Particle Generation Rate Research Articles

Mechanism behind the Controlled Generation of Liquid Metal Nanoparticles by Mechanical Agitation.

The size-controlled synthesis of liquid metal nanoparticles is necessary in a variety of applications. Sonication is a common method for breaking down bulk liquid metals into small particles, yet the influence of critical factors such as liquid metal composition has remained elusive. Our study employs high-speed imaging to unravel the mechanism of liquid metal particle formation during mechanical agitation. Gallium-based liquid metals, with and without secondary metals of bismuth, indium, and tin, are analyzed to observe the effect of cavitation and surface eruption during sonication and particle release. The impact of the secondary metal inclusion is investigated on liquid metals' surface tension, solution turbidity, and size distribution of the generated particles. Our work evidences that there is an inverse relationship between the surface tension and the ability of liquid metals to be broken down by sonication. We show that even for 0.22 at. % of bismuth in gallium, the surface tension is significantly decreased from 558 to 417 mN/m (measured in Milli-Q water), resulting in an enhanced particle generation rate: 3.6 times increase in turbidity and ∼43% reduction in the size of particles for bismuth in gallium liquid alloy compared to liquid gallium for the same sonication duration. The effect of particles' size on the photocatalysis of the annealed particles is also presented to show the applicability of the process in a proof-of-concept demonstration. This work contributes to a broader understanding of the synthesis of nanoparticles, with controlled size and characteristics, via mechanical agitation of liquid metals for diverse applications.

Investigation on the indoor environment during a whole cooking process under constant make-up air organization in a Chinese-style residential kitchen

A large amount of heat and particles generated from cooking have a negative impact on occupants’ thermal comfort as well as the indoor environment in the kitchen and adjacent rooms. Although heat generation rate and emission strength of particles are largely affected by oil temperature and cooking duration, there is a lack of quantification on the impact of the indoor environment during the whole cooking process. This study applies a computational fluid dynamics method to simulate air temperature, velocity and particle distribution in a kitchen and indoor environment during a 10-min and after 2-min cooking process with constant make-up air organization from window opening in a Chinese-style residential kitchen. User-defined function was defined for dynamic oil temperature and emission rate based on experimental results from previous studies and was interpreted in FLUENT for transient simulation within 12 min. Results indicate that heat spread remarkably after 30 s of oil heating. Air temperature experienced an increase of about 10.0°C around the occupant during the whole cooking process. For particle distribution, a large variation in the kitchen was observed after 4 min. Particle concentration in an indoor environment was increased dramatically after 6 min. Further studies on the transmission mechanism of heat and particles from the kitchen space to an adjacent room under different cooking scenarios are needed for the design and control of kitchen ventilation.

Analysis of gas–solid two-phase flow and structure optimization of mobile shot blasting machine recovery device

To address the problem of low efficiency of recycling process waste by gas–solid two-phase flow of the shot blasting machine recycling device, a method and structure by increasing the negative pressure value and optimizing the outlet pipe position are proposed. Computational fluid dynamics (CFD), discrete element method (DEM) and discrete phase model (DPM) were used to study the waste recovery efficiency at different pressure outlet conditions and outlet pipe locations. The validity of the model was verified by velocity tests at the outlet and inlet compared with simulations. The effect of particle size and particle generation rate on solid particle recovery efficiency was further investigated by analyzing the flow field distribution of the recovery unit. The results show that the maximum velocity of the gas phase in the recovery device increases with the increase of the absolute value of the outlet pressure, when the outlet pressure is -6500 Pa, the maximum velocity is 67.59 m/s. When the absolute value of the outlet pressure is greater than 6000 Pa, a small amount of steel shot particles is discharged from the recovery bin under the action of the outlet pressure, resulting in the loss of steel shot particles. After the outlet pipe position optimization, the steel shot particle recovery efficiency increased by 10% and the waste particle recovery efficiency increased by 18.9%.

