Fraction Of Large Particles Research Articles

Properties of a powder metallurgy low-alloy steel with varying fractions of large iron particles

ABSTRACT Interiors of coarse iron particles in blends used in powder metallurgy may not be adequately alloyed during sintering. Relatively low concentrations of alloying elements reduce the hardenability of these regions, which may not transform to martensite during quenching. Since an actual microstructure (martensite, pores and pearlite) may differ from the desired microstructure (martensite and voids), it is reasonable to speculate that large and relatively soft pearlitic inclusions may worsen mechanical properties. This conjecture was tested, and it was experimentally found that unless the fraction of coarse particles exceeded a certain threshold, the properties of a medium-density powder metallurgy steel did not vitiate significantly. It also was observed that when the fraction of large particles dropped below a certain limit, apparent density, compressibility and flowability degraded to an extent rendering the corresponding mixtures unusable.A contradiction between a visual perception and a verdict of a statistical analysis is commented upon.

External heat-exchange in a quasi-bidisperse granular fluidized bed

This paper presents the results of an experimental study of the external heat-exchange with a tube bundle in a bidisperse fluidized bed with marked bimodality of particle size distribution. The dependence of the maximum heat-transfer coefficient on the rate of air filtration was determined. The influence of the mass fraction of large particles on the heat-transfer coefficient in a mixture has been established. An interpolation dependence for calculating the optimal filtration rate of the investigated mixtures has been obtained. It is recommended to use the average surface diameter of the polydisperse particles mixture as the equivalent diameter when calculating the heat-exchange. A method for calculating the maximum heat-transfer coefficient and the optimal filtration rate for a quasi-bidisperse fluidized bed has been developed. The method is based on the use of heat transfer coefficients and optimal filtration rates of individual components of the bidisperse mixture. The equations for calculating the maximum heat-transfer coefficient and the optimal filtration rate in the bidisperse mixture has been obtained.

Aerosol particle formation in the upper residual layer

Abstract. According to current estimates, atmospheric new particle formation (NPF) produces a large fraction of aerosol particles and cloud condensation nuclei in the Earth's atmosphere, which have implications for health and climate. Despite recent advances, atmospheric NPF is still insufficiently understood in the lower troposphere, especially above the mixed layer (ML). This paper presents new results from co-located airborne and ground-based measurements in a boreal forest environment, showing that many NPF events (∼42 %) appear to start in the topmost part of the residual layer (RL). The freshly formed particles may be entrained into the growing mixed layer (ML) where they continue to grow in size, similar to the aerosol particles formed within the ML. The results suggest that in the boreal forest environment, NPF in the upper RL has an important contribution to the aerosol load in the boundary layer (BL).

The Interactions Between Oxide Film Inclusions and Inoculation Particles TiB2 in Aluminum Melt

In this work, a systematic study on the interactions between aluminum oxide films and TiB2 grain refiner particles and their effect on grain refinement behavior have been conducted. Oxide films were introduced into a commercial purity aluminum melt by adding AA 6061 alloy chips while the grain refiner particles were introduced by adding Al-3T-1B master alloy. Strong sedimentation of TiB2 grain refiner particles was observed in aluminum melt without chip addition during long-time settling. Most of the TiB2 particles were settled and accumulated at the bottom of crucible. In contrast, the sedimentation of TiB2 particles is much less in the melt with the addition of oxide films. A large fraction of TiB2 particles were found to be adhered to the oxide films located at the top part of the crucible, which inhibited the sedimentation of grain refiner particles. TP-1 type tests were also done to study the grain refinement efficiency of Al-3Ti-1B master alloy under different melt cleanliness and settling time. It is found that sedimentation of TiB2 particles greatly reduces the grain refinement efficiency. The introduction of oxide films seems to slightly alleviate the fading effect. This is owing to the strong adherence between the oxide films and TiB2 particles, which leads to a retardation of particle sedimentation.

