Numerical Performance Simulation of a Wetted Wall Bioaerosol Sampling Cyclone

Computational fluid dynamics techniques are used to study the performance of an axial flow bioaerosol sampling cyclone that continuously collects particles onto a flowing liquid film. A special shell-volume concept was developed to study formation and development of the liquid film on the inner wall of the cyclone. For a previous version of the cyclone, simulations demonstrated the presence of a ring of liquid in the region just upstream of the liquid skimmer that was suspected of causing liquid carryover into the exhaust air stream and degradation in aerosol collection efficiency. This ring was eliminated by re-design of the cyclone. For the upgraded version of the cyclone, CFD was used to successfully predict aerosol collection efficiency and cyclone pressure drop. The simulations reveal a complex flow evolution inside the cyclone. Stream-tubes are used to describe a significant narrowing of the width of the airflow as it enters the cyclone and an inward displacement of the flow as it travels in the axial direction. The particle deposition occurs primarily in a region that is subtended approximately by the length of the rectangular entrance slot and the first half turn of the flow in the cyclone. Cutpoint Stokes number is about 0.05 and the cutpoint particle size is about 1 μm aerodynamic diameter. At a flow rate of 1250 L/min, the pressure drop across the cyclone is 5.6 kPa (22 inches of water).     

In-Line Impactor Inlet for Bioaerosol Sampling

The proof of concept of a novel in-line real impactor (IRI) for preseparation of large particles in ambient inlets was demonstrated with a 1,250 L/min design. Numerical simulations predicted a cutpoint Stokes number 0.3 for a ratio of jet-to-plate spacing to jet width (S/W) of 2.0 and 0.5 for a ratio of 4.0. This variation in cutpoint Stokes number allows minor adjustments in cutpoint for a given device size. Experimental benchmark tests support the prediction of a shift in cutpoint with S/W. Inlet systems with flow rates of 100 and 400 L/min were designed by Stokes scaling of the 1,250 L/min IRI and integrating the lower flow devices with an existing inlet aspiration section and an insect screen. Experiments with the inlet system were conducted in a wind tunnel with particles from 3 to 20 μm aerodynamic diameter (AD) and wind speeds of 2, 8, and 24 km/h. A nominal cutpoint of approximately 11 μm AD was selected to accommodate bioaerosol sampling needs, and the wind tunnel results showed the average cutpoints of the 100 and 400 L/min inlet systems at the three wind speeds were 11.2 and 11.6 μm AD, respectively. Stand-alone tests with the 100 and 400 L/min IRIs were conducted where dry dusts (Arizona road dust/fine and coarse) were impacted on three types of collection surfaces (dry, grease-coated, and oil-soaked porous surfaces) to characterize solid particle carryover. The oil-soaked porous media allowed the least carryover of large solid particles.     

Aerosol-to-Hydrosol Transfer Stages for Use in Bioaerosol Sampling

Single-jet and multijet aerosol-to-hydrosol transfer stages (AHTSs) with cutpoints of 2 and 0.8 μm aerodynamic diameter, respectively, were designed and evaluated. The devices are intended to take the coarse particle flow stream (minor flow) from a virtual impactor and concentrate the aerosol particles into a low flow rate of liquid. The design air flow rate for each system is 1 L/min, and the collection liquid flow needs to be ≥ 0.3 mL/min with a surfactant added to prevent loss of hydrosol particles on internal surfaces of the devices. Satisfactory performance was achieved when distilled water with 0.1% Tween 20 was used as the collection fluid. The effectiveness (average fractional efficiency) for the single-jet device is 94% over the size range of 2.5 to 10 μ m aerodynamic diameter, and that of the multijet AHTSs is 90% over the size range of 1 to 10 μ m aerodynamic diameter. The systems have an ideal air power consumption of 1.4 mW and 4.5 mW, respectively. If an AHTS were operated in a heated enclosure and sampled air at ?28°C, less than 1 W of heating would be required to prevent freezing. Preliminary results of bioaerosol testing with 0.7 μm AD single spores of Bacillus globigii var. niger show efficiencies over 100%. These values are probably due to the different expression of viability of the spores in the reference samples and those in the output liquid of the AHTSs.     

