Particle Number Density Distribution Research Articles

Analysis of application range of simplified models for field to thermo-field to thermionic emission processes from the cathode

Electron emission plays a dominant role in plasma–cathode interactions and is a key factor in many plasma phenomena and industrial applications. It is necessary to illustrate the various electron emission mechanisms and the corresponding applicable description models to evaluate their impacts on discharge properties. In this study, detailed expressions of the simplified formulas valid for field emission to thermo-field emission to thermionic emission typically used in the numerical simulation are proposed, and the corresponding application ranges are determined in the framework of the Murphy–Good theory, which is commonly regarded as the general model and to be accurate in the full range of conditions of the validity of the theory. Dimensionless parameterization was used to evaluate the emission current density of the Murphy–Good formula, and a deviation factor was defined to obtain the application ranges for different work functions (2.5‒5 eV), cathode temperatures (300‒6000 K), and emitted electric fields (105 to 1010 V·m−1). The deviation factor was shown to be a nonmonotonic function of the three parameters. A comparative study of particle number densities in atmospheric gas discharge with a tungsten cathode was performed based on the one-dimensional implicit particle-in-cell (PIC) with the Monte Carlo collision (MCC) method according to the aforementioned application ranges. It was found that small differences in emission current density can lead to variations in the distributions of particle number density due to changes in the collisional environment. This study provides a theoretical basis for selecting emission models for subsequent numerical simulations.

Neural networks for separation of cosmic gamma rays and hadronic cosmic rays in air shower observation with a large area surface detector array

The Tibet ASγ experiment has been observing cosmic gamma rays and cosmic rays in the energy range from teraelectron volts to several tens of petaelectron volts with a surface detector array since 1990. The derivation of cosmic gamma-ray flux is made by finding the excess distribution of the arrival direction of air showers above background cosmic rays. In 2014, the underground water Cherenkov muon detector (MD) was added to separate cosmic gamma rays from the background on the basis of the muon-less feature of the air showers of gamma-ray origin; hybrid observations using these two detectors were started at this time. In the present study, we developed methods to separate gamma-ray-induced air showers and hadronic cosmic-ray-induced ones using the measured particle number density distribution to improve the sensitivity of cosmic gamma-ray measurements using the Tibet air shower array data alone before the installation of the MD. We tested two approaches based on neural networks. The first method used feature values representing the lateral spread of the secondary particles, and the second method used the shower image data. To compare the separation performance of each method, we analyzed Monte Carlo air shower events in the vertically incident direction with mono-initial-energy gamma rays and protons. When discriminated by a single feature, the feature with the highest separation performance has an area under the curve (AUC) value of 0.701 for a gamma-ray energy of 10 TeV and 0.808 for 100 TeV. A separation method with a multilayer perceptron (MLP) based on multiple features has AUC values of 0.761 for a gamma-ray energy of 10 TeV and 0.854 for 100 TeV, which represents an improvement of approximately 5% in the AUC value compared with the single-feature case. We also found that the feature values that effectively contribute to the separation vary depending on the energy. A separation method with a convolutional neural network (CNN) using the shower image data has AUC values of 0.781 for a gamma-ray energy of 10 TeV and 0.901 for 100 TeV, which are approximately 5% higher than those of the MLP method. We applied the CNN separation method to Monte Carlo gamma-ray and cosmic-ray events from the Crab Nebula in the energy range 10–100 TeV. The AUC values range from 0.753 to 0.879, and the significance of the observed gamma-ray excess is improved by 1.3 to 1.8 times compared with the case without the separation procedure.

Investigation of the influence mechanism of hypersonic flow field environment on thermionic emission efficiency

Electron Transpiration Cooling (ETC) is a novel thermal management method that utilizes thermionic emission to cool the leading edge surface of a hypersonic vehicle. Establishing a thermionic emission model that accounts for the hypersonic flow field environment is imperative for accurately predicting the thermal protection efficiency of ETC. A thermionic emission dual-sheath model was established, considering the impact of hypersonic non-equilibrium flow field particle parameters on thermal electron emission. The virtual cathode threshold was obtained through numerical analysis, considering the influence of particles parameters and surface thermoelectric material parameters in a non-equilibrium flow field. Distribution of sheath potential and particle number density in plasma sheath region were performed at a range of conditions. Thermionic emission efficiency approached 80 % as the plasma ion density was 5 × 20 m−3 and the work function was 2 eV. The results showed that plasma ions have different degrees of neutralization effect on thermal emission electrons depending on the flow field and surface material characteristics, ultimately causing varying surface thermionic emission efficiency.

