A New Aircraft Inlet for Sampling Interstitial Aerosol: Design Methodology,Modeling, and Wind Tunnel Tests

The design of a new aerosol sampler, called the blunt-body aerosol sampler (BASE), to sample interstitial particles inside clouds while avoiding the problem of cloud droplet shatter artifacts is introduced. The primary design feature of the inlet is a blunt body that houses an aerosol inlet toward its aft end. The housing is designed to be blunt enough to deflect large cloud particles traveling around the body while being streamlined enough to maintain an attached boundary layer under aircraft flow conditions. The attached flow requirement ensures that shatter particles formed from the impaction of cloud droplets on the blunt body are retained close to the surface of the body. A region of large particle shadow is, thus, created in the aft of the blunt-body housing, where an aerosol inlet can sample interstitial particles in the absence of cloud particles. Computational fluid dynamics (CFD) simulations are used to optimize the shape of the blunt body, and the final sampler design is predicted to sample particles smaller than 2 μm from the freestream while being uninfluenced by cloud droplet shatter particles of the same size. Wind tunnel tests were performed on a prototype model to confirm the attached nature of the boundary layer flow around the blunt body and to establish the size-dependent behavior of shatter particles in the vicinity of the housing. While the experiments provide initial validation of the interstitial inlet design concept, some discrepancies were observed between the wind tunnel tests and CFD predictions, suggesting a need for improvements in simulations, inlet design, and/or test methodology. Initial analyses of field data obtained from the first aircraft deployment of BASE confirm that sampling of shatter-free interstitial aerosol is possible with the inlet, but full performance characterization of BASE will require significant additional aircraft-based experiments under a range of cloud conditions.

Copyright 2013 American Association for Aerosol Research     

Wake Capture,Particle Breakup,and Other Artifacts Associated with Counterflow Virtual Impaction

Counterflow virtual impaction is used to inertially separate cloud elements from inactivated aerosol. Previous airborne, ground-based, and laboratory studies using this technique exhibit artifacts that are not fully explained by the impaction theory. We have performed laboratory studies that show small particles can be carried across the inertial barrier of the counterflow by collision and/or coalescence or riding the wake of larger particles with sufficient inertia. We have also performed theoretical calculations to show that aerodynamic forces associated with the requisite acceleration and deceleration of particles within a counterflow virtual impactor can lead to breakup. The implication of these processes on studies using this technique is discussed.     

激波与亚毫米液滴相互作用的二维和三维数值模拟研究

为了研究激波与亚毫米液滴相互作用过程,基于现有的实验结果,利用Fluent平台,采用VOF (Volume of Fluid)多相流模型和k-ε湍流模型,通过二维数值模拟分析了不同韦伯数(We)对亚毫米液滴变形演化过程的影响规律,通过三维数值模拟揭示了亚毫米液滴爆炸式破碎机理。结果表明,韦伯数对液滴变形有促进作用,韦伯数越大,液滴在压缩变形阶段所需要的时间越短;在气动力不变的条件下,相同液滴直径条件下,马赫数越大,液滴所受的气动力越大,液滴质心位移的无量纲加速度越大,低韦伯数下液滴的横向展开速率随韦伯数增大而减小,而高韦伯数下液滴的横向展开速率随韦伯数增大而增大。数值模拟结果与对比实验结果相近,有效阐明了韦伯数对亚毫米液滴变形的作用。     

On Measuring the Critical Diameter of Cloud Condensation Nuclei Using Mobility Selected Aerosol

Cloud condensation nuclei (CCN) instruments determine the so-called “critical diameter” for activation of particles into cloud droplets at a fixed water supersaturation. A differential mobility analyzer is often used to size-select particles for purposes of scanning for the critical diameter. Usually the diameter where 50% of the particles have activated to cloud droplets is assumed to be equal to the critical diameter. We introduce a model that describes the transfer of polydisperse charge-equilibrated particles through an ideal differential mobility analyzer followed by transit through an ideal CCN instrument. We show that if the mode diameter of the polydisperse size distribution exceeds the critical diameter of the particles, multiply-charged particles may lead to nonmonotonic CCN counter response curves (plots of CCN-active fraction vs. mobility diameter) that exhibit multiple peaks, rather than a simple sigmoidally-shaped curve. Hence, determination of the 50% activation diameter is ambiguous. Multiply-charged particles significantly skew the CCNc response curves when sampling particles with critical diameters exceeding 0.1 μ m from particle size distributions with mode diameters also larger than the critical diameter. We present a method for inversion of CCN counter data that takes multiple-charging effects into account, and demonstrate its application to laboratory data. Our calculated CCN counter response curves are in good agreement with observations, and can be used to infer the critical activation diameter for a specified supersaturation.     