A Circumferential Slot In-Line Virtual Impactor

An In-line Virtual Impactor is presented, which has an application as a pre-separator for sampling inlets, where the device scalps large particles from the aerosol size distribution. Numerical simulation was the principal tool employed in the design process, with physical experiments used to verify computational predictions. Performance investigations were primarily carried out for a configuration that provides a nominal cutpoint particle size of 10 μ m aerodynamic diameter at an inlet flow of 111 L/min and a major flow exhaust of 100 L/min; however, the concept is scalable in terms of both flow rates and cutpoint sizes. An inverted dual cone configuration contained within a tube provides a characteristic circumferential slot of width 2.54 mm (0.100 inches) and a slot length of 239 mm (9.42 inches) at the critical zone. The upper cone causes the flow to accelerate to an average throat velocity of 3.15 m/s, while the lower cone directs the major flow toward the exit port and minimizes recirculation zones that could cause flow instabilities in the major flow region. The cutpoint Stokes number is 0.73; however, the cutpoint can be adjusted by changing the geometrical spacing between the acceleration nozzle exit plane and a flow divider. When the system is operated at the major exhaust flow rate of 100 L/min, the pressure drop is 45 Pa. Good agreement is obtained between numerically predicted and experimentally observed performance.     

A Model for the Prediction of the Collection Efficiency Characteristics of a Small,Cylindrical Aerosol Sampling Cyclone

Velocity data from a previous study were nondimensionalized and used in conjunction with a computer program which solves the equations for particle trajectory to predict the collection efficiency for the cyclone. Results for the prediction of cutpoint at the same Reynolds number as that for which the velocities were measured, both for a large cyclone of 88.9 mm diameter and another geometrically similar at one half the scale, are excellent. The model predicts cutpoints of 10 μm and 5.1 μm for the large and small cyclone, respectively, while the actual cutpoints determined from aerosol tests were 9.9 μm and 5.2 μ m. The efficiency curve generated by the model was steeper (geometric standard deviation of 1.1) than the efficiency curve determined through the aerosol testing (geometric standard deviation of 1.4). A simplification of the Dirgo and Leith equation fitting Barth's design curve is suggested which provides a significantly better fit of the aerosol data (geometric standard deviation of 1.3). At 1.5N _{R Q}, where N _{R Q} = (4pg)/(πμD _c), the error in prediction of the cutpoint in the large cyclone is less than 8% while at 04N _{R Q} the error is less than 2%. Although results are good over a limited range of Reynolds numbers, the model is strictly applicable only for flows which are dynamically similar to those studied here.     

Circumferential-Slot Virtual Impactors with Stable Flow

Flow instabilities in a virtual impactor designed for bioaerosol concentration (size range about 2 to 10 μ m AD) can seriously degrade performance. When the flow in a 100 L/min circumferential slot virtual impactor (CSVI) was unstable, the transmission efficiency was 30% for 7.2 μ m AD aerosol particles, but when the instability problems were corrected, the transmission efficiency was increased to above 90%. Three-dimensional CFD simulations have been used to examine flows in two CSVIs, a nominal 10 L/min device in which the flow was stable, and the 100 L/min device in which the flow was initially unstable. From the CFD flow patterns in the 100 L/min device, the principal instability was in the minor flow region and was caused by overly rapid flow deceleration, too large of a volume, and too low of a jet velocity in that region. Changes were made to the geometry of the CSVI, and CFD was used as the diagnostic tool to determine when stable flow was achieved. Also, for the 100 L/min unit, wake effects from alignment posts that hold together the two halves of the CSVI propagated into the receiver section, and CFD analyses were used to modify the post locations to optimize the transmission efficiencies for the stable units. Numerical and experimental results show the dynamic ranges (ratio of the largest Stokes number for which the transmission efficiency is 50% to the cutpoint Stokes number) are about 100 for both devices. The peak value of the transmission efficiency for the 100 L/min unit is 95% and that for the 10 L/min device is 97%.     