Finite element simulation of the Portevin–Le Chatelier effect in highly reinforced metal matrix composites

ABSTRACT The Portevin–Le Chatelier effect in an AlCu/AlO composite with a large volume fraction (60%) of ceramic particles is investigated from a numerical point of view using a strain-ageing visco-plastic material model for the metallic matrix and several different finite element meshes based on real microstructures. Even for this very large volume fraction, some serrations are observed on the experimental tensile stress/strain curves, together with the propagation of bands of localised plasticity. The finite element model succeeds in reproducing the serrations and the propagation of bands experimentally observed, and allows a fine analysis of the spatio-temporal evolution of the bands of localised plastic strain rate. Different volume fractions are then virtually generated, tested, and compared to outline the influence of this second phase on the Portevin–Le Chatelier effect.

Increasing feed intake in domestic goats (Capra hircus): Measured effects on chewing intensity are probably driven by escape of few, large particles from the forestomach

On the one hand, oral processing – mastication – is considered a relatively inflexible component of mammalian feed acquisition that constrains instantaneous intake rates. On the other hand, experimental data shows that the level of feed intake affects faecal particle size and hence net chewing efficiency in ruminants, with larger particles occurring in the faeces at higher intakes. Here, we report the effect of an increased feed intake during maintenance (L1), late (200% of L1) and peak lactation (300% of L1) of a consistent diet (hay:concentrates 50:50) in eight domestic goats on various measures of digestive physiology including faecal mean particle size (MPS). Increasing intake led to an increased gut fill, a reduction in digesta retention times, and an increase in faecal MPS (from 0.57 to 0.72 mm). However, this was an effect of the large particle fraction (>2 mm) being disproportionately excreted at higher intakes; if MPS was assessed on the basis of particles below the typical escape threshold (≤1 mm), there was no difference between intake levels. These findings suggest that the effect of intake on the calculated net chewing efficiency in ruminants may rather be an effect of increased large particle escape from the forestomach than a reduced chewing intensity per bolus during ingestion or rumination.

Size segregation of binary particles in a moving granular bed filter for hot gas clean-up

High-temperature dust removal is one of the important procedures for integrated coal gasification combined cycle. In this work, a moving granular bed filter that can separate the mixture of particles of different sizes spontaneously was designed. We studied the flow characteristics of binary particles in the filter based on the discrete element method (DEM). We gave the formation process of the boundary between the stationary layer and the flowing layer, which should have a reference value for the control of the stationary layer and the design of the filter inclined groove. Typical separation trajectories of large and small particles during size segregation were also given. The degree of size segregation decreased with the increase of the mass flux of particles. Real-time control of the large particle layer thickness in the filter area can be achieved by changing the mass fraction of the inlet large particles.

Cross-scale characterization of sandstones via statistical nanoindentation: Evaluation of data analytics and upscaling models

Cross-scale mechanical characterization of two sandstones was performed using big data statistical nanoindentation involving continuous stiffness measurement to profile the depth-dependent changes in Young's modulus and hardness, with a particular focus on the evaluation of three different data analytics and four upscaling models. A massive number (i.e., >1000) of indents with depths of up to ~6000 nm were made on each of the two sandstones with highly contrasting microstructures and dissimilar compositions, followed by intensive data processing using the recently developed data analytics consisting of data segmentation to extract subdatasets at various depths, statistical deconvolution of each subdataset to identify mechanically distinct phases, re-assembly of results from different subdatasets, and indentation surround effect modeling to extract the properties of both individual phases and bulk rocks. Moreover, the composition and microstructure of the two sandstones were analyzed by quantitative X-ray diffraction and electron microscopy with elemental mapping, respectively. The new development is the three different statistical deconvolution techniques, including probability density function (PDF), cumulative distribution function (CDF), and Gaussian mixture modeling (GMM), used for data processing. Moreover, four micromechanical homogenization and upscaling models were also employed to derive Young's moduli of the two bulk rocks using experimental results. Surprisingly, the three deconvolution techniques yield similar results, and GMM can provide more accurate phase separation due to the use of two-dimensional datasets. Yet, none of the four upscaling models works well for the two rocks. These results manifest well the rocks' mineralogical composition and microstructure: while one consists of primary, inter-contacted, coarse-grained skeleton with secondary fine-grained particles filling the pores, the other is mainly made of primary fine-grained clay matrix with a large fraction of coarse particles as isolated inclusions.