Numerical Modeling of Hydrate Particle Deposition in Pipes With Diameter Reduction

Summary High-pressure and low-temperature conditions in deepwater gas production and transportation may cause severe hydrate blockage. At present, the research on hydrate migration and deposition primarily focuses on through-diameter pipes, and there are very few related studies on pipes with diameter reduction. In this study, we used the computational fluid dynamics (CFD)-discrete element coupling method. Considering the interaction between hydrate particles, particles and the pipe wall, and particles and fluid, this study establishes a 3D pipe model under different reducing ratios, simulates the migration and deposition law, and reveals the migration and deposition mechanism of hydrate particles at the reduced diameter of pipes. The results revealed that the reduced diameter affected the flow field distribution, led to the emergence of an eddy zone, which strongly hindered migration, and quickly led to the deposition of hydrate particles. Additionally, this study also investigated the effects of gas velocity, reducing ratio, particle diameter, particle generation rate, and flow direction on the deposition efficiency of hydrate particles at the reduced diameters of pipes. Finally, the calculation formula for the eddy zone at the reduced diameter of the pipe was obtained via regression. The equivalent deposition ratio (EDR*) was proposed as a new parameter to characterize the influence of reduced diameter on hydrate particle deposition and provides a new correlation calculation method for it. The results of this study can provide a valuable reference for the efficient control and design of hydrates.

On the breakdown process of capacitively coupled plasma in carbon tetrafluoride

Capacitively coupled plasma (CCP) in CF4 has been widely used in the semiconductor industry. However, its breakdown process at low pressure has rarely been studied. In this paper, we study the whole CCP breakdown process in CF4 using the 1D implicit particle-in-cell/Monte Carlo collision (PIC/MCC) method. The detailed evolution of the plasma parameters is given, and both the particle balance and power evolution are discussed. The electron density initially grows exponentially, driven by the penetrating electric field. Both the ionization in the discharge gap and the boundary interaction are significant for electron avalanches. The formation of a sheath maximizes the ionization rate and the heating power, which thoroughly changes the field structure. In the post-breakdown phase, the growing negative ion density shrink the sheaths and changes the heating mode from the α mode to the drift-ambipolar mode. The particle generation rate and heating power show a growth trend after a brief decline. The growth of the recombination rate slowly balances the gain and loss of ions, which finally stabilizes the discharge.

Electrical breakdown in dual-frequency capacitively coupled plasma: a collective simulation

Most plasma sources have to undergo a breakdown process, during which, energy is injected, and particles are ionized. However, we still know little about this fast evolution process. In this work, a one-dimensional direct implicit particle-in-cell/Monte-Carlo collision (PIC/MCC) program is used to study the breakdown process of a capacitively coupled plasma (CCP) driven by dual radio frequencies. The results show that the breakdown process can be divided into three phases: the pre-breakdown, transition, and post-breakdown phases. In the pre-breakdown phase, the plasma density and heating power grow exponentially. The electric field can penetrate the whole discharge region without any shielding, resulting in a higher-than-average electron energy. Secondary electron emission is critical to grow the electron numbers under these discharge conditions. During the transition phase, the formation of sheaths maximizes the electron generation rate and heating power. The formation of sheaths also causes a drastic change in the electrical characteristics of CCP devices. In the post-breakdown phase, the plasma parameters gradually evolve until a steady state is reached. The decreasing rate of generation and the increasing rate of particle loss gradually equalize. The trends of the power gain and plasma loss are similar to the curves for the particle generation and loss rates, and a dynamic equilibrium is finally reached in the last steady state.

An Experimentally Validated Analytical Model for Aerosol Number Concentration Reduction in Classrooms

To predict the aerosol number concentration decay in modern classrooms, this study derived an analytical model that addresses various indoor factors, viz., the filtration efficiency of air ventilation systems, effects of indoor air cleaners, particle deposition on walls, and particle emission from occupants. We also conducted experimental measurements to determine the wall-loss coefficient and the occupants’ particle generation rate, and the modeling results agreed with the experimental data reasonably well. Additionally, we investigated the behavior of the particle concentration decay in different ventilation scenarios. The model has been incorporated into web-based software that is freely available to the public. © The Author(s).

Fume particle size distribution and fume generation rate during arc welding of cast iron.