Design and Calibration of a Multi-Channel Aerosol Sampler for Tropopause Region Studies from the CARIBIC Platform

An aircraft-based, multi-channel aerosol sampler for studies of the upper troposphere and lowermost stratosphere from the CARIBIC (Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container) platform was designed and calibrated. The sampler operates with an impaction technique at a flow rate of 10.4 lpm and consists of sixteen sampling channels. Samples are collected in a time sequence. Each channel contains two sample types that are used for quantitative measurement of concentrations, using particle-induced X-ray emission (PIXE), and single particle analysis with electron microscopy. The minimum detection limits for PIXE analysis after 1.5 h sampling are, for example, 2.0, 0.14, and 0.02 ng/m³ STP (standard temperature and pressure) for sulfur, potassium, and nickel. Calibration included penetration studies of a cyclone arrangement used to define the upper size limit in the sampling to 2.0 μ m diameter and the collection efficiency of the impactor. Both components of the sampling system showed penetration and collection efficiency close to 100%, respectively, in the particle size range of interest. The impactor cut-off was found to be dependent on the ratio of the impactor upstream-to-downstream pressure for ratios well below the critical pressure drop (i.e., the pressure where the jet reaches sonic velocity) being 0.15 μ m and 0.08 μ m for ratios 0.41 and 0.2.     

Wind Tunnel Evaluation of an Aircraft-Borne Sampling System

The U.S. Environmental Protection Agency (EPA), the Florida Department of Environmental Protection (FLDEP), and Texas A&M University collaborated in the design, construction, and testing of a unique, highly crosslinked, Teflon-coated inlet and manifold gas and aerosol sampling system that is being used in EPA aircraft atmospheric pollution characterization studies. The aircraft-borne ambient sampling system, which consists of a Teflon-coated shrouded probe coupled to a Teflon-coated aluminum manifold, is designed to collect reactive gases (e.g., mercury and halide species) and aerosols for subsequent analysis and characterization. The shrouded inlet probe was tested for particle transmission ratios in a high-speed aerosol wind tunnel. An existing wind tunnel was upgraded from a maximum wind speed of 13.4 m/s (48 km/h or 30 miles/h) to 50.5 m/s (182 km/h or 113 miles/h) to test this probe. The wind tunnel was evaluated for compliance with the criteria of ANSI 13.1 to establish the acceptability of its use in testing probes. The results demonstrated that the velocity and tracer gas concentration profiles were within the specified limits. A wellcharacterized ThermoAndersen Shrouded Probe (Model RF-2-112) was also tested to check tunnel performance and test methodology. The results obtained from these tests are in close agreement with earlier published data.

When operated at a sampling flow rate of 90 L/min, the aircraft-borne shrouded probe showed a transmission ratio of about 0.76 at 45 m/s (162 km/h or 100 miles/h) for 10 μ m aerodynamic diameter particles. To improve the transmission ratio of the sampling probe, the sampling flow rate was reduced to 80 L/min and the air speed increased to 50.5 m/s, which increased the transmission ratio to about 0.9 for 10 μ m particles. Further reduction of the flow rate to 60 L/min increased the transmission to 1.2. The Teflon-coated manifold, which is located downstream of the shrouded probe, was statically tested for transmission ratio at flow rates of 90 L/min and 30 L/min. The results were a transmission ratio of about 0.80 for 10 μ m aerodynamic diameter particles. The combination of the shrouded probe operated at 60 L/min with a transmission ratio of 1.2 and the manifold with its transmission of 0.8 will give an overall transmission of about unity for 10 μ m aerodynamic diameter particles at a flight speed of 50.5 m/s.

These findings suggest that shrouded probes can be used for low speed (～ 100 miles/h) aircraft applications. The transmission ratio of these probes is a significant improvement over the conventional aircraft-mounted, sharp-edged isokinetic diffuser-type inlets.     