同轴双通道喷嘴雾化滴径沿轴向分布特性

Experiments were performed on the breakup of water droplets in a coaxial twin-fluid air-blast atomizer by a high-speed gas stream.With the measurements of Sauter mean diameter（SMD）distributions at a downstream distance of the atomizer,the droplet size was found first to decrease,reaching a minimum,and then to increase monotonically with the downstream distance.Correlation of L_c where the minimum droplet size appeared as a function of aerodynamic Weber number and liquid Reynolds number was analyzed.The growth rate of droplet size passes by L_c downstream increased with increasing water velocity,and the relationship between SMD and x/D_g accurately reflected the increasing trend of droplet size.     

液-液水力旋流器中的液滴破碎

在总结前人研究成果的基础上,分析了液-液水力旋流嚣中液滴破碎产生的原因,指出流场的湍流特性是产生液滴破碎的主要原因。对水力旋流器中的湍流度、雷诺切应力及颗粒的湍动能进行了分析,给出了水力旋流器中液滴破碎可能性较大的几个部位,并对旋流器边壁液滴破碎的可能性进行了讨论。分析了水力旋流器中液滴破碎的机理,阐明了液滴破碎判据——临界Weber数的表达式,并对理想球形液滴的破碎进行了分析。     

Particle Generation and Resuspension in Aircraft Inlets when Flying in Clouds

Using on-line analysis of single particles, we have observed both generation and resuspension of particles when ice crystals, cloud droplets, or dust impact an aircraft inlet. Large numbers of particles smaller than 1 μ m with a composition suggesting stainless steel were measured when flying a stainless steel inlet through cirrus clouds. Smaller numbers of metal particles were also observed when flying through dust or water clouds. A different instrument, sampling through a different inlet, found zinc particles when sampling in cirrus clouds. Laboratory experiments have verified that high-speed ice crystals can abrade stainless steel. Collision of ice crystals with the inlet wall also resuspended previously deposited particles. A notable example came when a flight through the space shuttle exhaust plume deposited large numbers of unique particles in our inlet. Some of the same types of particles were observed when the aircraft flew into an ice cloud the following day. The generation of particles by impaction of ice crystals and dust in inlets may have affected some published results about ice nuclei and metal particles in the upper troposphere. The newly generated particles cannot be distinguished from atmospheric particles by size alone.     

Influence of surfactants on growth of individual aqueous coarse mode aerosol particles

Understanding the links between aerosol and cloud and radiative properties remains a large uncertainty in predicting Earth's changing energy budget. Surfactants are observed in ambient atmospheric aerosol particles, and their effect on cloud droplet growth is a mechanism that was, until recently, neglected in model calculations of particle activation and droplet growth. In this study, coarse mode aqueous aerosol particles were created containing the surfactant Igepal CA-630 and NaCl. The evaporation and condensation of these individual aqueous particles were investigated using an aerosol optical trap combined with Raman spectroscopy. For a relative humidity (RH) change from 70% to 80%, droplets containing both Igepal and NaCl at atmospheric concentrations exhibited on average more than 4% larger changes in droplet radii, compared to droplets containing NaCl only. This indicates enhanced water uptake in the presence of surfactants, but this result is unexpected based on the standard calculation of the effect of surfactants, using surface tension reduction and/or hygroscopicity changes, for particles of this size. One implication of these results is that in periods with increasing RH, surfactant-containing aqueous particles may grow larger than similarly sized aqueous NaCl particles without surfactants, thus shifting atmospheric particle size distributions, influencing particle growth, and affecting aerosol loading, visibility, and radiative forcing.