This study measured the fume particle size distribution and fume generation rate during arc welding of cast iron and estimated the generation rate of respirable dust. In addition, the generation rate of particles with a diameter of 0.3 µm or less was estimated. In this experiment, three types of filler materials (mild steel wire, stainless steel wire, and mild steel covered electrodes) with main constituents of Fe or Fe-Cr-Ni, as proposed previously by the authors, were used. The welding methods were gas metal arc welding and shielded metal arc welding. The fumes measured in this research contained 73–91% respirable dust, and the fume generation rates were in the range of 1.96–12.2 mg/s. The results of this study were as follows: (i) the welding current affects the generation rate of respirable dust, and it is highly likely that the higher the fume generation rate, the more respirable dust is generated; (ii) the generation rates of respirable dust at low and high current were highest when mild steel covered electrodes and stainless steel wire was used, respectively; and (iii) the generation rate of particles with a diameter of 0.3 µm or less was highest when stainless steel wire is used.

Survey of particle production rates from process activities in pharmaceutical and biological cleanrooms

The lack of measurement data on particle generation rates from process activities for companies involved in the life sciences industry, such as pharmaceutical and biotechnology firms, has the potential to undermine cleanroom design, particulate concentration control, and energy efficiency. Based on this situation, this paper surveys the particle production rates from process activities in pharmaceutical and biological cleanrooms using field measurements for the particle sizes of interest (0.3 µm, 0.5 µm, and 1 µm). The measured and collected data were correlated to the surveyed activities to obtain a range of particle generation rates for different cleanroom operations. In terms of the recommended ACH values and measured ACH values, the results indicate that although most cleanrooms operate below the recommended ranges, in most cases they satisfy or exceed the required ISO standard cleanliness levels. In addition, the particle generation rates are process-specific and that suggests process-specific guidelines for different cleanrooms will help engineers, owners and contractors arrive at a better design solution. It is worth mentioning that nonperfect mixing of airflow and particles was observed in several cleanrooms, resulting in large variances in the measurements at different sampling locations. This observation suggests that a correction factor might be needed for the ventilation effectiveness for Class 7 and 8 cleanrooms. Although the determination of the correction factor is beyond the study of this paper, the data presented in this paper are critical to understand the severity of the non-perfect mixing in Class 7 and 8 cleanrooms and thus provides insights on the determination of the correction factor. The research reported in this paper was conducted through the ASHRAE 1399 research project, with a focus on developing scientific cleanroom design guidelines that can assist engineers, owners, and contractors to arrive at appropriately sized and efficient cleanroom systems.

Aerosol generation rates for showerheads

Aerosolization of water from discharging water appliances provides a transmission medium for Legionnaires’ disease. The quantity of aerosolized droplets influences the infection of Legionnaires’ disease. This study investigates the aerosol generation rates of four sample showerheads experimentally in a mechanically ventilated test chamber, assisted by computational fluid dynamics simulations. The results show that aerosol mass generation rate decreases with the showerhead resistance factor but increases with the water supply pressure, nozzle area ratio, flow rate, spray jet velocity, momentum and force. There is no significant correlation between aerosol mass generation rate and water spray uniformity ( p > 0.05, t-test). Furthermore, the aerosol mass generation rates and aerosol particle generation rates determined for the sample showerheads are in the ranges of 1.42 × 10−5 to 5.52 × 10−5 g s−1 and 0.35 × 106 to 1.35 × 106 particles s−1, respectively. Practical application: The proposed expression of aerosol generation rate can be the referenced guidance for future showerhead design to limit the aerosol generation rate.