Performance Characteristics of the Aerosol Collectors of the Autonomous Pathogen Detection System (APDS)

This research analyzes the physical performance characteristics of the aerosol collectors of the autonomous pathogen detection system (APDS) that was recently developed by the Lawrence Livermore National Laboratory. The APDS is capable of continuous and fully autonomous monitoring for multiple airborne threat organisms and can be used as part of a monitoring network for urban areas and major public gatherings. The system has already been successfully tested with airborne Bacillus anthracis and Yersinia pestis biowarfare agents. The APDS aerosol collection system consists of a PM-style cap to remove large particles and a low-pressure drop virtual impactor preconcentrator positioned in front of a wetted-wall cyclone. The aerosol collectors operate at flow rates as high as 3750 l/min and collect airborne particles into 4 ml of liquid for subsequent detection. In our tests we determined the overall collection efficiency of the system by measuring the difference between inlet and outlet particle concentrations. The tests were performed with polydisperse oleic acid and monodisperse polystyrene latex (PSL) particles (0.6–3.1 µ m), and for three values of the major air flow rates in the virtual impactor (1760, 2530, and 3300 l/min), two values of the product, or cyclone, flow rates (375 and 450 l/min), and two different volumes of collection liquid (4 and 6 ml). We found that the cutoff size (d₅₀ ) of the entire collection system varied from 1.5 to 2.0 µ m when collecting PSL particles, with 3.1 µ m PSL particles being collected with efficiency of approximately 85%. When collecting oleic acid particles the d₅₀ of the entire system varied from 1.1 to 1.6 µ m. The concentration rates of the aerosol collection system were found to increase with increasing overall collection flow rate and approached one million per minute at the highest tested flowrates. Such high concentrating rates and high air sample volumes make the APDS collection system highly suitable for detecting low concentrations of airborne pathogens.     

Sampling efficiencies of two modified viable cascade impactors

Prevention of airborne contagious diseases depends on successful characterization of aerosols in the environment. The use of cascade impactors to characterize ambient aerosols is one of the most commonly used methods, providing data on both particle size and concentration. In this study, the use of a cascade impactor recently described in the literature using 8 mL of liquid in Petri dishes (CI-L) was compared with a new method that uses wet membrane filters on top of wax filled Petri dishes (CI-WWMF). Sampling efficiencies of the cascade impactors were evaluated using 0.5, 1, 3, and 5 μm polystyrene latex (PSL) microspheres and aerosol consisting of single spores of Bacillus atrophaeus var. globigii (BG). The sampling efficiency of the CI-L was 6%, 11%, 17%, 21%, and 58% for 0.5, 1, 3, 5 μm PSL microspheres and BG spores, respectively. Higher overall sampling efficiencies of 71%, 91%, 60%, 64%, and 104% were observed for the same size and type of particles for the CI-WWMF. This study indicates that using wet filters on top of wax-filled Petri dishes (CI-WWMF) in a viable cascade impactor is more efficient than the CI-L method for size-selectively collecting biological aerosols from the environment. The CI-WWMF method is useful when a liquid medium is required for identifying and quantifying organisms using polymerase chain reaction (PCR) and immuno-assay techniques.

Copyright © 2017 American Association for Aerosol Research     

Use of an Andersen Bioaerosol Sampler to Simultaneously Provide Culturable Particle and Culturable Organism Size Distributions

A method is provided for estimating size distributions in terms of both culturable particles (CP) and culturable organisms (CO) from a single sample collected with an Andersen bioaerosol impactor. Half of the agar surface of each Petri dish is covered with a polycarbonate filter substrate through which liquid from the agar can wick and thereby form a flat wetted collection surface, and the other half of the agar surface is uncovered. Quantification of CO distribution results involves washing the collected particles from the filter surfaces and plating suitable dilutions of de-agglomerated particles, followed by incubating the organisms, and counting the resulting colonies; whereas, quantification of CP results is based on direct culturing of the agar media. Experiments were conducted with near-monodisperse clusters of Bacillus atrophaeus (aka BG) aerosols covering APS-determined geometric number mean sizes from 1.83 to 8.7 μm aerodynamic diameter (AD). There is little difference between the Andersen-determined median sizes of the CO and CP distributions of the near-monodisperse clusters, which supports the utility of the CO method. The method was applied to sampling bioaerosol in a previously occupied classroom and the results showed median sizes of 1.7 and 3.1 μm AD for the CP and CO distributions, respectively. Total bioaerosol concentrations were 0.53 and 3.4 CO/L, respectively, so the average CP contained 6.4 CO.