Spatially resolved measurement of the distribution of solid and liquid Si nanoparticles in plasma synthesis through line-of-sight extinction spectroscopy.

In many high-temperature gas-phase nanoparticle synthesis processes, freshly nucleated particles are liquid and solidify during growth and cooling. This study presents an approach to determine the location of the liquid-to-solid phase transition and the volume fraction and number density of particles of both phases within a gas phase reactor. Spectrally-resolved line-of-sight attenuation (LOSA) measurements are applied to a silicon nanoparticle aerosol generated from monosilane in a microwave plasma reactor. A phantom-based analysis using particle number density, particle size, and temperature distribution from direct numerical simulation (DNS) of the reacting flow indicates that the contributions from the two particle phases can be decoupled under practical conditions, even with noisy data. The approach was applied to analyze spatially and spectrally resolved LOSA measurements from the hot gas flow downstream of the plasma zone where both solid and liquid silicon particles coexist. Extinction spectra were recorded along a line perpendicular to the flow direction by a spectrometer with an electron-multiplying charge-coupled device (EMCCD) camera, and two-dimensional projections were deconvolved to obtain radial extinction coefficient distributions of solid and liquid particles across the cross-section of the flow. Particle number densities of both particle phases were retrieved simultaneously based on the size-dependent extinction cross-sections of the nanoparticles. The particle-size distribution was determined via thermophoretic sampling at the same location with subsequent transmission electron microscopy (TEM) analysis. The particle temperature distribution was determined from the particle's thermal radiation based on line-of-sight emission (LOSE) measurements. The approach for phase-selective data analysis can be transferred to other materials aerosol systems as long as significant differences exist in extinction spectra for the related different particle classes.

Heterogeneous condensation in the free jet of the coaxial nozzle

<p indent="0mm">Contrails frequently appear in the activities of air vehicles. The Euler-Euler approach based on the quadrature method of moments with the <italic>k</italic>-<italic>ω</italic> SST turbulence model is used to simulate the free jet of the coaxial nozzle with heterogeneous condensation to explain the shape, number density, and size distribution of contrails. The investigation analyzes the formation mechanism of the contrail and the flow characteristics of the free jet and obtains the parameters of the contrail. The results reveal that considering the turbulence effect on the particle phase, the diffusion of the particles and the gaseous species has a similar performance in the main section of the jet. With heterogeneous condensation, which consumes a component of the vapor, the particle sizes sharply increase, causing the vapor mass fraction of the jet to considerably decrease and the particle number density and volume fraction distributions to become dislocated. The absolute content of vapor is too small; therefore, the condensation heat release only increases the jet temperature by <sc>0.5 K,</sc> whose effect on the jet development can be ignored. The formed ice crystals almost do not disappear inside the computational domain because the increase rate of the particle sizes is far greater than the reduction rate in the computational domain. The particle size distribution develops from dispersion to concentration, finally presenting an approximately normal distribution, whose particle sizes are mainly concentrated in <sc>0.8–2.0 μm,</sc> accounting for approximately 83% of the particle total in the contrail, as the contrail shape develops in the computational domain from a ring shape to a pear shape, finally presenting a cone shape.

Nucleation and Growth of Iron (II) Oxide Nanoparticles in Thermal Arc Plasma and Their Interaction Study With SARS-CoV-2: A Computational Approach

A computational model for nucleation and growth of iron (II) oxide nanoparticle (IONP) in thermal plasma has been developed. A nondimensional form of the aerosol general dynamic equations (GDEs) along with a discrete volume sectional model assumption is used to numerically solve the coupled system of GDEs. The variation in supersaturation ratio and the mean particle diameter of IONPs with respect to temperature across the plasma reactor has been presented. The scatter plot showing the distribution of particle number density of certain size across the reactor chamber is shown. In silico molecular docking study was performed to reveal the putative interaction of the IONPs with severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) virus. The results revealed significant binding affinity of IONPs with 6LZG (spike receptor-binding domain complexed with its receptor ACE2) and 5RH4 (main protease) of SARS-COV-2 by forming hydrogen and hydrophobic bonds with nearby amino acid residues. The interactions of IONPs are associated with the conformational changes in the protein which could be used to treat and control SARS-CoV-2 infection.