Characterization of a Piezoelectric Inkjet Aerosol Generator for the Study of Bioaerosol Survivability

ABSTRACT In laboratory-based bioaerosol researches, the ability to reliably generate bioaerosols with minimal damage to the microorganisms is critical. In this work, a thermally consistent inkjet-based aerosol generation technique was adopted to generate bioaerosols, and its ability to preserve microbial survivability was assessed. The performance of the piezoelectric inkjet aerosol generator was evaluated using a potassium sodium tartrate solution and an aqueous suspension of 3-µm polystyrene latex spheres. An Aerodynamic Particle Sizer was employed to measure the aerosol number concentration and the size distribution. The main operating parameters of the inkjet print head for generating viable aerosols were the driving voltage, pulse duration, dispersion air flow, pulse frequency, and volume fraction of the solids in the nebulizing liquid. In the bioaerosol survivability experiments, yeast was used as the test microorganism. The nebulized yeast cells were sampled at different distances from the outlet of the inkjet aerosol generator and thus exposed to varying degrees of relative humidity for different residence times. Three bioaerosol samplers, namely, a BioSampler, a gelatin filter cassette, and a Marple cascade impactor with Mylar film, were used to collect the airborne yeast samples. Yeast survivability was determined by methylene blue viability staining. This study successfully demonstrated the feasibility and the advantages of using a piezoelectric inkjet print head to generate viable aerosols. Applying a square wave with appropriate combinations of voltage and pulse duration enabled the inkjet system to produce a liquid with rheological properties similar to those of water, and the particle generation rate was manipulated by adjusting the pulse frequency. Moreover, dispersion air was required to reduce coagulation. Although the number of droplets containing PSL spheres increased with the volume fraction of the spheres (Fv), a high Fv also reduced the probability of singlets forming.

Laboratory Validation and Initial Field Testing of an Unobtrusive Bioaerosol Detector for Health Care Settings

ABSTRACT Microorganisms can be transmitted from infected to healthy people as an aerosol. Military bioaerosol detectors currently used by soldiers or first responders may potentially be utilized in health care settings as part of a strategy to prevent the spread of airborne infectious diseases. The goal of this study was to conduct initial laboratory and field validation of an inexpensive and unobtrusive TACBIO detector and compare its performance with that of an expensive bioaerosol detection instrument, the Ultraviolet Aerodynamic Particle Sizer (UV-APS). The laboratory validation test used three bacterial clusters (Bacillus thuringiensis [Bt], Bacillus anthracis Sterne [BaS], and Bacillus atrophaeus var. globigii [Bg]) generated at controlled rates by an ink jet aerosol generator (IJAG). The detection efficiency of the UV-APS was ≥ 99% for all particle generation rates and species. The TACBIO detector exhibited a slightly lower detection efficiency but was still able to detect > 88% of Bt and BaS and 62.7–81.7% of Bg. Field validation conducted with simultaneous UV-APS and TACBIO sampling in an occupied hospital clinic showed both instruments closely tracking each other in detecting fluorescent particles > 1.5 µm in diameter. During a 6 hour sampling period, fluorescent particle (> 1.5 µm) concentrations showed wide short term variation connected to nearby human activity while smaller nonfluorescent particles displayed more gradual changes. These results indicate the usefulness of an unobtrusive environmental aerosol sampler in health care settings, motivating future field characterization and validation studies.

The mass transfer at Taylor cones

Electrospraying is a highly attractive method for generation of small particles and also for measurement applications of particles in suspensions. In the cone jet mode, the particle generation rate increases with decreasing flow rate. For applications in the field of aerosol technology flow rates of less than 10µl/h are attractive as particle sizes of less than 200nm can be obtained in this flow rate range. At the same time generation frequency of drops reaches the values required for size measurements by Scanning Mobility Particle Sizers (SMPS). A special challenge in operation of electrosprays at small flow rates is the change of liquid composition due to mass transfer processes between the Taylor cone and the surrounding gas phase. Typically used organic solvents such as alcohols, acetone or acetonitrile evaporate fast (or even evaporate completely) and mixtures containing such volatile species will change their composition significantly due to evaporation from the Taylor cone. Based on key experiments we showa)that the evaporating solvent can be reduced by as much as a factor of 20 by controlling the gas phase saturation. This allows spraying volatile solvents such as e.g. methanol at flow rates down to less than 1µl/h and makes the attractive sub-10µl/h flow rate range accessible to quite volatile solvents as well.b)that changes in the liquid composition by not considering the mass transfer (evaporation) problem lead to significant differences in liquid properties between the feed liquid and the Taylor cone. For example, an increase of surface tension by a factor of 2 is observed for water and t-butanol feed liquid.c)that employing a saturated sheath gas flow allows for very effective manipulation of the solvent composition in the Taylor cone. Based on water as feed liquid we show that a sufficient amount of organic solvent is easily transferred to the Taylor cone liquid from the gas phase to allow spraying the water without electron scavenging gases.Measurement results of the Taylor cone liquid composition are in good agreement with predictions by the presented new model describing the mass transfer between the Taylor cone and the surrounding gas phase. The efficiency of the mass transfer, the predictability of the process and the opportunity of comparably quick changes of the Taylor cone liquid by changing the gas phase saturation state result in a powerful tool for the entire field of electrospraying at low rates.