Copyright 2012 American Association for Aerosol Research     

Experimental Study of Particle Collection by Small Cyclones

A new set of experimental data on the particle collection characteristics of small cyclones is reported. The collection efficiency for particles ranging from 2 to 10 μm in diameter was measured systematically for nine cyclones at flow rates ranging from 8.8 to 18.4 L/min. Special emphasis was given to the effects of the exit tube size and of the cyclone body size on the particle collection efficiency. The size ratio of the exit tube to the cyclone body was varied from 0.24 to 0.80. The experimental results show that the stiffness of the particle collection cutoff with size does not change noticeably with a change in the cyclone body size while operation of a cyclone at a low flow rate can cause the particle collection characteristics to become less stiff. It was also found that the exit tube diameter influences the particle collection efficiency substantially, with results showing that as the exit tube size is decreased, the collection efficiency increases. A large cyclone body size increases the efficiency. However, when the cyclone body is increased excessively, the collection efficiency appears to decrease somewhat. The experimental data were compared with existing cyclone theories and Barth's (1956) theory was found to be in good agreement. Finally, the exit tube was found to affect substantially the pressure drop of cyclones. As the exit tube size increased, the pressure drop decreased. However, when the exit tube size was further increased until it approached the body size, the pressure drop increased again.     

Liquid Consumption of Wetted Wall Bioaerosol Sampling Cyclones: Characterization and Control

Advances in microfluidic, lab on chip, and other near-real-time biological identification technologies have driven the desire to concentrate bioaerosols into hydrosol sample volumes on the order of tens of microliters (μL). However, typical wet biological aerosol collector outputs are an order or two of magnitude above this goal. The ultimate success of bioaerosol collectors and biological identifiers requires an effective coupling at the macro-to-micro interface. Liquid collection performance was studied experimentally for a family of dynamically scaled wetted wall bioaerosol sampling cyclones (WWC's). Steady-state liquid collection rates and system response times were measured for a range of environmental conditions (temperatures from 10°C to 50°C and relative humidities from 10% to 90%), liquid input rates, and WWC airflow configurations. A critical liquid input rate parameter was discovered that collapsed all experimental data to self-similar empirical performance correlations. A system algorithm was then developed from empirical correlations to provide control over the liquid output rate and resulting concentration factor for a cyclone with an airflow rate of 100 L/min. Desired liquid output rates of 25 to 50 μL/min were maintained while sampling outdoor air over diurnal ranges of environmental conditions. These flow rates are associated with concentration factors on the order of 1,000,000 to 2,000,000 and liquid outputs that are a steady stream of 10 to 30 drops/min of 7 to 10 μL droplets. These developments should allow wetted wall cyclones to be effectively coupled to advanced biological identification systems.     

Design and development of a self-contained personal electrostatic bioaerosol sampler (PEBS) with a wire-to-wire charger

Here, we present a concept of a personal electrostatic bioaerosol sampler (PEBS), which is an open channel collector consisting of a novel wire-to-wire particle charger and a collection section housing a double-sided and removable metal collection plate and two quarter-cylinder ground electrodes. The charger consists of a tungsten wire (25.4 mm long and 0.076 mm in diameter) connected to high voltage and positioned in the center of the charging section (a cylinder 50.8 mm long and 25.4 mm in diameter); a ring of stainless steel wire 0.381 mm in diameter surrounds the hot electrode at its midpoint and is grounded. The newly designed wire-to-wire charger produces lower ozone concentrations compared to traditional wire-to-plate or wire-to-cylinder charger designs. The particles captured on the collection plate are easily eluted using water or other fluids. The sampler was iteratively optimized for optimum charging and collection voltages, and collection electrode geometry. When tested with polystyrene latex particles ranging from 0.026 µm to 3.1 µm in diameter and 10 L/min collection flow rate, the sampler's collection efficiency was approximately 70%–80% at charging and collection voltages of +5.5 kV and ?7 kV, respectively. The PEBS showed this collection efficiency at sampling times ranging from 10 min to 4 h. Preliminary tests with Bacillus atrophaeus bacterial cells and fungal spores of Penicillium chrysogenum showed similar collection efficiency. The use of a unique wire-to-wire charger resulted in ozone production below 10 ppb. Due to low ozone emissions, this sampler will allow maintaining desirable physiological characteristics of the collected bioaerosols, leading to a more accurate sample analysis.