Measurement report: Comparison of wintertime individual particles at ground level and above the mixed layer in urban Beijing

Abstract. Beijing has been suffering from frequent severe air pollution events, with concentrations affected significantly by the mixed-layer height. Major efforts have been made to study the physico-chemical properties, compositions, and sources of aerosol particles at ground level. However, little is known about the morphology, elemental composition, and mixing state of aerosol particles above the mixed layer. In this work, we collected individual aerosol particles simultaneously at ground level (2 m above ground) and above the mixed layer in urban Beijing (within the Atmospheric Pollution and Human Health in a Chinese Megacity, APHH-Beijing, 2016 winter campaign). The particles were analyzed offline by transmission electron microscopy coupled with energy dispersive X-ray spectroscopy. Our results showed that the relative number contribution of mineral particles to all measured particles was much higher during non-haze periods (42.5 %) than haze periods (18.1 %); in contrast, internally mixed particles contributed more during haze periods (21.9 %) than non-haze periods (7.2 %) at ground level. In addition, more mineral particles were found at ground level than above the mixed-layer height. Around 20 % of individual particles showed core–shell structures during haze periods, whereas only a few core–shell particles were observed during non-haze periods (2 %). The results showed that the particles above the mixed layer were more aged, with a larger proportion of organic particles originating from coal combustion. Our results indicate that a large fraction of the airborne particles above the mixed layer come from surrounding areas influenced by coal combustion activities. This source contributes to the surface particle concentrations in Beijing when polluted air is mixed down to the ground level.

Departure of the thermal escape rate from the jeans escape rate for atomic hydrogen at Earth, Mars, and Pluto

The recent observations of the Pluto’s upper atmosphere by the UV spectrometer Alice on the New Horizons spacecraft mission have shown that it is not in slow hydrodynamic escape as predicted by some fluid models but not by kinetic models. This instrument also detects the Lyman-alpha emission of atomic hydrogen. On Pluto, the hydrogen atoms are produced by the photodissociation of methane and reside in an extended corona around Pluto. Similar to the case at Earth and Mars, the Jeans escape should be the dominant escape process for hydrogen on Pluto due to the low value of the escape parameter at the exobase. However, because of this escape, the velocity distribution at the exobase is truncated at high velocities and the Jeans’s escape rate needs to be reduced by a factor B. The goal of this study is to calculate the value of B for the hydrogen on Pluto and check if a plane parallel model, valid to estimate B on Earth and Mars is also valid to calculate B on Pluto.We compute B with a plane parallel model for the planets’ exospheres, and with a more realistic spherical model to check the validity of the plane parallel model. We find very good agreement between the two models for the current exobase temperatures at Earth, Mars and Pluto. The departure of the thermal hydrogen escape rate from the predicted Jeans escape rate is larger for Mars and Earth than Pluto, even though the escape parameter is lower on Pluto than Mars and Earth. This difference is due to the presence of a minimum in this correction factor for an escape parameter near 3. This minimum is due to the large fraction of particles with a velocity larger than the escape velocity at low escape parameter, leading to an upward-directed velocity distribution close to the Maxwellian distribution at the exobase. The factor B can be decomposed as the product of two terms: one associated with the departure of the distribution velocity from a Maxwellian distribution at the exobase, and the second, associated with the few collisions above the exobase, reducing the escape rate. The first term has a minimum as a function of exobase temperature, while the second term is a monotonically decreasing function of exobase temperature to an asymptotic value.

Variability in the mass absorption cross section of black carbon (BC) aerosols is driven by BC internal mixing state at a central European background site (Melpitz, Germany) in winter

Abstract. Properties of atmospheric black carbon (BC) particles were characterized during a field experiment at a rural background site (Melpitz, Germany) in February 2017. BC absorption at a wavelength of 870 nm was measured by a photoacoustic extinctiometer, and BC physical properties (BC mass concentration, core size distribution and coating thickness) were measured by a single-particle soot photometer (SP2). Additionally, a catalytic stripper was used to intermittently remove BC coatings by alternating between ambient and thermo-denuded conditions. From these data the mass absorption cross section of BC (MACBC) and its enhancement factor (EMAC) were inferred for essentially water-free aerosol as present after drying to low relative humidity (RH). Two methods were applied independently to investigate the coating effect on EMAC: a correlation method (MACBC, ambient vs. BC coating thickness) and a denuding method (MACBC, ambient vs. MACBC, denuded). Observed EMAC values varied from 1.0 to 1.6 (lower limit from denuding method) or ∼1.2 to 1.9 (higher limit from correlation method), with the mean coating volume fraction ranging from 54 % to 78 % in the dominating mass equivalent BC core diameter range of 200–220 nm. MACBC and EMAC were strongly correlated with coating thickness of BC. By contrast, other potential drivers of EMAC variability, such as different BC sources (air mass origin and absorption Ångström exponent), coating composition (ratio of inorganics to organics) and BC core size distribution, had only minor effects. These results for ambient BC measured at Melpitz during winter show that the lensing effect caused by coatings on BC is the main driver of the variations in MACBC and EMAC, while changes in other BC particle properties such as source, BC core size or coating composition play only minor roles at this rural background site with a large fraction of aged particles. Indirect evidence suggests that potential dampening of the lensing effect due to unfavorable morphology was most likely small or even negligible.