Scale-dependent statistics of inertial particle distribution in high Reynolds number turbulence

Multiscale statistical analyses of inertial particle distributions are presented to investigate the statistical signature of clustering and void regions in particle-laden incompressible isotropic turbulence. Three-dimensional direct numerical simulations of homogeneous isotropic turbulence at high Reynolds number ($Re_\lambda \gtrsim 200$) with up to $10^9$ inertial particles are performed for Stokes numbers ranging from $0.05$ to $5.0$. Orthogonal wavelet analysis is then applied to the computed particle number density fields. Scale-dependent skewness and flatness values of the particle number density distributions are calculated and the influence of Reynolds number $Re_\lambda$ and Stokes number $St$ is assessed. For $St \sim 1.0$, both the scale-dependent skewness and flatness values become larger as the scale decreases, suggesting intermittent clustering at small scales. For $St \le 0.2$, the flatness at intermediate scales, i.e. for scales larger than the Kolmogorov scale and smaller than the integral scale of the flow, increases as $St$ increases, and the skewness exhibits negative values at the intermediate scales. The negative values of the skewness are attributed to void regions. These results indicate that void regions at the intermediate sales are pronounced and intermittently distributed for such small Stokes numbers. As $Re_\lambda$ increases, the flatness increases slightly. For $Re_\lambda \ge 328$, the skewness shows negative values at large scales, suggesting that void regions are pronounced at large scales, while clusters are pronounced at small scales.

Insights from a new method providing single-shot, planar measurement of gas-phase temperature in particle-laden flows under high-flux radiation

Two-colour laser-induced fluorescence (LIF) of toluene has been demonstrated to provide in situ, spatially resolved, planar measurements of the gas-phase temperature in a particle-laden flow with strong radiative heating at fluxes up to 42.8 MW/m2. Toluene was seeded in trace quantities into the gas flow laden with particles of mean diameter 173 μm at a volumetric loading sufficiently high for particle–fluid and particle–particle interactions to be significant. The particle number density was also measured simultaneously using Mie scattering. The two-colour LIF method was found to resolve temperature with a pixel-to-pixel standard deviation of 17.8 °C for unheated measurements in this system despite significant attenuation of the probe laser and signal trapping of the fluorescence emissions from the densely loaded particles. Following heating of the particles using high flux radiation, the increase in the gas-phase temperature from convection was found to be spatially non-uniform with highly localised regions of temperature spanning from ambient to 150 °C. This gas-phase heating continued well downstream from the limits of the region with radiative heating, with the time-averaged gas temperature increasing with distance at up to 2,200 °C/m on the jet centreline. The temperature of the flow was non-symmetrical in the direction of the heating beam, because the particles attenuate the radiation through absorption and scattering. The addition of radiation at fluxes up to 42.8 MW/m2 did not significantly change the particle number density distribution within the region investigated here.Graphic abstract

P0736 / #1895: EVALUATION OF SPLASHGUARDCG PREVENTION OF AEROSOLISED PARTICLE DISPERSION IN A PAEDIATRIC MODEL OF SPONTANEOUS AND NON-INVASIVE VENTILATION