© 2017 American Association for Aerosol Research     

Aerosolization of Bacterial Spores with Pressurized Metered Dose Inhalers

Bioaerosol detection and identification systems need to be periodically checked for assurance that they are responsive to aerosol challenges. Herein, pressurized metered dose inhalers (pMDIs) containing ethanol suspensions of two simulants for B. anthracis spores are considered for providing suitable aerosols. Doses and shot weights from pMDIs with canisters having volumes equal to that of 200 metering-valve actuations were constant for ≤165 actuations, but drop beyond that range. There were statistically significant dose variations between replicate pMDIs and between two types of actuators used on the pMDIs. The storage half-lives of pMDIs filled with Bacillus atrophaeus (BG) and Bacillus thuringiensis subsp. israelensis (Bti) spore formulations are predicted to be 32 and 136 months, respectively, if the canisters are stored under refrigeration (4°C). The prediction is based on use of a logarithmic regression model relating CFU per actuation to storage time, with data taken at times of 1–12 months. Demonstration of the utility of the concept was provided by producing responses from a polymerase chain reaction (PCR) identifier with pMDI-generated BG and Bti aerosols that were collected with a 100 L/min wetted wall bioaerosol sampling cyclone.

Copyright 2013 American Association for Aerosol Research     

A Water Cyclone to Preserve Insoluble Aerosols in Liquid Flow—An Interface to Flow Cytometry to Detect Airborne Nucleic Acid

A miniature cyclone was designed to gently capture fine aerosols into a continuous liquid flow. The geometry of the cyclone was designed so that the friction of the turning air swirls a 100 μl volume of water at the base of the cone, creating a standing liquid vortex which coats the inside deposition surface. The collection efficiency of the cyclone was characterized as a function of insoluble particle size, both in stand-alone operation and preceded by aerosol growth by water vapor condensation. The aerosol growth lowered the smallest collected particle size and created synonymous sample-into-substrate material conditions at the point of impact. The cyclone collection efficiencies were higher than 88% for the fluorescent polystyrene latex bead diameter sizes 50–3000 nm. The cyclone was further interfaced to a flow cytometer to detect airborne nucleic acid (as a virus test aerosol) in the cyclone sample flow. The flow cytometer, which is commonly used for single cell identification via fluorescence, was modified to accept a continuous sample flow (nominal 60 μl min^?1) from the cyclone for real-time detection. A rod-shaped plant virus (Tobamovirus) and a protein-enveloped insect virus (Baculovirus) were aerosolized, collected by the cyclone, and stained inline using the nucleic acid dyes SYBR Green I, SYTO-9, and SYTO-24 (Molecular Probes, Inc.). In addition, an Environmental Scanning Electron Microscope (ESEM) was used to confirm the collection of single virus particles and qualitatively evaluate the degree to which the aerosolization and collection process affected the integrity of the virus.     

Cyclone Washer Adapted from an American‐Type Cyclone Separator

A modified cyclone washer was designed, fabricated, and its collection efficiency evaluated. This equipment consists of an American‐type cyclone separator with a triple cone and a spray nozzle was introduced into its cylindrical body. The study consisted of an experimental evaluation of the operating conditions at ambient and higher than ambient temperatures, varying chimney height and water flow rate, with the purpose of humidifying the dust. The collection efficiency of the cyclone washer was evaluated particles of micronized quartz with an average diameter of 7.48 μm and a density of 2.650 g/cm³. The amount of particles varied from 20–100 mg/m³ of air. An average efficiency of 97.07 ± 1.03 % was obtained with four spray nozzles, a chimney height of 0.645 m and 0.358 m³/s of gas.     