A field, laboratory, and literature review evaluation of the water retention curve of volcanic ash soils: How well do standard laboratory methods reflect field conditions?

AbstractAccurate determination of the water retention curve (WRC) of a soil is essential for the understanding and modelling of the subsurface hydrological, ecological, and biogeochemical processes. Volcanic ash soils with andic properties (Andosols) are recognized as important providers of ecological and hydrological services in mountainous regions worldwide due to their large fraction of small size particles (clay, silt, and organic matter) that gives them an outstanding water holding capacity. Previous comparative analyses of in situ (field) and standard laboratory methods for the determination of the WRC of Andosols showed contrasting results. Based on an extensive analysis of laboratory, experimental, and field measured WRCs of Andosols in combination with data extracted from the published literature we show that standard laboratory methods using small soil sample volumes (≤300 cm3) mimic the WRC of these soils only partially. The results obtained by the latter resemble only a small portion of the wet range of the Andosols' WRC (from saturation up to −5 kPa, or pF 1.7), but overestimate substantially their water content for higher matric potentials. This discrepancy occurs irrespective of site‐specific land use and cover, soil properties, and applied method. The disagreement limits our capacity to infer correctly subsurface hydrological behaviour, as illustrated through the analysis of long‐term soil moisture and matric potential data from an experimental site in the tropical Andes. These findings imply that results reported in past research should be used with caution and that future research should focus on determining laboratory methods that allow obtaining a correct characterization of the WRC of Andosols. For the latter, a set of recommendations and future directions to solve the identified methodological issues is proposed.

DEM study of segregation degree and velocity of binary granular mixtures subject to vibration

This paper presents a study on the Brazil-nut segregation of binary granular mixtures under vibration by the discrete element method. The ascending velocity of large particles (Va) and the overall segregation degree (Dsep) are correlated to the vibration velocity amplitude (Vb) and the dimensionless frequency (fd). Both Dsep and the average Va increase linearly with Vb under a given fd, while the coefficient shows a critical change at fd =1. Va conforms to a single-peak distribution, and the shape parameters of the distribution converge after Vb> 1.7. Both Va and Dsep can be correlated to (A/ds)‧fd0.5 universally, where A is amplitude and ds is the small particles size. The increase of the volume fraction of large particles leads to the increase of Dsep and Va, which is probably linked with the increase in granular temperature and the difference between the vibration patterns of large and small particles.

Experimental study of the influence of operational and geometric variables on the powders produced by close-coupled gas atomisation

The effect of several operational and geometric variables on the particle size distribution of powders produced by close-coupled gas atomisation is analysed from a total of 66 experiments. Powders of three pure metals (copper, tin and iron) and two alloys (bronze Cu-15 wt% Sn and stainless steel SS 316 L) have been produced. Nitrogen, argon and helium were used as atomising gases. It is shown that the gas-to-metal ratio of volume flow rates (GMRV) is more relevant than the ratio of mass flow rates (GMR) in order to analyse the effect of atomisation variables on the particle size. Kishidaka's equation, originally proposed for water atomisation, is modified to predict the median particle size in gas atomisation. The accuracy of the new equation is compared with that of Lubanska, and Rao and Mehrotra. Kishidaka's modified empirical correlation is the most accurate in predicting the median particle size of the powders produced in this work. The morphology of the produced powders is studied by scanning electron microscopy (SEM) and it is observed that the melt superheat can play an important role in the aggregation of fine particles (< 10 μm), which increases the fraction of large particles (> 100 μm).