AIMS & OBJECTIVES: The Splashguard (SG) is a plexiglass enclosure placed around the patient's head to limit the risk of transmission of COVID-19 during care. The SG was designed to prevent droplet transmission, but can concentrate aerosolized particles inside the SG. The objective was to evaluate the effectiveness of adding suction to prevent the risk of dispersal of aerosolized particles (AP) from critically ill patients on spontaneous ventilation (SV) and non-invasive ventilation (NIV). METHODS: The Topas Aerosol Generator (ATM-220) was connected to the airways of a high-fidelity manikin (SimBabyTM) with a breathing simulator (ASL 5000 tm), to generate a stable concentration of di-ethyl-hexyl-sebacat aerosol particles for 40 s within the SG. The NIV was applied using a Servo-I ventilator with a full-face mask (Figure). The temporal variation of the particle number density and size distribution was measured using a laser aerosol spectrometer (GRIMM 11-R) over the 0.5 μm to 1.0 mm range inside the SG, just above the manikin's head, with and without suction (92 L/min) inside the enclosure. RESULTS: While the aerosol generator is supplying aerosol to the Simbaby airways, the particle concentration increases steadily in the enclosure, with a larger peak in SV than in NIV. The PA peak was greater in SV than in NIV (1.4x106 vs 6.0x105, respectively). Aspiration decreased both the PA peak in SV and NIV, and the time to achieve a 50% reduction in AP concentration. CONCLUSIONS: The application of suction reduced the aerosol concentration inside the SG and may reduce the risk of airborne transmission when handling the SG.

Faucet aerator design influences aerosol size distribution and microbial contamination level

Faucet aerators have been linked to multiple opportunistic pathogen outbreaks in hospital, especially Pseudomonas aeruginosa, their complex structure promoting biofilm development. The importance of bacteria aerosolization by faucet aerators and their incidence on the risk of infection remain to be established. In this study, ten different types of aerators varying in complexity, flow rates and type of flow were evaluated in a controlled experimental setup to determine the production of aerosols and the level of contamination. The aerosol particle number density and size distribution were assessed using a particle spectrometer. The bacterial load was quantified with a 14-stage cascade impactor, where aerosol particles were captured and separated by size, then analysed by culture and flow cytometry. The water was seeded with Pseudomonas fluorescens as a bacterial indicator. Aerosol particle size and mean mass distribution varied depending on the aerator model. Devices without aeration or with laminar flow produced the lowest number and mass of aerosol particles when measured with spectrometry. Models with aeration displayed wide differences in their potential production of aerosol particles. A new aerator with a low flow, no air inlet in its structure, and a spray stream produced 12 to 395 times fewer aerosol particles containing bacteria. However, the impact of low flow on biofilm development and incorporation of pathogens should be further investigated. Repeated use of aerators resulted in fouling which increased the quantity of bacteria released through aerosol particles. An in-depth mechanical cleaning including complete dismantling of the aerator was required to recover initial performances. Aerators should be selected to minimize aerosol production, considering the ease of maintenance and the main water usage at each sink. Low flow aerators produced a lower number of contaminated aerosol particles when new but may be more susceptible to fouling and quickly lose their initial advantage.

Stability of a vertical Couette flow in the presence of settling particles

We consider the linear stability of a particle-laden vertical Couette flow between two flat plates. The flow is described in the framework of a two-fluid approach with a negligibly small volume fraction of the dispersed phase (the “dusty-gas model”). The carrier-fluid velocity profile in the main flow is modified by the presence of settling particles distributed non-uniformly between the plates. Two types of particle number density distributions are considered, namely, a single layer with a maximum at the flow center line and two symmetric layers located between the center line and the walls. Linearized governing equations are reduced to a modified Orr–Sommerfeld equation for the amplitude of a disturbance in the form of a normal mode. On the basis of a parametric study of the eigenvalues, it is shown that, in contrast to the case of two-phase Couette flow with zero particle velocity slip, the vertical disperse flow is unstable over a wide range of governing parameters, including small Reynolds numbers. In flows with both kinds of particle concentration profiles considered, there are two types of growing modes. A gravitational mode is triggered at low Reynolds numbers and typically small wavenumbers, while a shear mode with the wavelength of the order of the channel width and larger time-amplification rates is triggered at large Reynolds numbers. It is found that both modes are amplified with an increase in the particle mass loading or the ratio of the Reynolds number to squared Froude number, determining the carrier-fluid velocity profile. The results of the study can be used to control the stability of various technological processes involving particle-laden flows.