Copyright © 2018 American Association for Aerosol Research     

Aerosol etching by low-pressure impaction

Low-pressure impaction technology has been applied to a new etch process using aerosol, called aerosol jet etching (AJE). Fine droplets (0.1 to 0.3 urn) produced by spray-evaporation-condensation method impinge on a substrate in low-pressure impactor and etches its surface. Investigations were carried out on the control of etchant droplet size, the critical diameter for impaction and the performance of AJE on patterned etching. The patterned etching on SiO₂ film reveals some advantages over conventional wet etching: the economic use of etchant, the reduction of waste disposal and the increase in controllability of etching. To make maximum use of the advantage of AJE, techniques of further decreasing the droplet size and depositing this small droplets on substrates need to be developed.     

Aerosol Generation by a Spray-Drying Technique Under Coulomb Explosion and Rapid Evaporation for the Preparation of Aerosol Particles for Inhalation Tests

In this study, nanosized (<100 nm) aerosol particles with high mass concentrations for inhalation tests were generated by a spray-drying technique with combining Coulomb explosion and rapid evaporation of the droplets. Under typical spray-drying conditions, aerosol particles with average diameter of 50–150 nm were prepared from a suspension of NiO nanoparticles with a primary diameter of 15–30 nm. Under the Coulomb explosion method, the sprayed droplets were charged by being mixed with unipolar ions to break up the droplets, which resulted in the generation of smaller aerosol particles with diameters of 15–30 nm and high number concentrations. Under the rapid evaporation method, the droplets were heated immediately after being sprayed to avoid inertial impaction on the flow path due to shrinkage of the droplet, which increased the mass concentration of the aerosol particles. The combination of the Coulomb explosion and rapid evaporation of droplets resulted in the generation of aerosol particles with sizes less than 100 nm and mass concentrations greater than 1 mg/m³; these values are often necessary for inhalation tests. The aerosols generated under the combined method exhibited good long-term stability for inhalation tests. The techniques developed in this study were also applied to other metal oxide nanoparticle materials and to fibrous multiwalled carbon nanotubes.

Copyright 2014 American Association for Aerosol Research     

喷淋塔液滴粒径分布及比表面积的实验研究

以水和空气为实验介质,通过拍照法获得喷淋塔内液滴粒径分布,考察了不同喷淋量及空塔气速对塔内不同高度处液滴Sauter平均直径(SMD)的影响,并对液滴粒径分布进行了理论分析.结果表明,喷淋塔顶部液滴分布密集,底部稀疏,液滴群在下落过程中,平均粒径减小且趋于均匀化;塔顶处液滴SMD随喷淋量的增加而增大,处在塔中下部的液滴SMD则随喷淋量增大而减小,提高空塔气速,可减小平均粒径;理论分析认为,液滴粒径减小主要是由于发生了碰撞破碎的缘故,而塔内液滴大小不一是碰撞的主要原因;通过量纲1化拟合得到喷淋塔内液滴SMD经验关联式,其计算结果与实验值吻合较好;考虑液滴破碎的喷淋塔比表面积比不考虑破碎的比表面积大70%左右.     

Aerosol Formation During Water Ingress Into the Core of a Pebble Bed High-Temperature Reactor

In order to give an estimation of the safety of nuclear power plants, it is necessary to characterize the aerosol that develops in the event of a serious accident. Aerosol formation in a graphite-moderated high-temperature pebble bed reactor is possible when water or steam intrudes into the helium primary circuit. Because of the great temperature gradient between water droplets and the pebble surface, the evaporation of water is determined by the thin steam layer between the droplet and hot surface. The thin steam layer reduces the heat flux from the pebble to the water droplet. This effect is the well-known Leidenfrost phenomenon. The steam discharges from the steam layer at a high velocity, and is able to dislodge particles from the pebbles. The chemical reaction of the hot pebble graphite with steam results in a gasification by producing a gas mixture of H₂O, He, CO, C0₂, and CH₄. In order to investigate the process of aerosol formation a test device was set up containing 1500 spherical graphite elements with a diameter of 10 mm, which can be heated up to a temperature of 1500°K. The mass concentration of the thus-produced graphite particles in the aerosol increases rapidly, when the corrosion progress has reached a specific value. The size distribution of the equivalent aerodynamic diameter is shifted to larger diameters with rising corrosion time.     