Influence of Grain Refiners on the Wettability of Al2O3 Substrate by Aluminum Melt

It is well known that grain refiner additions in aluminum melts significantly reduce the filtration efficiency of ceramic foam filters (CFF). However, the mechanism remains unclear. In this work, the influence of grain refiners on the wettability of alumina substrate by aluminum melt was studied by both conventional sessile drop and improved sessile drop methods at different temperatures and vacuums. Commercial purity aluminum (CP-Al) and grain refiner master alloys Al-3Ti-1B, Al-5Ti-1B, Al-3Ti-0.15C were used. It is found that master alloy melts wet alumina substrate better than CP-Al. Generally, a lower temperature or lower vacuum results in a higher contact angle. The roles of grain refiner particles in improving the wettability were studied by analyzing the solidification structure of post wetting-test droplets using SEM. Strong sedimentation of grain refiner particles at the metal-substrate interface was observed, which is attributed to the higher density of grain refiner particles compared to the Al melt. Meanwhile, a large fraction of grain refiner particles agglomerates at the oxide skin of the aluminum droplets, showing a strong adhesion between the particles and oxide skin. Such adhering of grain refiner particles is proposed to enhance the rupture of the original oxide skin of the droplets and slow down the reoxidation process at the surface layer. Both adherence of grain refiner particles to surface oxide skin and sedimentation of particles at the metal-substrate interface are responsible for the wetting improvement.

The characterization of Taklamakan dust properties using a multiwavelength Raman polarization lidar in Kashi, China

Abstract. The Taklamakan desert is an important dust source for the global atmospheric dust budget and a cause of the dust weather in East Asia. The characterization of Taklamakan dust in the source region is still very limited. To fill this gap, the DAO (dust aerosol observation) was conducted in April 2019 in Kashi, China. The Kashi site is about 150 km from the western rim of the Taklamakan desert and is strongly impacted by desert dust aerosols, especially in spring time, i.e., April and May. According to sun–sky photometer measurements, the aerosol optical depth (at 500 nm) varied in the range of 0.07–4.70, and the Ångström exponent (between 440 and 870 nm) in the range of 0.0–0.8 in April 2019. In this study, we provide the first profiling of the 2α+3β+3δ parameters of Taklamakan dust based on a multiwavelength Mie–Raman polarization lidar. For Taklamakan dust, the Ångström exponent related to the extinction coefficient (EAE, between 355 and 532 nm) is about 0.01 ± 0.30, and the lidar ratio is found to be 45 ± 7 sr (51 ± 8–56 ± 8 sr) at 532 (355) nm. The particle linear depolarization ratios (PLDRs) are about 0.28–0.32 ± 0.07 at 355 nm, 0.36 ± 0.05 at 532 nm and 0.31 ± 0.05 at 1064 nm. Both lidar ratios and depolarization ratios are higher than the typical values of Central Asian dust in the literature. The difference is probably linked to the fact that observations in the DAO campaign were collected close to the dust source; therefore, there is a large fraction of coarse-mode and giant particles (radius &gt;20 µm) in the Taklamakan dust. Apart from dust, fine particles coming from local anthropogenic emissions and long-range transported aerosols are also non-negligible aerosol components. The signatures of pollution emerge when dust concentration decreases. The polluted dust (defined by PLDR532≤0.30 and EAE355-532≥0.20) is featured with reduced PLDRs and enhanced EAE355−532 compared to Taklamakan dust. The mean PLDRs of polluted dust generally distributed in the range of 0.20–0.30. Due to the complexity of the nature of the involved pollutants and their mixing state with dust, the lidar ratios exhibit larger variabilities compared to those of dust. The study provides the first reference of novel characteristics of Taklamakan dust measured by Mie–Raman polarization lidar. The data could contribute to complementing the dust model and improving the accuracy of climate modeling.