Numerical simulation of plasma and electromagnetic characteristics for an ion thruster discharge chamber

In this paper, a two-dimensional axisymmetric particle-in-cell plus Monte Carlo collision numerical model is developed to simulate the plasma and electromagnetic characteristics inside the discharge chamber of an ion thruster. This model tracks three major particle types: primary electrons, secondary electrons, and xenon ions. The neutral particles are treated as a background gas, and the collision mechanism between electrons and neutral gas is modeled in detail. Besides the external electric field and magnetic field, the time-varying electromagnetic fields caused by the movement of the charged particles are considered. Compared to the electrostatic model, the electromagnetic model can further study the electromagnetic radiation characteristics of the discharge chamber. The numerical results are presented and discussed, including the particle number density distributions, the particle energy distributions, and the time-varying electromagnetic fields. At last, the influence of the electromagnetic effect on ion dynamics is analyzed.

Experimental determination of distributions of soot particle diameter and number density by emission and scattering techniques**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0601900), the National Natural Science Foundation of China (Grant Nos. 51827808, 51821004, and 51406095), the Fundamental Research Funds for the Central Universities, China (Grant Nos. 2018ZD03 and 2017ZZD005), and Science and

A diagnostics method was presented that uses emission and scattering techniques to simultaneously determine the distributions of soot particle diameter and number density in hydrocarbon flames. Two manta G-504C cameras were utilized for the scattering measurement, with consideration of the attenuation effect in the flames according to corresponding absorption coefficients. Distributions of soot particle diameter and number density were simultaneously determined using the measured scattering coefficients and absorption coefficients under multiple wavelengths already measured with a SOC701V hyper-spectral imaging device, according to the Mie scattering theory. A flame was produced using an axisymmetric laminar diffusion flame burner with 194 mL/min ethylene and 284 L/min air, and distributions of particle diameter and number density for the flame were presented. Consequently, the distributions of soot volume fraction were calculated using these two parameters as well, which were in good agreement with the results calculated according to the Rayleigh approximation, demonstrating that the proposed diagnostic method is capable of simultaneous determination of the distributions of soot particle diameter and number density.

Effect of settling particles on the stability of a particle-laden flow in a vertical plane channel

The stability of a viscous particle-laden flow in a vertical plane channel in the presence of the gravity force is studied. The flow is described using a two-fluid “dusty-gas” model with negligibly small volume fraction of fines and two-way coupling of the phases. Two different profiles of the particle number density in the main flow are considered: homogeneous and non-homogeneous in the form of two layers symmetric about the channel axis. The novel element of the linear-stability problem formulation is a particle velocity slip in the main flow caused by the gravity-induced settling of the dispersed phase. The eigenvalue problem for a linearized system of governing equations is solved using the orthonormalization and QZ algorithms. For a uniform particle number density distribution, it is found that there exists a domain in the plane of Froude and Stokes numbers, in which the two-phase flow in a vertical channel is stable for an arbitrary Reynolds number. This stability domain corresponds to relatively sm...

Simultaneous particle size and 3D position measurements of pulverized coal flame with digital inline holography

3D measurements of burning coal particles in the flame are challenging and of great interest. Digital inline holography is applied to perform 3D measurement of coal particle size and 3D position in a laboratory-scale pulverized coal flame. Pulsed laser beam illumination is used to record the coal particle hologram with clear holographic fringes in the presence of high refractive index gradient and strong turbulence. Over 30,000 coal particles have been reconstructed and analyzed. Results show that the particle size, 3D positions and particle number density distribution can be determined simultaneously. Digital inline holography presents to be a powerful tool in the 3D measurement of burning particles in a turbulent and reacting medium.