Calibration and Use of the Aerodynamic Particle Sizer (APS 3300)

The Aerodynamic Particle Sizer (APS) has been in use at the National Institute for Occupational Safety and Health (NIOSH) for over two years, beginning with a prototype model and more recently with a commercial version (Model 3300). The APS has been tested and used in a variety of laboratory and field situations. It has been a very useful instrument for testing aerodynamic sizing devices and provided a much needed means of rapid aerodynamic sizing of particles. Limits to the accuracy of the APS in determining aerodynamic diameter of particles were investigated.

The calibration of the APS was originally carried out by using monodisperse di-octyl phthalate (DOP) oil aerosol in the 3–15 μm range. Using a laser imaging system, the flattening of droplets into oblate spheroids was observed for larger particles (20–100 μm). The APS was recalibrated with solid latex particles and the DOP particles were measured to determine the effect of the droplet flattening. A 15 μm droplet is measured as being 20% smaller by the APS. A semiempirical equation was developed to fit the droplet deformation data. Particle measurement in the APS takes place largely outside the Stokes regime. Therefore, it has been predicted by Wilson and Liu (1980) that the measured diameters will be dependent on density. Monodisperse particles of density 1.15 and 2.15 were generated. In the range of 8–14 μm there was a difference of up to 8% in the measured size for particles of the same aerodynamic diameter. Particle coincidence can modify the measured size distribution in a different way than for other optical particle counters. The APS has circuitry to reduce the effects of coincidence that can also modify the meausred distribution. Calculations were carried out to simulate the effect of coincidence. Several potential problems and improvements for the APS were found.     

A Cylindrical Thermal Precipitator with a Particle Size-Selective Inlet

We designed a thermal precipitator in a cylindrical configuration with a size-selective inlet, and investigated its performance in experiments using differential mobility analyzer (DMA)-classified particles of sodium chloride (NaCl) and polystyrene latex (PSL). Our investigation was performed in two parts: (1) using the size-selective inlet to determine the best inlet-to-wall distance for optimal impaction of 1 μm particles; (2) using a simple inlet tube to measure particle collection via thermophoresis over a size range from 40 nm to 1000 nm. The results showed that the inlet had a particle cut-off curve, with a 50% particle cut-off Stokes number of 0.238, resulting in removing particles with sizes larger than 1 μm at an aerosol flow rate of 1.5 lpm. The thermophoretic particle collection efficiency in the prototype was measured without the size-selective inlet installed. The size dependence of the collection efficiency was negligible for particles with diameters ≤300 nm and became noticeable for those with diameters >300 nm. An analytical model was further developed to estimate the particle collection efficiency due to thermophoresis of the prototype under various aerosol flow rates and temperature gradients. For particles with diameters less than 400 nm, reasonable agreement was obtained between the measured data and the collection efficiency calculated from the developed analytical model. It was further concluded that the derived formula for the calculation of thermophoretic particle collection efficiency could serve as the backbone for future design of thermal precipitators in any configuration, when combined with the proper formula for the dimensionless thermophoretic particle velocity.

Copyright 2012 American Association for Aerosol Research     

Evaluation of a Perpendicular Inlet for Airborne Sampling of Interstitial Submicron Black-Carbon Aerosol

The majority of airborne aerosol measurements employ forward-facing inlets with near-isokinetic sampling; these inlets have known artifacts when sampling in clouds such that data taken in cloud must typically be discarded. Here we report first results from a perpendicular inlet for sampling interstitial submicron black-carbon (BC) containing aerosol. The inlet, consisting of a flat plate to stabilize flow prior to perpendicular sampling, was evaluated using a single particle soot photometer (SP2) aboard the NASA WB-57F aircraft during the Midlatitude Airborne Cirrus Properties Experiment (MACPEX) of 2011. The new inlet rejects large particles and is free of aerosol artifacts when sampling in ice clouds while allowing sampling of submicron BC-containing aerosol with the same unit efficiency as a validated isokinetic inlet, thus allowing for airborne sampling of interstitial BC aerosol.

