Particle size distribution control and related properties improvements of tungsten powders by fluidized bed jet milling

Fine grinding process of different particle size tungsten powders was carried out by fluidized bed jet milling. The results showed that the jet milling treatment caused deagglomeration of tungsten powders, which led to particles sufficient dispersion and narrow particle size distribution. Grinding gas pressure of 0.70 Mpa made the particles achieve high speed which promoted the particles collision contributing to particle dispersion and shape modification. For tungsten powder with particle size of 3 μm FSSS, a higher packing density with 5.54 g/cm³ was obtained, compared with the 3.71 g/cm³ of the original powder. For tungsten powder with particle size of 1 μm FSSS, the big agglomerates disappeared and the particle size distribution become narrower, while small aggregates about 2–3 μm still exist after the jet milling process. For tungsten powder with particle size of 5 μm and 10 μm FSSS, different medium diameter particle size and narrow particle size distribution of monodisperse tungsten powders can be produced by the optimized jet milling parameters. More important, the effective dispersion, favorable shape modification and precise classification have been achieved in the simple process.     

Potential new experiments on fabrication of cast particulate composites in space

Recent developments in fabrication of cast metal ceramic particle composites by liquid metallurgy techniques are outlined. Difficulties encountered in preparing cast composites in the ground environment (including non-uniform distribution and agglomeration of dispersed particles and relatively poor bonding between dispersoids and matrix) and how these can be overcome in a microgravity environment have been discussed. This paper also reviews experiments performed by various space agencies including NASA and ESA on fabrication of composites in space. Some new experiments concerning fabrication of cast composites like dispersion of submicron ceramic particles in molten metals, preparation of cermets with very large volume fractions of ceramic particles and dispersion of flake-type ceramic particles to achieve grain refinement have been proposed.     

Numerical Study of Particle Dispersion in the Wake of Two Tandem Square Cylinders Using Discrete Vortex Method

This study has been conducted to investigate numerically the particle dispersion in the wake of dilute particle-laden gas flows past two identical square cylinders in tandem arrangement at Reynolds number of 10,000,000. In the numerical method, the discrete vortex method is employed to calculate the gas flow fields, and the Lagrangian approach is applied to track individual solid particles. A dispersion function is defined to represent the lateral dispersion scale of particles. The wake vortex patterns and the distributions of particles with various Stokes numbers ranging from 0.01 to 10 are obtained. The numerical results reveal that: (1) the particles with St = 0.01 can distribute both in the vortex core and around the vortex periphery, whereas the particles with St = 1.0, 10 congregate mainly around the vortex periphery; (2) the particles with St = 0.01, 0.1 are trapped by the vortices into the gap between the two square cylinders, while very few particles with St = 1.0, 10 are distributed within the gap; (3) the particle's dispersion intensity along the lateral direction decreases greatly as St is increased from 0.01 to 10.     

Explosion characteristics tests for micrometer-sized Ag dust cloud with a concentration of 0.8 g/L

A dust explosion is triggered by the rapid combustion of burnable particulate matter suspended in air. Two blank tests and three Ag-dust-cloud explosion tests were performed in a 20-L spherical steel chamber using a 5 kJ pyrotechnic igniter as the energy source in each test. In the explosion tests, no apparent increase in pressure above that found in the blank tests was observed for Ag dust cloud with a concentration of 0.8 g/L. In addition, fine solid particles remaining after the tests showed no evidence of a combustion reaction at the end of the tests. In this research, the thermal decomposition of Ag powders was measured using a differential scanning calorimeter, and the thermal analysis technique to estimate hazards in relation to the samples was employed. The change in enthalpies (H) of the Ag powders were approximately 180, 93, and 53 J/g, and the onset temperatures (T₀) were approximately 211.76, 201.41, and 240.46 °C for P3032, P3012, and P3072, respectively. These results support the conclusion that Ag samples with concentrations below 0.8 g/L do not pose a dust explosion hazard.     

