John S. Haglund

A circumferential slot virtual impactor

A virtual impactor aerosol concentrator has been developed that uses circumferential slots for acceleration of aerosol particles and for collection of the coarse fraction. This allows for accurate and economical machining of small slot widths, which leads to low-pressure losses for the separation process. One important application of the device is in the concentration of bioaerosols, especially for military field applications where minimization of power consumption is necessary. A prototype configuration of the circumferential slot virtual impactor (CSVI), which was designed using numerical methods, was constructed and tested. The device has a curvilinear slit nozzle with a diameter of 150.3 mm (5.918 in), which provides a total slot length of 472 mm. Its slot width was 0.499 mm (0.0197 in). According to Loo and Cork, for circular-jet virtual impactors the misalignment between the axis of the acceleration jet and the receiver nozzle will cause an increase in wall losses of about 1.6% for each 1% of misalignment. Measurements were made of the nozzle dimensions in the critical region of the CSVI that showed 1.8% relative misalignment. When this prototype was operated at a flowrate of 122 l/min and a flow fraction (minor air flowrate/total air-flowrate) of 10%, the cutpoint was 2.2 μm aerodynamic diameter and the corresponding cutpoint Stokes number was 0.58. The collection efficiency was greater than 72% for particle sizes larger than twice the cutpoint, up to the largest particle size tested (10 μm aerodynamic diameter). The peak collection efficiency was greater than 95%. For virtual impactors, a critical performance parameter is the loss of particulate matter to the inner walls of the system. For the prototype system, where numerical methods had been used to generate designs that reduced wall losses, the losses at the cutpoint size of 2.2 μm aerodynamic diameter, are approximately 3%. For an operational condition of a total flowrate of 122 l/min and a coarse particle flow fraction of 10%, the pressure drop across the major flow stream (fine particle stream) was 63 Pa (0.25 in of water), with an ideal power consumption of 0.14 watts.

Wetted Wall Cyclones for Bioaerosol Sampling

A wetted wall bioaerosol sampling cyclone with an aerosol sampling flow rate of 1250 L/min and a continuous liquid outflow rate of about 1 mL/min was developed by upgrading an existing system. The aerosol-to-hydrosol collection efficiency curve for the upgraded device was shown to have a cutpoint of 1.2 μ m aerodynamic diameter (AD) and an average collection efficiency of 90% over the size range of 2 to 10.2 μ m AD. Tests with near-monodisperse cells and clusters of Bacillus atrophaeus (aka BG) spores showed an average aerosol-to-hydrosol collection efficiency of 98% over the size range from 1.7 to 9.8 μ m AD. Pressure drop across the cyclone, which is also the ideal specific power, was 5.5 kPa (22 inches H2O). Stokes scaling was used to design geometrically similar cyclones with nominal air sampling flow rates of 100 and 300 L/min. Extensive tests were performed with the 100 L/min unit and check tests with the 300 L/min. Results with the scaled units showed similar, although somewhat lower collection efficiencies than the 1250 L/min device, but with lower consumption of liquid and lower pressure losses. For the 100 L/min cyclone, the cutpoint of the aerosol-to-hydrosol efficiency curve was 1.2 μ m AD, and the average collection efficiency for single cells and clusters of BG spores was 86% over a size range of 1.2 to 8.3 μ m AD. Also, for the 100 L/min cyclone, typical output liquid flow rates were 100 μ L/min, and the pressure loss was 1.6 kPa (6.4 inches H2O).

Evaluation of Mixing Elements in an L-Shaped Configuration for Application to Single-Point Aerosol Sampling in Ducts

Experimental data for velocity and tracer gas concentration profiles were collected at several downstream locations of an L-shaped configuration with different mixing elements. Results were presented in the coefficients of variation (COVs) to help determine appropriate locations for single-point sampling downstream of each duct configuration. Comparisons between experimental data and numerical simulations for velocity COVs were also presented. Different mixing elements were applied to the L-shaped configuration to create additional mixing to help provide an acceptable sampling location with low pressure drop for single-point sampling in a duct at less than four duct diameters from the mixing element. The mixing elements included a 90° elbow, a tee, a horizontal generic-tee-plenum (H-GTP) system, and a vertical generic-tee-plenum (V-GTP) system. The COVs for velocity and gas concentration for the two GTP systems were all determined to be less than 13% as compared to a range of 6% to 89% for the 90o elbow and a tee at four duct diameters in round and square ducts. Tests with two different sizes of GTPs were conducted and the results showed the performance of the GTPs to be relatively unaffected by either size or velocity as reflected by the Reynolds number. The pressure coefficient was approximately 0.82 for the H-GTPs, as compared to 5.2 for a previously designed generic mixing system (GMS). The GTPs can be useful in the design of biological and chemical sampling systems in air-conditioning ducts and for mixing in aerosol wind tunnels where uniform aerosol concentrations are needed at the test section.

Evaluation of a High Volume Aerosol Concentrator

A virtual impactor sampler, which is designed to concentrate aerosols from a 1000 L/min ambient air sample into a 1 L/min exhaust airflow stream, was tested with near monodisperse aerosols in aerosol wind tunnels to characterize sampling performance. New methodology is introduced to correct results for the presence of doublet and satellite aerosol particles that can be present in the particle size distribution from a vibrating jet atomizer. Aerosol penetration from the free stream near the sampler inlet to the outlet of the device has a peak value of 78% at a particle size of 3.9 w m AD. Sampling effectiveness, which is the mean penetration over the size range of 2.5 to 10 w m AD, is 48%. There are 4 virtual impaction stages in the sampler, and examination of the regional losses shows that most of the aerosol deposition occurs on surfaces of the last 2 stages. The ideal power expenditure of the sampler (excluding electrical and frictional losses in the motor and bearing losses in the blower) is 58 watts as compared to the actual power consumption of 320 watts.