Size-resolved particle number emissions in Beijing determined from measured particle size distributions

Abstract. The climate and air quality effects of aerosol particles depend on the number and size of the particles. In urban environments, a large fraction of aerosol particles originates from anthropogenic emissions. To evaluate the effects of different pollution sources on air quality, knowledge of size distributions of particle number emissions is needed. Here we introduce a novel method for determining size-resolved particle number emissions, based on measured particle size distributions. We apply our method to data measured in Beijing, China, to determine the number size distribution of emitted particles in a diameter range from 2 to 1000 nm. The observed particle number emissions are dominated by emissions of particles smaller than 30 nm. Our results suggest that traffic is the major source of particle number emissions with the highest emissions observed for particles around 10 nm during rush hours. At sizes below 6 nm, clustering of atmospheric vapors contributes to calculated emissions. The comparison between our calculated emissions and those estimated with an integrated assessment model GAINS (Greenhouse Gas and Air Pollution Interactions and Synergies) shows that our method yields clearly higher particle emissions at sizes below 60 nm, but at sizes above that the two methods agree well. Overall, our method is proven to be a useful tool for gaining new knowledge of the size distributions of particle number emissions in urban environments and for validating emission inventories and models. In the future, the method will be developed by modeling the transport of particles from different sources to obtain more accurate estimates of particle number emissions.

Compaction characteristics of binary coal mixtures during binder less briquetting process

ABSTRACT Dried coal typically had certain Particle Size Distribution (PSD) resulting from the size reduction process. Effect of PSD to the compaction phenomenon was investigated on the binary mixtures of Indonesian middle-rank coal with different mass fraction of small and large particles (%L). Compacted density of each coal mixtures was measured during uniaxial briquetting process and their compaction characteristics were analyzed with Cooper-Eaton model. It was found that Cooper-Eaton model could fit excellently (R2 > 0.99) with the experimental data. Analysis of compaction characteristics of the model (Ai and ki ) and briquette density revealed that mixtures 0%L (PSD Small) and 100%L (PSD Large) were easier to compact compared to binary mixtures. Binary mixture 70%L was hardest to compact, evaluating from its lowest increase of density, from 893 to 1492 kg/m3. Relatively high coordination number at pressure 2 kPa on mixture 70%L (7.81 ± 0.20) might be the reason for high deformation difficulty. It was concluded that binary mixture of coal with the highest bulk density would results in briquette with the lowest density.

Formation and evolution of the interfacial structure in al/steel compound castings during solidification and heat treatment

In this work, Al7SiMg/steel compound castings were produced through a low-pressure die casting process. All steel inserts were galvanized, where half of them were flux-coated to further improve the wettability and remove interfacial oxide layers during casting. The reaction layer formed in the Al7SiMg/steel interface was examined using Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). In addition, Vickers Micro-hardness was measured across the interface. Results show that successful metallurgical bonding can be achieved between aluminum and galvanized steel, both with and without additional flux coating. A large fraction of intermetallic particles formed at the reaction layer, where ternary Al4·5FeSi particles were the dominating phase. The influence of T6 heat treatment (solution treatment at 540 °C, followed by artificial ageing) on the interfacial microstructure was also studied. After heat-treatment, the thickness of the interfacial layer increased significantly, due to the growth of β-Al4.5FeSi and Al–Fe binary particles into the bulk of steel. Consequently, cracks formed and propagated through the inner binary intermetallic layer. Formation mechanisms of various intermetallic phases at the interface during solidification and heat treatment have been discussed.

Mechanism of Variation in High-Temperature Grain Stability of Aluminum in Dissimilar Friction Stir Welds

Abstract In the dissimilar Friction Stir Welding (FSW) of aluminum to titanium, a large fraction of titanium particles is inhomogeneously distributed in the weld nugget and their distribution is highly complex. Such a distribution can have an immense influence on the grain stability of the weld nugget, which decides its mechanical properties at the high temperatures experienced in critical applications. The present investigation highlights the variation in grain structure at the top surface and center of the weld nugget. The results show that the microstructure at the surface of the weld contains a higher fraction of fine titanium particles, refined grains of aluminum and high-angle grain boundaries, and a lower intensity of shear texture components when compared to the center of the weld nugget. The variation in the grain stability of the weld was correlated with the qualitative variation in the strain rate and temperature in the weld. It is proposed that the formation and distribution of a high fraction of fine titanium particles results in superior grain stability of aluminum at the surface of the weld due to arrest of the grain boundary mobility against grain growth. This mechanism and methodology can be applied in developing metal matrix composites with superior mechanical properties as well.