Color Perception in Microgravity Conditions: The Results of CROMOS Parabolic Flight Experiment

The aims of this research are first to verify the actual difference in color perception between conditions in Earth’s (1×g) and parabolic flight’s microgravitational conditions (μg) and to improve the methodology used for data collection, testing the CROMOS software for color sensitivity investigation. Additionally this paper seeks to establish a larger awareness of microgravity vision and its design implications in the field of aerospace engineering. The analysis of variations in color perception between microgravity and 1×g can be applied to a range of fields concerning the space habitat (Fig. 1), the design of information (such as safety notices), or in the space station the analysis of chemical and biological reactions based on chromatography (for example, when subtle color variations are used as indicators in histological cell analysis).     

Characteristic Analyses of Plasma Air Cleaning Systems for the Removal of Indoor Air Pollutants

Tobacco smoke is one of the most common manmade aerosols. Yellow sand dust and pollen are the particular and regional pollutants generated by natural phenomena. These pollutants have different removal characteristics, respectively, when the air cleaning system is operated. It is well known that tobacco smoke particles are removed effectively with electrostatic precipitators. But it is necessary to evaluate whether the plasma air cleaning system has good performance of removing Yellow sand dusts and pollens simultaneously and also to establish the operation modes for efficient removal of those particular air pollutants by controlling the air flow rates passing the electrostatic precipitator and operating times of air cleaning system. In this study, the performance evaluation of plasma air cleaning systems is investigated with tobacco smoke particles, Yellow sand dusts, and pollens. For the multi-pass test in occupied spaces of 150 m³, the operation time required to reduce dust concentration from the initial concentration of 300 µg/m³ to 150 µg/m³, the criteria of indoor air quality in Korea, are 40 min for tobacco smoke, 28 min for Yellow sand dust, and 5 min for pollen when the flow rate is 17 m³/min. Also, the optimal operation modes for each pollutant are suggested for the efficient removal of indoor air pollutants. At first, most particles are removed by maximum flow operation. Second, the rest of the particles are removed by medium flow operation. Next, the plasma air cleaning systems are maintained by minimum flow for tobacco smoke mode and by repeating minimum flow and medium flow for Yellow sand and pollen modes. Edit to “Because the Yellow sand dust and the pollen flow into the room continuously and settle down … noise reduction.” Because the Yellow sand dust and the pollen flow into the room continuously and settle down. The plasma air cleaning system is suitable for the removal of the tobacco smoke, the Yellow sand dust, and the pollen for maintaining suitable indoor air quality, and, if it is operated through the suitable operation mode, energy efficiency will improve noise reduction.     

Investigation of the influence of free convection on the heat transfer of cylinders with different aspect ratios

Electrically heated cylindrical wires are used in research and industry for fluid velocity and turbulence measurements. At very low free-stream velocities (u≤0.1 m/s), hot-wire measurements are significantly influenced by buoyant convection. Below a certain Reynolds number Re* this effect degrades the accuracy of the measurements. To assess the contribution of free-convection heat transfer to the heat balance of hot-wires in cross flow, measurements under normal gravity and microgravity (µg) conditions are compared keeping all other parameters constant. Under gravity conditions, the acceleration of gravity, the hot-wire axis and the direction of the free stream are all perpendicular to each other. The microgravity experiments were carried out in the Drop-Tower Bremen in which the residual acceleration is less than 10^?5 g during a period of 4.7 s. The present investigation is concerned with a velocity range of 0≤u≤0.35 m/s corresponding to a Reynolds number range Re<0.1 in standard air. This range includes pure free convection for Re→0 and forced-convection-dominated heat transfer for Re=0.1. At intermediate Reynolds numbers both transport mechanisms must be considered.     

