Velocity Of Individual Particles Research Articles

Influence of the inner roughness of powder channels on the powder propagation behavior in laser metal deposition

The powder propagation behavior of powder nozzles for the laser metal deposition process has a significant influence on powder utilization rate and track geometry. A well-focused powder stream will lead to a higher process efficiency and lower material loss. Powder channels with different roughness and a constant diameter of 1.5 mm were placed by wire electrical discharge machining into a copper alloy printed by powder bed fusion. Nickel base powder with a size of −106 to +45 μm was delivered through the powder channels with varied carrier gas flow rates and varied powder mass flow rates. High-speed imaging was used to analyze the powder flow. From these recordings, the dispersion angle of the powder stream from single channels could be measured as well as the velocity of particles. Moreover, the relationship between individual particle velocity and individual particle flight angle was investigated. It was found that the inner roughness of powder channels has a major impact on powder propagation behavior. It could be shown that with a decrease in Ra from 2.16 to 0.27 μm the divergence angle decreased by around 61% while the particle velocity was increased by at least 28% for all varied parameters. Particles with a high velocity tend to have a lower particle flight angle.

Settling Velocities of Small Microplastic Fragments and Fibers.

There is only sparse empirical data on the settling velocity of small, nonbuoyant microplastics thus far, although it is an important parameter governing their vertical transport within aquatic environments. This study reports the settling velocities of 4031 exemplary microplastic particles. Focusing on the environmentally most prevalent particle shapes, irregular microplastic fragments of four different polymer types (9-289 μm) and five discrete length fractions (50-600 μm) of common nylon and polyester fibers are investigated, respectively. All settling experiments are carried out in quiescent water by using a specialized optical imaging setup. The method has been previously validated in order to minimize disruptive factors, e.g., thermal convection or particle interactions, and thus enable the precise measurements of the velocities of individual microplastic particles (0.003-9.094 mm/s). Based on the obtained data, ten existing models for predicting a particle's terminal settling velocity are assessed. It is concluded that models, which were specifically deduced from empirical data on larger microplastics, fail to provide accurate predictions for small microplastics. Instead, a different approach is highlighted as a viable option for computing settling velocities across the microplastics continuum in terms of size, density, and shape.

Time-resolved characteristics of oscillatory particle-laden air flow in a realistic human airway model

Human airways represent a complex flow system with a spatially and temporally variable character of air flow during respiration. In this paper, we experimentally studied the oscillatory flow of air with monodispersed micron-sized liquid particles in a transparent, anatomically realistic model of human upper airways and several bronchi generations using phase-Doppler anemometry (PDA). The PDA provided point-wise high-frequency measurements of axial velocities of individual aerosol particles in multiple positions of the airways (in the trachea and the upper bronchi) for three breathing regimes with a sinusoidal course. Typical time-resolved velocity plots at several positions within the model were documented and analysed using dimensionless criteria. Local mean air velocity and turbulence time-lines disclosed specific flow dynamic features in the multiple bifurcation system, namely the transit of vortical structures, oscillations induced by flow reversals, and inspiratory flow separations behind bifurcations. The results elucidated the laminar, transitional and turbulent flows during inspiratory and expiratory breathing phases. The character of the flow varies significantly with position in the airways, while the breathing regime has a generally low effect on the flow character. Inspection of the flow in the terminal branches indicated the need to add further branches for more realistic results there.

Transport and deposition behaviors of graphite aerosol in the steam generator building during an overpressure discharge accident of HTR-PM

The water-ingress accident is one of design basis accidents of high-temperature gas-cooled reactors (HTGR). The rapid change of the flow field and the direct brush of the high-velocity gas flow may cause a large fraction of graphite dust to resuspend, which is further discharged into the steam generator building when the overpressure relief protection is triggered. Due to the coupling with the radioactive fission products (FPs), the behaviors of the discharged particles are crucial for the reactor safety analysis and source-term evaluation in the overpressure discharge scenario. In this work, we numerically investigate the transport, deposition and releasing behaviors of graphite aerosol in the full-scale steam generator building of HTR-PM by the Eulerian-Lagrangian method. The mixture-ideal-gas law is used for describing the mixture between helium and air, and the CFD-DPM scheme is employed for tracking the trajectory and velocity of individual particles. Particularly, the effects of the irregular shape on the particle-fluid and particle-wall interactions are included. The simulation results show the particles are quasi-uniformly distributed in the steam generator building and the deposition and releasing rates with the time follow the exponential relationship at the late-stage. The distribution of all the deposited, escaped and airborne particles are analyzed. Due to the strong thermophoretic effect, a large fraction of particles are deposited on the cold walls. The extrapolation by the exponential relation indicates a final releasing fraction of 18.94%. Compared to the current conservative assumption that all the dust particles in the steam generator building will eventually enter the environments, our results demonstrate that the steam generator building of HTR-PM still has certain capability to retain the graphite dust during an overpressure discharge accident scenario.

