponding power density. We hereby present the results of our investigation on the factors that limit the lifetime of Hall thrusters, and three proven techniques for improving longevity of use including magnetic shielding (MS), wall-less technology, and aft-magnetic fields with large gradient

Comparative transcriptomic analyses of two sugarcane Saccharum L. cultivars differing in drought tolerance

Sugarcane (Saccharum spp.) is an important cash crop, and drought is an important factors limiting its yield. To study the drought resistance mechanism of sugarcane, the transcriptomes of two sugarcane varieties with different levels of drought resistance were compared under different water shortage levels. The results showed that the transcriptomes of the two varieties were significantly different. The differentially expressed genes were enriched in starch and sucrose metabolism, linoleic acid metabolism, glycolysis/gluconeogenesis, and glyoxylate and dicarboxylate metabolic pathways. Unique trend genes of the variety with strong drought resistance (F172) were significantly enriched in photosynthesis, mitogen-activated protein kinases signaling pathway, biosynthesis of various plant secondary metabolites, and cyanoamino acid metabolism pathways. Weighted correlation network analysis indicated that the blue4 and plum1 modules correlated with drought conditions, whereas the tan and salmon4 modules correlated with variety. The unique trend genes expressed in F172 and mapped to the blue4 module were enriched in photosynthesis, purine metabolism, starch and sucrose metabolism, beta-alanine metabolism, photosynthesis-antenna proteins, and plant hormone signal transduction pathways. The expression of genes involved in the photosynthesis-antenna protein and photosynthesis pathways decreased in response to water deficit, indicating that reducing photosynthesis might be a means for sugarcane to respond to drought stress. The results of this study provide insights into drought resistance mechanisms in plants, and the related genes and metabolic pathways identified may be helpful for sugarcane breeding in the future

Nanofluids Research: Key Issues

Nanofluids are a new class of fluids engineered by dispersing nanometer-size structures (particles, fibers, tubes, droplets) in base fluids. The very essence of nanofluids research and development is to enhance fluid macroscopic and megascale properties such as thermal conductivity through manipulating microscopic physics (structures, properties and activities). Therefore, the success of nanofluid technology depends very much on how well we can address issues like effective means of microscale manipulation, interplays among physics at different scales and optimization of microscale physics for the optimal megascale properties. In this work, we take heat-conduction nanofluids as examples to review methodologies available to effectively tackle these key but difficult problems and identify the future research needs as well. The reviewed techniques include nanofluids synthesis through liquid-phase chemical reactions in continuous-flow microfluidic microreactors, scaling-up by the volume averaging and constructal design with the constructal theory. The identified areas of future research contain microfluidic nanofluids, thermal waves and constructal nanofluids

Toward nanofluids of ultra-high thermal conductivity

The assessment of proposed origins for thermal conductivity enhancement in nanofluids signifies the importance of particle morphology and coupled transport in determining nanofluid heat conduction and thermal conductivity. The success of developing nanofluids of superior conductivity depends thus very much on our understanding and manipulation of the morphology and the coupled transport. Nanofluids with conductivity of upper Hashin-Shtrikman (H-S) bound can be obtained by manipulating particles into an interconnected configuration that disperses the base fluid and thus significantly enhancing the particle-fluid interfacial energy transport. Nanofluids with conductivity higher than the upper H-S bound could also be developed by manipulating the coupled transport among various transport processes, and thus the nature of heat conduction in nanofluids. While the direct contributions of ordered liquid layer and particle Brownian motion to the nanofluid conductivity are negligible, their indirect effects can be significant via their influence on the particle morphology and/or the coupled transport

Fluid-particle heat transfer in static assemblies: Effect of particle shape

The heat transfer in fluid flow through static assemblies of randomly distributed spheroids, including spheres, prolates, and oblates, is investigated using direct numerical simulation (DNS). Constant heat fluxes are imposed at individual particle surface by the Neumann boundary condition for temperature. The superficial Reynolds number (Re) and the solid volume fraction (phi) range from 2.5 to 200, and from 0.1 to 0.3, respectively. The simulation results for spheres and prolates agree well with the DNS data and correlations in literature and new correlations for prolates and oblates are proposed. With these results, the particle shape effect on the average Nusselt number (Nu(avg)) is described more comprehensively: At low phi (e.g., 0.1), the Nu(avg) for prolates and oblates are higher than that of spheres, and as Re increases, the Nu(avg) of oblates gradually catches up that of prolates and becomes comparable at Re approximate to 200. Moreover, oblates attenuate and intensify the heat transfer at low and high Re at high phi, respectively. (C) 2020 Published by Elsevier Ltd