Experimental Study on the Ignition Process of Single Coal Particles at Microgravity

In this paper, the first Chinese microgravity (μ-g) experimental study on coal combustion was introduced. An experimental system used to study the ignition process of single coal particles was built up, complying with the requirements of the 3.5 s drop tower in the National Microgravity Laboratory of China (NMLC). High volatile bituminous and lignite coal particles with diameter of 1.5 and 2.0 mm were tested. The ignition and combustion process was recorded by a color CCD and the particle surface temperature before and at the ignition was determined by the RGB colorimetric method. Comparative experiments were conducted at normal gravity (1-g). The experiments revealed that at different gravity levels, the ignition of all tested coal particles commenced in homogeneous phase, while the shape, structure, brightness and development of the flames, as well as the volatile matter release during the ignition process are different. At μ-g, the part of volatile was released as a jet, while such a phenomenon was barely observed at 1-g. Also, after ignition, flames were more spherical, thicker, laminated and dimmer at μ-g. It was confirmed that ignition temperature decreased as the particle size or volatile content increased. However, contradicted to existing experimental results, provided other experimental conditions except gravity level were the same, ignition temperature of coal particles was about 50–80 K lower at μ-g than that at 1-g.     

Relative Chargeability of Nanostructured and Conventional Particles by Tribocharging

The relative chargeability of particles composed of aggregates of nanoparticles by tribocharging was measured and compared with that of conventional particles. Particles were dispersed through a Teflon® tribocharger into a 20.8 m³ experimental chamber. The net charge-to-mass ratios of the dispersed particles were measured with a Faraday cup charge measuring device. Tribocharging with a Teflon® charger was able to charge the particles positively. NanoActive® TiO₂ gained the greatest net charge-to-mass ratio (1.21 mC/kg, s.d. = 0.07) followed by NanoActive® MgO (0.81 mC/kg, s.d. = 0.12) and ISO fine test dust (0.66 mC/kg, s.d. = 0.13). These net charge-to-mass ratios, however, were small compared to the Gaussian limit (<8%). Results suggest that commercially available tribochargers may not be suitable for imparting significant charge to nanostructured aggregates.     

Dusty plasma of a dc glow discharge: methods of investigation and characteristic features of behavior

Results are given of experimental investigations of a dusty plasma in the strata of a dc glow discharge. Characteristic forms of plasma-dust structures formed by dust particles of spherical and highly asymmetric shapes are described. Results are given of observation of self-excited dust-acoustic waves, as well as of high-amplitude waves generated under pulsed gasdynamic stimulation. A method of measuring charges of dust particles levitating in strata is described, as well as a method of measuring the radial field of forces acting on dust particles in a plasma trap. Mention is made of methods for the investigation of plasma-dust structures under conditions of microgravity.Translated from Teplofizika Vysokikh Temperatur, Vol. 42, No. 6, 2004, pp. 821–834.Original Russian Text Copyright © 2004 by V. I. Molotkov, O. F. Petrov, M. Yu. Pustylnik, V. M. Torchinskii, V. E. Fortov, and A. G. Khrapak.     

Effect of bismuth–tin alloy particle diameter on bonding strength of copper nanoparticles/bismuth–tin solder hybrid joints

The effect of the diameter of Bi–Sn alloy particles on the bonding strength of hybrid joints formed between SiC chips and direct-bonded copper (DBC) plates using a Cu nanoparticles/Bi–Sn solder was studied. The bonding strength was the highest at 40 MPa for a Bi–Sn alloy particle diameter of 10 µm. Further, the bonding strength was dependent on the area of the bonding layer adhering to the SiC-side fracture surface, as determined by the die-shear test. Ni, which was deposited on the SiC chips and DBC plates before the bonding process, remained near the interfacial area of the bonding layer in the joints formed using the 5 µm particles. In contrast, Ni diffused all over the bonding area, with the exception of the interfacial area where Cu–Sn compounds were formed, in the joints produced using the larger alloy particles. The distribution of Sn in the bonding layer became more uniform and the segregation of Bi at the interface became more pronounced as the particle size was reduced. Further, with an increase in the particle size, the Ag layers deposited on the surfaces of the SiC chips and DBC plates diffused into the bonding layer after the first firing step at 473 K, which was performed before the secondary firing step at 623 K. These results imply that the diameter of the Bi–Sn solder particles in hybrid joints affects the interfacial structure, as it governs the wetting behavior of the Bi–Sn solder and hence has a determining effect on the bonding strength.     

Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm³) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.     

