Joshua Allen Hubbard

Experimental Study of Electrostatic Aerosol Filtration at Moderate Filter Face Velocity

Aerosol collection efficiency was studied for electrostatically charged fibrous filters (3M Filtrete™, BMF-20F). In this study, collection efficiencies at moderate filter face velocities (0.5–2.5 m/s) representative of some high volume sampling applications was characterized. Experimental data and analytical theories of filter performance are less common in this flow regime since the viscous flow field assumption may not be representative of actual flow through the filter mat. Additionally, electrostatic fiber charge density is difficult to quantify, and measurements of aerosol collection efficiency are often used to calculate this fundamental parameter. The purpose of this study was to assess the relative influence of diffusion, inertial impaction, interception, and electrostatic filtration on overall filter performance. The effects of fiber charge density were quantified by comparing efficiency data for charged and uncharged filter media, where an isopropanol bath was used to eliminate electrostatic charge. The effects of particle charge were also quantified by test aerosols brought into the equilibrium Boltzmann charge distribution, and then using an electrostatic precipitator to separate out only those test particles with a charge of zero. Electrostatically charged filter media had collection efficiencies as high as 70–85% at 30 nm. Filter performance was reduced significantly (40–50% collection efficiency) when the electrostatic filtration component was eliminated. Experiments performed with zero charged NaCl particles showed that a significant increase in filter performance is attributable to an induction effect, where electrostatic fiber charge polarizes aerosol particles without charge. As filter face velocity increased the electrostatic filtration efficiency decreased since aerosol particles had less time to drift toward electrostatically charged fibers. Finally, experimental data at 0.5 m/s were compared to theoretical predictions and good agreement was found for both electrostatic and nonelectrostatic effects.

Experimental study of impulse resuspension with laser Doppler vibrometry.

Particle resuspension due to mechanical impulse was studied for spherical polymethylmethacrylate (pmma) particles ranging from 1.7 to 14.4 μm in diameter on titanium dioxide (TiO2) and silicon dioxide (SiO2) wafers. Dry powders were dispersed, electrostatically neutralized, and allowed to deposit under the influence of gravity. Contaminated surfaces were then mechanically excited with a 5 MHz piezoelectric transducer where surface accelerations (∼106 m/s2) and resuspension ratios were quantified with laser Doppler vibrometry (LDV) and digital microscopy, respectively. For TiO2, experiments were performed over a broad range of relative humidity (25 to 95%) to assess the effects of capillary condensation. Resuspension was a monotonically decreasing function of relative humidity. Existing theories were used to separate data into two adhesion regimes based on capillary bridge formation: van der Waals (vdW) and capillary dominated adhesion. For relative humidity above 60%, resuspension forces were nondimensionalized by the theoretical capillary force. Resuspension data for all particle sizes and relative humidity were described by a single sigmoid function dependent on the dimensionless resuspension force. Below 60% relative humidity, resuspension forces were nondimensionalized by the vdW force calculated with Johnson–Kendall–Roberts adhesion theory. The experimental work of adhesion (pmma-TiO2) was optimized such that the dimensionless resuspension curves, for capillary and vdW forces, had equivalent dimensionless resuspension forces at 50% resuspension. The calculated value, 0.047 J/m2, was within the range of values expected from other published works. Resuspension was not observed for particles on SiO2 substrates. This result was attributed to electrostatic surface charge patches where particle charge and surface resistivities were measured to analyze the relative influence of electrostatic adhesion forces. Copyright 2012 American Association for Aerosol Research

High-Volume Aerosol Filtration and Mitigation of Inertial Particle Rebound

The performance of electrostatically charged blown microfiber filter media was characterized for high-volume sampling applications. Pressure drop and aerosol collection efficiency were measured at air pressures of 55.2 and 88.7 kilopascals (kPa) and filter face velocities ranging from 2.5 to 11.25 meters per second (m/s). Particle penetration was significant for particles above 0.5 micrometers (μm) in aerodynamic diameter where the onset of particle rebound was observed as low as 200 nanometers (nm). Particle retention was enhanced by treating filters in an aqueous solution of glycerol. Adding this retention agent eliminated electrostatic capture mechanisms but mitigated inertial rebound. Untreated filters had higher nanoparticle collection efficiencies at lower filter face velocities where electrostatic capture was still significant. At higher filter face velocities, nanoparticle collection efficiencies were higher for treated filters where inertial capture was dominant and particle rebound was mitigated. Significant improvements to microparticle collection efficiency were observed for treated filters at all air flow conditions. At high air pressure, filter efficiency was greater than 95% for particles less than 5 μm. At low air pressure, performance enhancements were not as significant since air velocities were significantly higher through the fiber mat. Measured single fiber efficiencies were normalized by the theoretical single fiber efficiency to calculate adhesion probability. The small fiber diameter (1.77 μm) of this particular filter gave large Stokes numbers and interception parameters forcing the single fiber efficiency to its maximum theoretical value. The adhesion probability was plotted as a function of the ratio of Stokes and interception parameter similar to the works of others. Single fiber efficiencies for inertial nanoparticle collection were compared to existing theories and correlations. Copyright 2014 American Association for Aerosol Research

Aerosol filtration testing for enhanced performance of radionuclide monitoring stations

Aerosol filtration was studied to improve U.S. Radionuclide Monitoring Station (RMS) performance for Nuclear-Treaty-Verification. We characterized the performance of three filter materials which employed electrostatically charged filter fibers to enhance nanoparticle collection. Particle-pre-charging, a well-established industrial technique, was tested and enhanced aerosol collection efficiencies. Test results indicated it may be possible to reduce baseline radionuclide sensitivity to approximately 55–60% of current values by increasing the volume of air sampled. Engineering analysis suggested that particle-pre-charging may be a viable technical approach for fielded RMS systems.

A laboratory exposure system to study the effects of aging on super-micron aerosol particles

A laboratory system was constructed that allows the super-micron particles to be aged for long periods of time under conditions that can simulate a range of natural environments and conditions, including relative humidity, oxidizing chemicals, organics and simulated solar radiation. Two proof-of-concept experiments using a non-biological simulant for biological particles and a biological simulant demonstrate the utility of these types of aging experiments. Green Visolite®, which is often used as a tracer material for model validation experiments, does not degrade with exposure to simulated solar radiation, the actual biological material does. This would indicate that Visolite® should be a good tracer compound for mapping the extent of a biological release using fluorescence as an indicator, but that it should not be used to simulate the decay of a biological particle when exposed to sunlight. The decay in the fluorescence measured for B. thurengiensis is similar to what has been previously observed in outdoor environments.