Theoretical assessment of particle generation from sodium pool fires

Potential sodium discharge in the containment during postulated Beyond Design Basis Accidents (BDBAs) in Sodium-cooled Fast Reactors (SFRs) would have major consequences for accident development in terms of energetics and source term. In the containment, sodium vaporization and subsequent oxidation would result in supersaturated oxide vapours that would undergo rapid nucleation creating toxic aerosols. Therefore, modelling this vapour nucleation is essential to proper source term assessment in SFRs. In the frame of the EU-JASMIN project, a particle generation model to calculate the particle generation rate and their primary size during an in-containment sodium pool fire has been developed. Based on a suite of individual models for sodium vaporization, oxygen natural circulation (3D modelling), sodium-oxygen chemical reactions, sodium-oxides-vapour nucleation and condensation, its consistency has been partially validated by comparing with available experimental data. As an outcome, large temperature and vapour concentration gradients set over the sodium pool have been found which result in large particle concentrations in the close vicinity of the pool.

Air ion and aerosol study in rural dwellings

Concentration and size distribution of airborne particles (size range 10–1100nm), using a SMPS+C Scanning Mobility Particle Sizer, and concentrations of positive and negative cluster ions (size range 0.36–1.6nm), using several CDI-06 Gerdien-type integral air ion detectors, have been monitored in three rooms in rural dwellings of Serbia and Slovenia, during periods when nobody was present in the room and while smoking and heating were taking place. The highest particle generation rate was 2.4×1011min−1 while cigarette smoking was taking place and was 1.5×1011min−1 during heating with a cast iron stove with wood burning at 150°C, and 1.1×1010min−1, during heating with an Alpine-type oven at 40–50°C. The related particle loss rate constants were 0.0603min−1, 0.0442min−1, and 0.0067min−1. The estimated mean values of the effective ion attachment rate βeff vary between (2.2–5.4)10−6cm3s−1. A correlation between ions concentration and particles concentration and their sizes has been sought, and findings are discussed and shown.

Modelling the risk of airborne infectious disease using exhaled air.

In this paper we develop and demonstrate a flexible mathematical model that predicts the risk of airborne infectious diseases, such as tuberculosis under steady state and non-steady state conditions by monitoring exhaled air by infectors in a confined space. In the development of this model, we used the rebreathed air accumulation rate concept to directly determine the average volume fraction of exhaled air in a given space. From a biological point of view, exhaled air by infectors contains airborne infectious particles that cause airborne infectious diseases such as tuberculosis in confined spaces. Since not all infectious particles can reach the target infection site, we took into account that the infectious particles that commence the infection are determined by respiratory deposition fraction, which is the probability of each infectious particle reaching the target infection site of the respiratory tracts and causing infection. Furthermore, we compute the quantity of carbon dioxide as a marker of exhaled air, which can be inhaled in the room with high likelihood of causing airborne infectious disease given the presence of infectors. We demonstrated mathematically and schematically the correlation between TB transmission probability and airborne infectious particle generation rate, ventilation rate, average volume fraction of exhaled air, TB prevalence and duration of exposure to infectors in a confined space.