Experimental investigation of face mask filtration in the 15–150 μm range for stationary flows

The effectiveness of face masks for preventing airborne transmission has been debated heavily during the COVID-19 pandemic. This paper investigates the filtration efficiency for four different face mask materials, two professional and two homemade, for different airflow conditions using model experiments and artificially generated water droplets. The size range chosen represents particles with the largest volume that can be suspended in air. The particles are detected using double pulsed interferometric particle imaging, from which it is possible to estimate the positions, velocity, and size of individual particles. It is found that all the tested face masks are efficient in preventing particles from transmission through the mask material. In the presence of leakage, particles larger than approximately 100μm are completely removed from the air stream. The filtration efficiency decreases with the decreasing particle size to approximately 80% for 15μm particles. The size dependency in the leakage is mainly due to the momentum of the larger particles. The results show that even simple face mask materials with leakage prevent a large portion of the emitted particles in the 15–150 μm range.

Modelling the particle trajectory and melting behaviour of non-spherical ice crystal particles

Existing ice crystal icing codes commonly neglect non-spherical particle rotation behaviour. This leads to uncertainties in modelling ice crystal icing as ice crystals are typically non-spherical. This paper develops a two-dimensional framework to model the particle trajectory and melting behaviour of rotating non-spherical ice crystal particles. Non-spherical particle translational and rotational motions are resolved using a framework of unit complex numbers. The effect of rotation on particle heat and mass transfer is implemented using a sub-model dependent on both rotational and translational particle Reynolds numbers. A test case of flow through a swan neck duct is presented. The model is validated through comparison to the discrete phase model in ANSYS Fluent and experimental water droplet impingement test data. Comparison of the developed framework with other simplified particle tracking methods (without modelling the non-spherical particle rotation) is first conducted. These results show large differences in the trajectory, velocity and melt ratio of individual particles and in overall particle impingement behaviours, especially for high aspect ratio particles. Particle rotation can indirectly affect single particle’s melting behaviour through its effect on particle trajectory and velocity. Both aspect ratio and porosity are seen to enhance particle melting behaviour and affect particle impingement behaviour. The numerical uncertainties in the DNS-based correlations of the particle forces and torques employed are also discussed, as well as the performance of the developed framework for the case of a flow past a test article.

Identification of collective particle motion in a rotating drum using a graph community detection algorithm

The discrete element method (DEM) is the method of choice in many cases of simulation and analysis of processes in granular matter, providing the data at the level of individual particles. In most applications and experiments, however, the bulk behavior is of interest; therefore, larger scale structures must be identified from the DEM simulation data. We present the method for detection of particle groups involved in collective motion based on network analysis. Knowing the positions and velocities of individual particles, a “velocity similarity graph” is built, where the graph vertices represent the particles. The vertex pairs are connected by the edge if the distance between the respective particles is small enough. The edge weight is calculated to be inversely proportional to the difference in the respective particle velocities, that is, the vertex pairs representing nearby particles having similar velocities are connected by edges of larger weight. If a group of particles moves in a coordinated matter, the particles in this group will have similar velocities; therefore, the corresponding vertices in the graph will be connected by edges of larger weight in the representing graph. Having produced the velocity similarity graph, identification of particle groups becomes equivalent to the problem of “community detection” in graph analysis. The algorithms and techniques developed for community detection in graphs can be thereby applied for identification of particle groups involved in coordinated motion in granular matter. We illustrate this approach by an example of granular media filled in a rotating cylinder.

A particle-tracking image pyrometer for characterizing ignition of pulverized coal particles

In this study, a particle-tracking image pyrometer (PTIP) system, based on a high-resolution high-speed camera and a digital single lens reflex (SLR) camera, was proposed to investigate the ignition characteristics of pulverized coal particles. The goal of the PTIP system was to obtain reliable simultaneous measurements of the velocity, diameter, and temperature of individual burning particles. A modified particle-tracking velocimetry (PTV) algorithm was developed to collect the image sequences of individual particles, and it could weed out defocused particles and other odd particles by low-frequency filtration. Moreover, a template matching method was adopted to distinguish particles undergoing different combustion states (homogeneous volatile combustion or heterogeneous coal/char combustion). The experimental validation was carried out under an O2/CO2 atmosphere in an optically accessible flat flame-supported entrained flow reactor. Particles of Shenhua (SH) bituminous coal, in the size range of 70–133 μm, were combusted in the flue gas with gas temperature of 1400 K and O2 concentration of 20%. The diameter-temperature patterns of particles along their trajectories were discussed. Several criteria based on statistical analysis were adopted to divide the combustion process and obtain the ignition delay time (IDT) and devolatilization duration time (DDT).