Iron Burning in Pressurised Oxygen Under Microgravity Conditions

An investigation of cylindrical iron rods burning in pressurised oxygen under microgravity conditions is presented. It has been shown that, under similar experimental conditions, the melting rate of a burning, cylindrical iron rod is higher in microgravity than in normal gravity by a factor of 1.8 ± 0.3. This paper presents microanalysis of quenched samples obtained in a microgravity environment in a 2.0 s duration drop tower facility in Brisbane, Australia. These images indicate that the solid/liquid interface is highly convex in reduced gravity, compared to the planar geometry typically observed in normal gravity, which increases the contact area between liquid and solid phases by a factor of 1.7 ± 0.1. Thus, there is good agreement between the proportional increase in solid/liquid interface surface area and melting rate in microgravity. This indicates that the cause of the increased melting rates for cylindrical iron rods burning in microgravity is altered interfacial geometry at the solid/liquid interface.     

DUST STRANDS IN CYCLONE SEPARATORS AT LOW DUST CONCENTRATION

Abstract

State of the art in calculating a cyclone separator is the application of the equilibrium theory and taking the formation of dust strands into account as well. The latter process does not depend on particle size mainly. An ideal flow pattern for the formation of dust strands is the so called Dean-vortex: it is being realized favorably in the axial flow cyclone. A dust strand can be produced down to a raw gas concentration C₀ ≈ 10^?5. Then, it is being exhausted through one or few holes in the mantle of the axial cyclone applying bleeding of about 10 % of the volume flow Separating its dust in a bin cyclone and recirculating the binflow gas to the main, axial cyclone completes this high performance cyclone separator. Dimensional analysis shows that the clean gas concentration c₁ mainly depends on the swirl W_tan/w_ax, the raw gas concentration c₀and on Reynolds number. For usual dust conditions a clean gas concentration c₁ ≤ 50 mg/m³ is feasible.     

Parabolic flight experiments with PK-4

PK-4 is an experiment designed to investigate complex plasmas (low-temperature plasmas containing microparticles, e.g. dust grains) in a combined dc/rf discharge under microgravity conditions on board of the International Space Station. Within the 35th and 36th ESA parabolic flight campaigns first experiments under microgravity conditions in a specially designed experiment set-up have been performed. The particle flow inside the tube, the appearance of dust waves, and lane formation in interpenetrating particle clouds have been observed.     

Particle conveying under microgravity in a vibrating vessel

This paper presents a numerical study on the conveying of particles in a vibrating vessel under microgravity. Such a vessel is composed of parallel plates with sawtooth wavy surfaces, which are specifically designed to convey particles using simple vibration. The numerical model was validated by good agreement between the simulated and experimental results. Then the effects of key variables, including the vessel geometry, vibration amplitude and frequency and gravity level, were systematically investigated by a series of controlled simulations. The results confirm the optimised design from the previous experiments, and numerically demonstrate that using such a system a steady conveying operation can be achieved under microgravity. The convey rate is positively affected by the vibration amplitude and frequency in a complicated way, which cannot be simply described by the commonly used vibration intensity or velocity amplitude. The gravity level also has a significant effect on the convey rate when it is over 0.001g. The convey rate can be estimated by the product of the average solid fraction and velocity. And the effects of the variables can be better understood through the analyses on these two parameters. Finally, a predictive model is proposed to estimate the convey rate under different operational conditions. The findings are useful for the design of particle conveying techniques for outer space applications.     

Microgravity Effects on Chronoamperometric Ammonia Oxidation Reaction at Platinum Nanoparticles on Modified Mesoporous Carbon Supports

The effect of microgravity on the electrochemical oxidation of ammonia at platinum nanoparticles supported on modified mesoporous carbons (MPC) with three different pore diameters (64, 100, and 137 Å) was studied via the chronoamperometric technique in a half-cell. The catalysts were prepared by a H₂ reductive process of PtCl\(_{6}^{\mathrm {4-}}\) in presence of the mesoporous carbon support materials. A microgravity environment was obtained with an average gravity of less than 0.02 g created aboard an airplane performing parabolic maneuvers. Results show the chronoamperommetry of the ammonia oxidation reaction in 1.0 M NH₄OH at 0.60 V vs. RHE under microgravity conditions. The current density, in all three catalysts, decreased while in microgravity conditions when compared to ground based experiments. Under microgravity, all three catalysts yielded a decrease in ammonia oxidation reaction current density between 25 to 63% versus terrestrial experimental results, in time scales between 1 and 15 s. The Pt catalyst prepared with mesoporous carbon of 137 Å porous showed the smallest changes, between 25 to 48%. Nanostructuring catalyst materials have an effect on the level of current density decrease under microgravity conditions.     