Investigation of Cut-Off Sizes and Collection Efficiencies of Portable Microbial Samplers

This research investigated the physical collection efficiencies and cut-off sizes of SMA MicroPortable, BioCulture, Microflow, Microbiological Air Sampler (MAS-100), Millipore Air Tester (MAT), SAS Super 180, and RCS High Flow portable microbial samplers when collecting Polystyrene Latex particles ranging from 0.5 to 9.8 μm in aerodynamic size. Traditional collection efficiency measurements often directly compare particle concentrations upstream and downstream of the sampler without considering the particle losses. Here, we developed a new approach which tests collection efficiencies of the sampler with and without agar collection plate loaded. This method thus allows estimating the effective collection efficiency, i.e., the fraction of incoming particles deposited onto the agar collection medium only. The experimental cut-off sizes, or d ₅₀, of the investigated samplers ranged from 1.2 μm for the RCS High Flow, 1.7 μm for the MAS-100, 2.1 μm for SAS Super 180, to 2.3 μm for MAT; for other three samplers they were close to or above 5 μm. In most cases the theoretical d ₅₀ was lower than the experimental value, which was likely due to the dissipation of impactor jets and the influence of cross-flow in the multi-nozzle impactors. For most samplers, we observed a notable difference between the collection efficiency obtained by the traditional measurement method and the effective collection efficiency. In general, all samplers collected 10% or less of 0.5 μm particles onto the agar medium. This study indicates that the use of most of the tested bioaerosol samplers may result in a substantial underestimation of bacterial concentrations, especially of single bacterial cells with diameter 0.5–1.0 μm. On the other hand, most of the investigated samplers would be more efficient when collecting larger fungal spores.     

Design and Characterization of a New Coarse Particle Collector Based on Microtrap Impactor Technology

A microtrap inertial impactor has been developed and characterized for use as an area or personal sampler. The microtrap impactor utilizes a high-density multijet plate to direct airflow and a matched multiwell plate to impact and collect particles for extraction with a reduced pressure drop relative to inertial impactors with fewer jets. Reported here is the characterization of the microtrap impactor using a fluidized bed aerosol generator and a small volume nebulizer to generate particles of Arizona Road Dust, potassium chloride, and oleic acid. Collection efficiency was determined by measuring particle size distributions with an aerodynamic particle sizer. Two geometries of the microtrap were tested suitable for a two-stage coarse particle sampler, with 1–4 μm and a 4–10 μm stages. The 1 μm cut-point microtrap stage has a collection efficiency above 97% for particles greater than 2 μm in diameter (at a 10 L/min flow rate and a pressure drop of 0.12 kPa). This stage's collection efficiency was constant for a period of time up to 10 h under typical ambient conditions without any coating on the impaction surface. The microtrap impactor provides an improvement in area sampling due to its high collection efficiency at a low pressure drop across the device, and its use of an uncoated impaction surface allowing for the extraction and analysis of biological samples.

© 2013 American Association for Aerosol Research     

防返混锥对双循环旋风器性能的影响研究

前面的研究表明,双循环旋风分离器的设计使得大于3μm颗粒的分离效率接近100%。本文通过CFD模拟软件Fluent 6.2对带有防返混锥的双循环旋风分离器内的压力场和颗粒轨迹进行了数值模拟,并与实验结果进行了比较,模拟结果和实验结果基本一致。模拟得出防返混锥可使分离器的阻力系数增加12%,并减小灰仓内3μm以下颗粒的返混量。实验结果表明,进口气速在8～21m/s时,防返混锥可使主进口和回流口的阻力系数分别增加14.6%和11.8%;当进口平均气速在15～19m/s时,若采用主进口进料,防返混锥可使总分离效率提高0.15%～0.2%;若采用回流口进料,可提高1.5%～2%。