Inkjet Aerosol Generator as Monodisperse Particle Number Standard

The AIST-inkjet aerosol generator (IAG) can generate highly monodisperse solid or liquid aerosol particles in the particle diameter range from 0.3 to 20 μm at precisely known particle generation rates. The device has been developed for evaluating the counting efficiencies of optical and condensation particle counters. Particle generation efficiency of the IAG is defined as the number of aerosol particles generated by one voltage pulse sent to an inkjet head. The 95% confidence interval of the efficiency were 0.998 ± 0.006 within the 0.4 to 10 μm particle diameter range. The efficiencies remained close to unity when the droplet generation rates were within 20–500 s−1 and 100–900 s−1 using ultrapure-water and isopropyl alcohol (IPA) as the solvent of the inkjet solution, respectively. The operating aerosol flowrate range of the IAG is currently 0.5 and 1.0 L/min. The coefficients of variations (C.V.) of the size distributions were 2 to 3% indicating the generated particles were highly monodisperse. The generated particle sizes were defined as the volume equivalent diameter, Dve. The uncertainty analysis on the factors affecting Dve indicated that 95% confidence interval of the Dve is expected to be ±5%. The uncertainty of Dve was entirely caused by the uncertainty of the average mass of a droplet. The reproducibility of particle sizes within 0.5 to 10 μm was evaluated using an aerodynamic particle sizer. The C.V. of the measured particle sizes were less than 6% and 4% when NaCl particles and ionic liquid droplets were generated, respectively.Copyright 2014 American Association for Aerosol Research

The role of space charge compensation for ion beam extraction and ion beam transport (invited)

Depending on the specific type of ion source, the ion beam is extracted either from an electrode surface or from a plasma. There is always an interface between the (almost) space charge compensated ion source plasma, and the extraction region in which the full space charge is influencing the ion beam itself. After extraction, the ion beam is to be transported towards an accelerating structure in most cases. For lower intensities, this transport can be done without space charge compensation. However, if space charge is not negligible, the positive charge of the ion beam will attract electrons, which will compensate the space charge, at least partially. The final degree of Space Charge Compensation (SCC) will depend on different properties, like the ratio of generation rate of secondary particles and their loss rate, or the fact whether the ion beam is pulsed or continuous. In sections of the beam line, where the ion beam is drifting, a pure electrostatic plasma will develop, whereas in magnetic elements, these space charge compensating electrons become magnetized. The transport section will provide a series of different plasma conditions with different properties. Different measurement tools to investigate the degree of space charge compensation will be described, as well as computational methods for the simulation of ion beams with partial space charge compensation.

Monitoring and control of particulate matter in indoor air: A review

The review discusses the sources and concentrations of particulate matter in different microenvironments, their effect on human health, monitoring and their possible control measures particularly in office and residential buildings. Effect of various ventilation strategies on particulate concentration in air has been discussed. The calculations suggest that if the outdoor particle concentration is not more than 5-10 times the indoor particle generation rate, ventilation could be a very effective tool to control the indoor particle concentration. For coarse particles the choice of ventilation strategy is not very crucial, but for fine particulate slow rate of ventilation is more effective.

Long‐term evaluation of a compliant cushion form acetabular bearing for hip joint replacement: A 20 million cycles wear simulation

Soft bearing materials that aim to reproduce the tribological function of the natural joint are gaining popularity as an alternative concept to conventional hard bearing materials in the hip and knee. However, it has not been proven so far that an elastic cushion bearing can be sufficiently durable as a long term (∼20 years) articulating joint prosthesis. The use of new bearing materials should be supported by accurate descriptions of the implant following usage and of the number, volume, and type of wear particles generated. We report on a long-term 20 million cycle (Mc) wear study of a commercial hip replacement system composed of a compliant polycarbonate-urethane (PCU) acetabular liner coupled to a cobalt-chromium alloy femoral head. The PCU liner showed excellent wear characteristics in terms of its low and steady volumetric wear rate (5.8-7.7 mm(3)/Mc) and low particle generation rate (2-3 × 10(6) particles/Mc). The latter is 5-6 orders of magnitude lower than that of highly cross-linked polyethylene and 6-8 orders of magnitude lower than that of metal-on-metal bearings. Microscopic analysis of the implants after the simulation demonstrated a low damage level to the implants' articulating surfaces. Thus, the compliant PCU bearing may provide a substantial advantage over traditional bearing materials.