Kuramoto model with run-and-tumble dynamics

This work considers an extension of the Kuramoto model with run-and-tumble dynamics-a type of self-propelled motion. The difference between the extended and the original model is that in the extended version angular velocity of individual particles is no longer fixed but can change sporadically with a new velocity drawn from a distribution g(ω). Because the Kuramoto model undergoes phase transition, it offers a simple case study for investigating phase transition for a system with self-propelled particles.

Numerical Analysis on Transport of Particle Assemblage in Fibrous Media

We have simulated the motion of particle assemblage passing through fibrous media. On the assumption that the particle size is of the order of micrometers, the velocity of individual particles was calculated by use of the Stokesian dynamics that describes the Stokes flow in the presence of multiple particles. The results show that a part of particles is captured by fibers while others avoid them and permeate the media. From the velocity, traveling distance and direction of the particles which are not captured, the diffusion caused by the hydrodynamic interaction was observed. The results indicated that the motion of each particle greatly differs depending on the particle size particularly for the large fiber volume fraction, which would be closely related to the hydrodynamic diffusive behavior.

Can terminal settling velocity and drag of natural particles in water ever be predicted accurately?

Abstract. Natural particles are frequently applied in drinking water treatment processes in fixed bed reactors, fluidised bed reactors, and sedimentation processes to clarify water and to concentrate solids. When particles settle, it has been found that, in terms of hydraulics, natural particles behave differently when compared to perfectly round spheres. To estimate the terminal settling velocity of single solid particles in a liquid system, a comprehensive collection of equations is available. For perfectly round spheres, the settling velocity can be calculated quite accurately. However, for naturally polydisperse non-spherical particles, experimentally measured settling velocities of individual particles show considerable spread from the calculated average values. This work aims to analyse and explain the different causes of this spread. To this end, terminal settling experiments were conducted in a quiescent fluid with particles varying in density, size, and shape. For the settling experiments, opaque and transparent spherical polydisperse and monodisperse glass beads were selected. In this study, we also examined drinking-water-related particles, like calcite pellets and crushed calcite seeding material grains, which are both applied in drinking water softening. Polydisperse calcite pellets were sieved and separated to acquire more uniformly dispersed samples. In addition, a wide variety of grains with different densities, sizes, and shapes were investigated for their terminal settling velocity and behaviour. The derived drag coefficient was compared with well-known models such as the one of Brown and Lawler (2003). A sensitivity analysis showed that the spread is caused, to a lesser extent, by variations in fluid properties, measurement errors, and wall effects. Natural variations in specific particle density, path trajectory instabilities, and distinctive multi-particle settling behaviour caused a slightly larger degree of the spread. In contrast, a greater spread is caused by variations in particle size, shape, and orientation. In terms of robust process designs and adequate process optimisation for fluidisation and sedimentation of natural granules, it is therefore crucial to take into consideration the influence of the natural variations in the settling velocity when using predictive models of round spheres.

Simulation of Velocity Evolution of a Cold Collision-less Non-Magnetised Plasma by Particle-in-Cell Method

This work presents simple numerical simulation algorithm to analyse the velocity evolution of high density non-magnetized glow discharge (cold) collision-less plasma using Particle-in-Cell (PIC) method. In the place of millions of physical electrons and background ions, fewer particles called super particles are used for simulation to capture the plasma properties such as particle velocity, particle energy and electrical field of the plasma system. The plasma system which is of interest in this work is weakly coupled plasma having quasi-neutrality nature. Simulation results showed symmetric velocity distribution about zero with slight left skewness, indicating static system. The order of directional velocity of individual particle seems to agree with the input electron temperature of the considered plasma system. The particle and field energy evolution were observed having fluctuations about zero which indicates that the system is equilibrating. This work marks the preliminary work to study the transport of plasma species in plasma column of gliding arc discharge.