Application of μRaman Microscopy to the Identification of Individual Airborne Particles: Preliminary Results from Kozani's Area,Northern Greece

In the present study, Raman spectroscopy combined with a con-focal microscope has been applied for the identification of airborne particulate matter collected from Kozani's area, northern Greece. This technique provided a rapid and accurate identification of the molecular composition of micrometer sized (down to 2 µm) airborne particles. Different experimental conditions (laser excitation wavelength of 532 and 780 nm, slit and pinhole aperture, 50× and 100× objectives, number of exposures, and time of each exposure) were employed in order to obtain the optimal analytical parameters. The slit aperture and the 532 nm laser source were preferable. Removal of the background interference caused by the blank Teflon filter was performed for the acquisition of Raman spectra of minute (<10 µm) airborne particles, whereas no background correction was necessary for larger particles (>10 µm). Several distinct mineral phases were determined, such as: the common geogenic minerals calcite, gypsum, anhydrite, titanium oxides and microcline (feldspar); the anthropogenic minerals, black carbon and nitrates; and the lepidolite and smectite (phyllosilicates) attributable to Saharan dust episodes.     

Analysis of triboelectric charging of particles due to aerodynamic dispersion by a pulse of pressurised air jet

Triboelectric charging of powders causes nuisance and electrostatic discharge hazards. It is highly desirable to develop a simple method for assessing the triboelectric charging tendency of powders using a very small quantity. We explore the use of aerodynamic dispersion by a pulse of pressurised air using the disperser of Morphologi G3 as a novel application. In this device particles are dispersed by injection of a pulse of pressurised air, the dispersed particles are then analysed for size and shape analysis. The high transient air velocity inside the disperser causes collisions of sample particles with the walls, resulting in dispersion, but at the same time it could cause triboelectric charging of the particles. In this study, we analyse this process by evaluating the influence of the transient turbulent pulsed-air flow on particle impact on the walls and the resulting charge transfer. Computational Fluid Dynamics is used to calculate particle trajectory and impact velocity as a function of the inlet air pressure and particle size. Particle tracking is done using the Lagrangian approach and transient conditions. The charge transfer to particles is predicted as a function of impact velocity and number of collisions based on a charge transfer model established previously for several model particle materials. Particles experience around ten collisions at different velocities as they are dispersed and thereby acquire charges, the value of which approaches the equilibrium charge level. The number of collisions is found to be rather insensitive to particle size and pressure pulse, except for fine particles, smaller than about 30 µm. As the particle size is increased, the impact velocity decreases, but the average charge transfer per particle increases, both very rapidly. Aerodynamic dispersion by a gas pressure pulse provides an easy and quick assessment of triboelectric charging tendency of powders.     

A NUMERICAL MODEL FOR STEADY NONPOINT SOURCE DISPERSION OF WIND-BLOWN FINE PARTICLES FROM STORAGE PILES AND EXPERIMENTAL STUDY

This study is concerned with modeling of the loss of fine dust from storage piles and their dispersion in the atmosphere. The results of downwind particulate dispersion are tested by means of a two-dimensional wind tunnel model using tracer particles. The study shows that a wind barrier located two to three pile heights upstream will effectively reduce the wind blowing of fine particles from storage piles and the downwind particle density. The tracers used in the experiment are smoke, magnesia, latex, and glass particles. The particle sizes studied range from 15 μm to 75 μm. Experimental results indicate that the effects of particle settling due to gravity force will be negligible when the particle settling velocity is less than 0.03 m/s. If particle size is less than 15 μm, particles will most likely remain in a suspension state over a long distance. Finite difference techniques are used for steady state numerical simulation of particulate dispersion. The effects of the particle sizes, wind velocity, and the ground conditions on the downwind particle density distribution are presented.