Copyright 2013 American Association for Aerosol Research     

The calibration of a timbrell aerosol spectrometer

An experimental method for the calibration of a Timbrell aerosol spectrometer is described and the influence of the aerosol and winnowing air flow rates on performance investigated. The Timbrell spectrometer size classifies airborne particles according to their aerodynamic diameters, winnowing them in a recirculating flow of clean air while they fall under the influence of gravity in a horizontal settling chamber. Size separation takes place under Stokesian conditions and the particles deposit on to a series of microscope slides. The results of the calibration with monodisperse polymer latex microspheres and polydisperse spherical particles of polyvinyl acetate show that the instrument achieved acceptable size resolution (12%) at an aerosol sampling flow rate of 1 cm³ min⁻¹, with a ratio of aerosol to winnowing flow rate of 6.7 × 10⁻³. Under the conditions used the instrument was shown to be most sensitive for size classifying particles in the range 4–12 μm aerodynamic diameter. The Timbrell aerosol spectrometer is a useful device both as a reference method for the determination of aerodynamic diameters of airborne particles and as a means of collecting such particles for subsequent examination by other aerosol analysis equipment.     

Note on the movement of insecticide droplets through and out of a crop canopy after emission below canopy level

Coarse aerosol sprays with number median diameters in the range 30–40 μm were emitted at 0.25 m above the ground within the canopy of a cotton crop planted at 0.93 m row separation and standing 1.0 m tall. Movement of spray was monitored by vertical stepped paper tower samplers with narrowest section of width 0.13 cm, giving collection by inertial impaction down to 10 μm in a 1 m/sec wind. Horizontal paper targets were used to sample droplets depositing by sedimentation. Droplet number counts indicated that under conditions of moderate instability and associated convective turbulence, droplets were drawn rapidly out of the crop to a height of 2 m (the upper level sampled). The peak concentration of the spray cloud was centred at a height of between 1.0 and 2.0 m beyond 1.0 m downwind of the emission point in the first test, using droplet spectrum with volume median diameter of 41 μm, and beyond 2.5 m downwind of the emission point in the second test with spectrum of 57 μm volume median diameter. Inertial impaction at the 0.25 and 0.5 m levels was slight, except on the samplers within 0.5 m of the sprayer, and sedimentation was only significant at and below 0.5 m in the first two rows sampled. The moderate wind speed of 2.5 m/sec at canopy level produced marked inertial impaction over the first 4.5 m downwind of the sprayer at the 1.0 m level.     

New PM10 Inlet Design and Evaluation

ABSTRACT

A new PM₁₀ inlet for a beta-gauge sampler was designed based on the particle cup impactor concept. The performance of the inlet was evaluated at near-zero wind velocity in the test chamber and at the wind velocities of 2 and 8 km/hr in the wind tunnel. The performance indicated that particles with aerodynamic diameters of 10 μm or larger were collected in the particle cup and the inlet proved to meet the basic requirement of PM₁₀ sampling.     

Measurements of Kelvin-Equivalent Size Distributions of Well-Defined Aerosols with Particle Diameters > 13 nm

During the 1979 workshop of the working group on ultrafine aerosols, different experimental techniques for measuring the number concentration and size of ultrafine aerosol particles were compared. In the present paper we report on a comparison of different particle size measuring techniques for ultrafine aerosols. Well-defined monodisperse aerosols with electrical mobility particle diameters ranging from 13 to 100 nm were generated using an electrical aerosol classifier. Kelvin-equivalent size distributions of these aerosols were determined by means of a process-controlled expansion chamber, the size-analyzing nuclei counter (SANC). To this end the considered aerosol was humidified and the number concentration of the droplets growing in the expansion chamber was measured for stepwise increase in supersaturation. At a quite well defined critical supersaturation, a significant increase in the measured droplet concentration, and thus the onset of heterogeneous nucleation, was observed. By means of the Kelvin-Gibbs equation this critical supersaturation is related to the Kelvin-equivalent diameter of the aerosol particles. Measurements were made on NaCl and dioctyl phthalate (DOP) aerosols. For NaCl particles the Kelvin diameter was found to be larger by a factor of about 4 than the electrical mobility diameter, as determined by the electrostatic aerosol classifier. This is explained by the solubility of the NaCl particles. For DOP particles, however, the Kelvin diameter agrees quite well with the electrical mobility diameter. The Kelvin size distributions were found to be quite narrow, indicating a high monodispersity of the generated aerosol as well as a satisfactory size resolution of the SANC. Thus different experimental techniques, based on completely different principles, yielded similar measurement results.