Aerosol sampling and identification is vital for assessment and control of particulate matter pollution, airborne pathogens, allergens and toxins, and their effect on air quality, human health, and climate change. Assays capable of accurate identification and quantification of chemical and biological airborne components of aerosol provide very limited sampling time resolution and relatively dilute samples. A low-cost micro-channel collector (mCC) which offers fine temporal and spatial resolution, high collection efficiency, and delivers highly concentrated samples in very small liquid volumes was developed and tested. The design and optimization of this mCC was guided by computational fluid dynamics (CFD) modeling. Collection efficiency tests of the sampler were performed in a well-mixed aerosol chamber using aerosolized fluorescent microspheres in the 0.5-6 mm diameter range. Samples were collected in the mCC and eluted into 100 mL liquid aliquots; bulk fluorescence measurements were used to determine the performance of the collector. Typical collection efficiencies were above 50% for 0.5 mm particles and 90% for particles larger than 1 mm. The experimental results agreed with the CFD modeling for particles larger than 2 mm, but smaller particles were captured more efficiently than predicted by the CFD modeling. Nondimensional analysis of capture efficiencies showed good agreement for a specific geometry but suggested that the effect of channel curvature needs to be further investigated.
The rate of particle removal from a surface by air jet impingement has been evaluated for 3 different types of trace explosives. Samples of research development explosive (cyclotrimethylenetrinitramine), trinitrotoluene, and C-4 were each transferred to glass surfaces and then subjected to a short burst of air from a jet with varying diameter, standoff distance, and backpressure to achieve a range of shear stresses at the surface. TNT was observed to be easiest to remove, while C-4 required the greatest shear force to resuspend. An analytical model has been developed to predict removal of spherical particles as a function of particle diameter and nondimensionalized downstream distance from a gas jet. This model was fitted to experimental data from the removal of ceramic microspheres of various sizes. The removal rate of these ceramic microspheres was observed to be much greater than that of the 3 types of explosive particles, despite the particles' similar sizes.
A rectangular slit micro-aerodynamic-lens (μADL) aerosol concentrator operating at atmospheric pressure has been developed. A single stage version has shown concentration ratios of up to 40:1 for 1 μm aerosol particles while particles larger than 2 μm can be concentrated by more than 100:1 in a single stage. The design of this device has been guided by unsteady 3D CFD modeling using detached eddy simulations (DES), and has been validated experimentally using polystyrene spheres and salt crystals of known aerodynamic diameters. The pressure drop in the device does not exceed 1.5 kPa in the major flow and 0.3 kPa in the minor flow at a total flow of 10 slpm. INTRODUCTIONAtmospheric aerosols from a variety of sources including natural, anthropogenic, and industrial processes, and from chemical and biological weapons (CBW) are potential threats to human health. Particle sensors designed to detect and characterize atmospheric aerosols are often flow limited; therefore, in order to provide a sufficient number of particles to the detector in an acceptably short period of time, it is necessary to sample large volumes of air at a high flow rate and concentrate the aerosol to provide a concentrated sample at a flow rate that the detector can handle. Concentrating particles in the range of 1 to 10 μm diameter is of particular interest since this size range corresponds to the size of many bio-aerosols; particles of this size have high inhalation and lung deposition efficiency; see, for example, Hinds (1999). Several approaches are currently used for aerosol particle concentration and collection in the target size range, most of which are based on the inertial properties of the aerosol particles. Devices employing these approaches include cyclones, inertial impactors, settling chambers, virtual impactors, liquid impingers, inertial particle separators, etc. Each of the devices is able to separate aerosol particles from major flow, and some
A two-wavelength transmissometer employing a He-Cd laser (lambda(1) = 0.325 microm) and a He-Ne laser (lambda(2) 3.39 microm) has been developed for measuring the Sauter mean diameter of mineral ash droplets in high-temperature high-velocity coal-fired combustion flows. From transmission measurements at the two wavelengths, it is shown that mean diameters in the 0.3-3.5-,microm range may be inferred with a weak sensitivity to particle refractive index and size distribution shape. The volume concentration or loading of the aerosol may then be determined from the measured transmission at either wavelength. The instrument has been used to measure the mean size and loading of ash droplets in a pulverized coal-fire channel flow at temperatures to ~2900 K and velocities of up to 400 m-sec(-1) for combustion MHD power generation applications.
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