Heat Transfer Study on A Bioaerosol Sampling Cyclone

Abstract Computational fluid dynamics, supplemented by theoretical analyses, is used to study turbulent heat transfer in a wetted wall bioaerosol sampling cyclone. The cyclone is designed to operate at temperatures as low as -20ºC so heat must be applied to prevent the liquid (water) from freezing. For the particular application of interest, which is a cyclone with a flow rate of 1250 L/min, the electrical power for heating is limited to 350 W and the local temperature of the heated wall must be properly controlled. The inner wall of the cyclone has a liquid film, which is formed from impact of droplets that are created from atomization of the liquid upstream of the cyclone body. Calculations showed that the mean diameter of the droplets was about 42 μm and they would not freeze in air at temperatures as low as -40ºC. A commercially available CFD package, Fluent, was used in the numerical simulations to predict the turbulent heat transfer coefficients at the cyclone surface, to design heaters, and to study the temperature response of the cyclone wall. A skimmer, which is used to separate the collection liquid from the air stream, was re-designed based on CFD findings. Compared with an earlier cyclone, the power consumption for heating is significantly reduced, yet the new system can be operated at lower temperatures with higher air flow rates. CFD predictions were experimentally tested and good agreement was obtained.

Aerosol Deposition on Electroformed Wire Screens

Insect screens, which are usually an integral component of an air sampling inlet, can cause inadvertent deposition of larger aerosol particles. Numerical and experimental studies were performed to characterize aerosol deposition on commercially available electroformed wire screens for aerodynamic particle diameters between 3 and 20 μ m, Stokes numbers between 0.49 and 20, wire widths between 35 and 160 μ m, and screen open area fractions of 0.56 to 0.90. With increasing values of Stokes numbers, the actual collection efficiency increases gradually to a maximum value that is asymptotic to the fraction of open area. Deposition is characterized in terms of a standardized screen efficiency, which is the actual efficiency divided by the areal solidarity (1–fraction of open area). A correlation equation has been developed for the electroformed mesh screens, which relates the standardized efficiency to the fraction of open area, the Stokes number, the interception parameter, and the Reynolds number based on wire size. Data obtained from experimental studies with two screen types and numerical studies with those, plus two additional screen types, over a wide range of Stokes numbers and wire Reynolds number (Re w ) from about 0.5 to 30, collapse to a single correlation curve, which is valid for the range of variables tested. The pressure drop across the screens is low, on the order of 1 Pa for face velocity values that are on the order of 1 m/s. A regression analysis was used to obtain coefficients that fit the results of the numerical experiments to an existing pressure loss model.

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 (WWCs). 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.

Flattening Coefficients for Oleic Acid Droplets on Treated Glass Slides

Flattening coefficients for liquid oleic acid droplets were determined for droplets containing varyig concentrations of uranine when impacted on surfactant-treated glass slides. Flattening coefficients varied by surfactant and by uranine concentration. The results of this work suggest that interfacial forces between oleic acid droplets and surfactant-coated slides change with surfactant chemistry enough to have significant impacts on results from evaluation of size-selective aerosol sampler performance. The effects of uranine concentration and surfactant type should not be extrapolated from data generated using alternative surfactants when using microscopy to verify the size of monodisperse liquid particles utilized for sampler performance testing. Copyright 2012 American Association for Aerosol Research

Large Particle Penetration During PM10 Sampling

The objective of the present study was to characterize the performance of a federal reference method (FRM) PM10 size-selective inlet using analysis methods designed to minimize uncertainty in measured sampling efficiencies for large particles such as those most often emitted from agricultural operations. The performance of an FRM PM10 inlet was characterized in a wind tunnel at a wind speed of 8 km/h. Data were also collected for 20 and 25 μm particles at wind speeds of 2 and 24 km/h. Results of the present sampler evaluation compared well with those of previous studies for a similar inlet near the cutpoint, and the sampler passed the criteria required for certification as a FRM sampler when tested at 8 km/h. Sampling effectiveness values for particles with nominal diameters of 20 and 25 μm exceeded 3% for 8 and 24 km/h wind speeds in the present study and were statistically higher than both the “ideal” PM10 sampler (as defined in 40 CFR 53) and the ISO (1995) standard definition of thoracic particles (p < 0.05) for 25 μm particles leading to the potential for significant sampling bias relative to the “ideal” PM10 sampler when measuring large aerosols. Copyright 2014 American Association for Aerosol Research

Effect of convergence angle on impactor performance

ABSTRACT Two nozzles, modified and original, were tested in a sampler that was placed in a wind tunnel and penetration efficiencies, √Stk50, and slope of the performance curve were determined by challenging the sampler with fluorescent-tagged monodisperse test aerosol particles having known concentration. It was shown that a change in convergence angle of the modified nozzle can affect impactor performance. The √Stk50 for original and modified nozzles were 0.57 and 0.49, respectively. The slope of the efficiency curve for original and modified nozzles was 1.52 and 1.36, respectively.