Development of a soot particle model with PAHs as precursors through simulations and experiments

In the present study, the polycyclic aromatic hydrocarbons (PAHs) and soot particulates emitted from a gasoline direct injection (GDI) engine were sampled and analyzed. The results show that the vapor-phase PAHs and particulate-bound PAHs exist in GDI engine exhaust emissions. Soot particles are formed by the agglomeration of the quasi-spherical primary carbon particles, and the size of the PAH cluster is close to that of the core of the primary carbon particles. To predict soot particulates evolution in the engine cylinder, a reduced toluene reference fuel (TRF)-PAH chemical mechanism consisting of iso-octane, n-heptane and toluene as gasoline surrogate fuels for the GDI engine combustion simulation was developed. The reduced mechanism contains 232 reactions and 85 species, including 17 species and 40 elementary reactions, related to the PAH formation, which could well capture the oxidation characteristics of ignition delays and laminar flame speeds, as well as PAH emissions from the GDI engine. Then, a mathematical soot growth model coupled with the reduced TRF-PAH mechanism was developed based on the method of moment. The obtained experimental soot emissions were well predicted by this soot model. The distribution of particle number density was consistent with the distribution of A4 and C2H2, and the growth and distribution of soot mass was determined by the particle surface growth due to C2H2, the nuclear reaction of A4 and the particle surface oxidation reaction.

Studying the influence of export pressure on plasma's arc length and equivalent particle number density by moiré and emission tomography

The influence of export pressure on plasma's arc length and equivalent particle number density is investigated by integrating moiré and emission tomography. Three argon arc plasmas, which are generated in the same experimental condition but with different export pressures, are chosen as practical examples for experiment. Based on the projection information obtained by emission tomography, the arc length of the three argon arc plasmas are obtained and compared. At the same time, the equivalent particle number density distributions of the three argon arc plasmas are reconstructed from the experimentally measured refractive index distribution by moiré tomography. The experimental results manifest that (1) the arc length increases with the increase of export pressure, while (2) the equivalent particle number density of the chosen cross section decreases with the increase of export pressure. In a word, this study will provide some reference for studying the optical property of plasma and better applying plasma in various scientific and technical fields.

Effects of fermionic dark matter on properties of neutron stars

By assuming that only gravitation exists between dark matter (DM) and normal matter (NM), we study the effects of fermionic DM on the properties of neutron stars using the two-fluid Tolman-Oppenheimer-Volkoff formalism. It is found that the mass-radius relationship of the DM admixed neutron stars (DANSs) depends sensitively on the mass of DM candidates, the amount of DM, and interactions among DM candidates. The existence of DM in DANSs results in a spread of mass-radius relationships that cannot be interpreted with a unique equilibrium sequence. In some cases, the DM distribution can surpass the NM distribution to form DM halo. In particular, it is favorable to form an explicit DM halo, provided the repulsion of DM exists. It is interesting to find that the difference in particle number density distributions in DANSs and consequently in star radii caused by various density dependencies of nuclear symmetry energy tends to disappear as long as the repulsion of accumulated DM is sufficient. These phenomena indicate that the admixture of DM in neutron stars can significantly affect the astrophysical extraction of nuclear equation of state by virtue of neutron star measurements. In addition, the effect of the DM admixture on the star maximum mass is also investigated.

Modeling of aggregation kernel using Monte Carlo simulations of spray fluidized bed agglomeration

The present work attempts to consider the microscopic mechanisms of spray fluidized bed agglomeration while modeling the macroscopic kinetics of the process. A microscale approach, constant volume Monte‐Carlo simulation, is used to analyze the effects of micro‐processes on the aggregation behavior and identify the influencing parameters. The identified variables, namely the number of wet particles, the total number of particles, and the number of droplets are modeled and combined in the form of an aggregation kernel. The proposed kernel is then used in a one‐dimensional population balance equation for predicting the particle number density distribution. The only fitting parameters remaining in the population balance system are the collision frequency per particle and a success fraction accounting for the dissipation of kinetic energy. Predictions of the population balance model are compared with the results of Monte‐Carlo simulations for a variation of significant operating parameters and found to be in good agreement. © 2014 American Institute of Chemical Engineers AIChE J, 60: 855–868, 2014

Stability of a weakly interacting Fermi gas in a strong magnetic field

Using the quantum statistical theory, the stability of a weakly interacting Fermi gas trapped in a strong magnetic field is investigated, the expression of stability condition is given, and the mechanisms of influences of interparticle interaction and magnetic field on the stability are analyzed. The results show that repulsive interaction will increase both the chemical potential and the space domain needed for system stability. But, the increasing of magnetic field strength will reduce the space domain needed for the system stability, and lower the stability of the particle number density distribution.