67P/Churyumov–Gerasimenko’s dust activity from pre- to post-perihelion as detected by Rosetta/GIADA

ABSTRACT We characterized the 67P/Churyumov–Gerasimenko’s dust activity, by analysing individual dust particle velocity and momentum measurements of Grain Impact Analyser and Dust Accumulator (GIADA), the dust detector onboard the ESA/Rosetta spacecraft, collecting dust from tens to hundreds of kilometres from the nucleus. Specifically, we developed a procedure to trace back the motion of dust particles down to the nucleus, identifying the surface’s region ejecting each dust particle. This procedure has been developed and validated for the first part of the mission by Longobardo et al. and was extended to the entire GIADA data set in this work. The results based on this technique allowed us to investigate the link between the dust porosity (fluffy/compact) and the morphology of the ejecting surface (rough/smooth). We found that fluffy and compact particles, despite the lack of correlation in their coma spatial distribution (at large nucleocentric distances) induced by their different velocities, have common ejection regions. In particular, the correlation between the distributions of fluffy and compact particles is maintained up to an altitude of about 10 km. Fluffy particles are more abundant in rough terrains. This could be the result of past cometary activity that resurfaced the smooth terrains and/or of the comet formation process that stored the fluffy particles inside the voids between the pebbles. The variation of fluffy particle concentration between rough and smooth terrains agrees with predictions of comet formation models. Finally, no correlation between dust distribution on the nucleus and surface thermal properties was found.

On the influence of rotational motion on MRI velocimetry of granular flows - Theoretical predictions and comparison to experimental data.

Continuum dynamics of granular materials are known to be influenced by rotational, as well as translational, motion. Few experimental techniques exist that are sensitive to rotational motion. Here we demonstrate that MRI is sensitive to the rotation of granules and that we can quantify its effect on the MRI signal. In order to demonstrate the importance of rotational motion, we perform discrete element method (DEM) simulations of spherical particles inside a Couette shear cell. The variance of the velocity distribution was determined from DEM data using two approaches. (1) Direct averaging of the individual particle velocities. (2) Numerical simulation of the pulsed field gradient (PFG) MRI signal acquisition based on the DEM data. Rotational motion is found to be a significant effect, typically contributing up to 50% of the signal attenuation, thus amplifying the calculated velocity variance. A theoretical model was derived to relate an MRI signal to the angular velocity distribution. This model for the signal was compared to previously published experimental data as well as simulated MRI results and found to be consistent.

Use of particle tracking velocimetry in timber material and connection testing

Particle tracking velocimetry (PTV) is a quantitative field measuring technique originally developed to track and measure the velocity of individual particles in fluid flows. In this study, PTV was applied for the first time in three different experimental studies in the field of structural timber engineering: (1) dowel embedment testing in cross laminated timber (CLT), (2) large-scale dowelled CLT connection testing, (3) material testing of small clear wood and laminated veneer lumber (LVL). The suitability of PTV was first assessed by comparing the PTV displacements to those obtained by potentiometers. Furthermore, the behaviour of the timber substrate was analysed with PTV in both embedment and large-scale connection tests. It was found that PTV was able to not only accurately measure dowel displacements, but also capture crack growth in the timber and compute the resulting displacement and strain fields in the dowel embedment area. Lastly, PTV was applied to small clear specimen testing to capture brittle fracture in shear, tension perpendicular to the grain, and in cleavage. This study demonstrated that PTV is able to provide reliable contact-free high-resolution measurements of displacement and strain fields on exposed timber surfaces and capture failure mechanisms such as brittle fractures and crack growth.

Spatial resolution requirements for active radiation detectors used beyond low earth orbit

Measurements of the incident fluence of HZE particles, as a function of LET, are used to determine absorbed dose as well as Quality Factors for assigning risk estimates to astronauts during manned space missions. These data are often based on thin solid state detectors that measure energy deposition, dE, and the assumption that the trajectory of the particle, dx, is equivalent to the thickness of the detector. Heavy ions often fragment while penetrating shielding materials in vehicles or habitats. Projectile fragments can be clustered spatially and temporally at the location of the thin detector which are then misclassified as a single particle. Eliminating the confounding effects of coincident events is the first step in extending the reach of flight instruments to identify the charge and velocity of individual particles. Identification of individual particles, in a fragmentation spectrum, will require that detection systems have sufficient segmentation to eliminate coincident events. The objective of this study was to reduce coincident events while avoiding over-design and complexity.Monte Carlo simulations, using Geant4, were performed for 4He, 12C, 28Si and 56Fe ions at energies of 300, 900 and 2400 MeV/n incident upon aluminum shields having areal densities of 5.4, 13.5, and 54 g/cm2. The identity, energy and spatial distribution of all particles downstream from the shielding were analyzed using a novel approach based on proximity distributions. Results indicated that pixel dimensions on the order of 1 mm were sufficient to reduce errors caused by coincident events for active space radiation detectors.

CFD–DEM simulation and experimental investigation of the flow behavior of lunar regolith JSC-1A

Rovers on Mars and the Moon analyze the local geology by collecting samples of the upper layer in containers and ovens. After the analysis, the complete discharge of samples from the reservoir must be ensured. Because of the low atmospheric pressure, reduced gravity, and different grain shapes of the bulk material, the discharge process is very different compared to that on Earth. In this study, the behavior of lunar regolith JSC-1A in closed containers during discharge was investigated by analyzing the flow in an hourglass under the Earth’s atmosphere. Reproducible fluidization of the top particle layer was observed during the outflow of the upper half of the hourglass. These particles were fluidized by the displacement flow initiated by falling particles in the completely closed container. This complex problem was simulated by coupling computational fluid dynamics (CFD) with the discrete element method (DEM). A CFD–DEM simulation with 1million particles was performed. Because billions of particles are present in the actual system, the use of a coarse graining approach was required. In addition, high-speed camera measurements were used to determine the velocities of individual particles to validate the simulation. The fluidization effect was successfully simulated using the coupled method.

Scale‐dependent nonequilibrium features in a bubbling fluidized bed

We investigate experimentally the nonequilibrium features in a pseudo 2‐D bubbling fluidized bed. Velocities of individual particles are measured by using a particle tracking velocimetry (PTV) method, and void fractions are obtained with the Voronoi tessellation. A bimodal shape of probability density function (PDF) for particle vertical velocity is found in not only time‐averaged but also time‐varying statistics, which is caused by the transition between the dense and dilute phases and breaks the local‐equilibrium assumption in continuum modeling of fluidized beds. The results of time‐varying radial distribution function and voidage distribution also confirm this finding. Moreover, the analysis of voidage, particle velocity, granular temperature and turbulent kinetic energy of particles shows that there is no scale‐independent plateau over the interface, and it seems hard to find a scale‐independent plateau to separate the micro‐ and meso‐scales of fluidized beds, which require sub‐grid meso‐scale modeling for continuum or coarse‐graining methods of gas‐fluidized systems. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2364–2378, 2018

A new data-processing approach to study particle motion using ultrafast X-ray tomography scanner: case study of gravitational mass flow

In most industrial products, granular materials are often required to flow under gravity in various kinds of silo shapes and usually through an outlet in the bottom. There are several interrelated parameters which affect the flow, such as internal friction, bulk and packing density, hopper geometry, and material type. Due to the low-spatial resolution of electrical capacitance tomography or scanning speed limitation of standard X-ray CT systems, it is extremely challenging to measure the flow velocity and possible centrifugal effects of granular materials flow effectively. However, ROFEX (ROssendorf Fast Electron beam X-ray tomography) opens new avenues of granular flow investigation due to its very high temporal resolution. This paper aims to track particle movements and evaluate the local grain velocity during silo discharging process in the case of mass flow. The study has considered the use of the Seramis material, which can also serve as a type of tracer particles after impregnation, due to its porous nature. The presented novel image processing and analysis approach allows satisfyingly measuring individual particle velocities but also tracking their lateral movement and three-dimensional rotations.

Electrical measurement of cross-sectional position of particles flowing through a microchannel

The first high-throughput system for the electrical detection of cross-sectional position and velocity of individual particles flowing through a rectangular microchannel is presented. Lateral position (along channel width) and vertical position (along channel height) are measured using two different sets of coplanar electrodes. In particular, the ratio of travel times measured with electrodes generating a current flow transverse or oblique with respect to particle trajectory yields lateral position. The relative prominence and transit time of a bipolar double-Gaussian signal obtained with a suited electrode configuration, respectively, supply vertical position and velocity. The operating principle is presented by means of finite element numerical simulations. The method is experimentally validated by comparing the electrical estimates of position and velocity of polystyrene beads with optical estimates obtained by processing high-speed images. The system is used to observe bead focusing at different particle Reynolds numbers. This system, providing a fully electrical characterization of single-particle motion, represents a powerful tool, e.g. to understand fluid motion at the microscale, in particle separation studies, or to assess the performance of particle focusing devices. Moreover, it can be simultaneously used to perform single-cell impedance spectroscopy, thus achieving an unprecedented multiparamteric characterization.