Vibrating Orifice Monodisperse Aerosol Generator Research Articles

Penetration of charged particles through metallic tubes

Several theoretical studies have shown that charged particles increased the particle deposition efficiency in cylindrical tubes by image force. Some experimental investigations have also found that the deposition loss increased with increasing aerosol charge. However, the amount of charges on the aerosols or the electrical mobility was relatively low in these previous experimental studies. In order to extend to higher aerosol charge, a TSI vibrating orifice monodisperse aerosol generator was modified to generate 1-μm DEHS aerosols with charge up to 24,000 elementary units. Metallic tubes were employed to exclude the effect of Coulombic force. Tube diameter, tube length, and average velocity in tubes were among the major operating parameters. Aerosol charge was monitored using a TSI electrometer, while a TSI aerodynamic particle sizer was utilized to measure both aerosol concentrations and size distributions upstream and downstream of the tubes. Aerosol deposition loss in the sampling train of the TSI electrometer was measured to back-calculate the average aerosol charge. A closed-form theoretical model, showing the deposition efficiency as a function of particle charge, was validated by the experimental data produced.The aerosol deposition efficiency increased with increasing aerosol charge and tube length, due to stronger image force and longer retention time, respectively. The deposition efficiency decreased with increasing average velocity because of shorter retention time. Under the same average velocity, the deposition loss decreased with increasing tube diameter because the fraction of aerosols near the inner wall was higher for small-diameter tube than for large-diameter tube. The model developed from parabolic flow showed good agreement with most of the experimental data, except for data of highly charged particles. The discrepancy was probably due to the space charge effect.

Determination of Uniformity of Filter Deposits

Certain measurement techniques (such as the asbestos method using phase contrast microscopy) require uniform deposits of the sample on a filter. The asbestos fiber analytical methods require such uniform deposition because the analysis only observes small, randomly chosen locations on the filter. In this study, a vibrating orifice monodisperse aerosol generator was used to generate methylene blue particles. The aerosols were dried by filtered compressed air and then neutralized by inducing a charge on the droplet stream that emerged from the vibrating orifice. An Aerodynamic Particle Sizer was used to measure the number concentration and size distribution of the generated aerosol particles. Meanwhile, the filter deposits were examined via image processing, combined with statistical methods for defining uniformity. In order to better define uniformity and make the indicator more universal, the uniformity was defined as the exponential of the negative CV (coefficient of variation) value which was a transfor...

Experimental Study on the Loading Characteristics of Needlefelt Filters with Micrometer-Sized Monodisperse Aerosols

In this work, three types of needlefelt filters, made of Polyester (PE), Ryton Sulfar (RS), and Polyaramid (PA), were tested to in- vestigate the aerosol loading characteristics of fabric filters when challenged with micrometer-sized monodisperse potassium sodium tartrate (PST) particles. A fibrous filter with packing density of 9%, thickness of 0.38 mm, and fiber diameter of 5.1 θ m was included for comparison. A vibrating orifice monodisperse aerosol generator was used to produce three different sizes (5, 10, and 20 θ m) of PST particles for aerosol loading experiment. An ultrasonic atomizing nozzle and a TSI constant output nebulizer were used to generate polydisperse PST particles for the aerosol penetration test. The aerosol penetration of submicrometer-sized particles through the filters was measured by using a Scanning Mobility Particle Sizer. An Aerodynamic Particle Sizer was used to measure the penetration fraction of aerosol particles larger than 0.8 θ m. The pressure drop across the filter was monitored by using pressure transducers, which were calibrated against an inclined manometer. Airflows of 5, 10 , 20, and 30 cm/s were used to study the flow dependency. The aerosol penetration results showed that the particles larger than 3 θ m did not penetrate the clean fabric filters tested in the present study. The loading curves (plots of pressure drop against sampling time) displayed three regions: an initial region of fast increase, a transition region, and a final linear region after the dust formation point. After the formation point of the dust cake, both fabric and fibrous filters shared the same slope (of the loading curves). The slope of different regions of the loading curves was determined by many factors, such as size of challenge aerosol, face found to be critical to the performance of the fabric filters. In order lower porosity, which caused an extra rise in pressure drop across velocity, surface treatment, and the compressibility of the dust cake forming on the filter. The method of final surface treatment was to avoid the unnecessary rise in air resistance, the melting clumps formed during final surface treatment should be as thin and narrow as possible, just enough to support the filter bag cleaning. From the standpoint of filter quality and energy consumption, the low filtration velocity has to be adopted whenever possible, because high filtration velocity not only led to lower filter quality (in particular for submicrometer-sized particles) but also created dust cake of lower porosity, which caused an extra rise in pressure drop across thet dust cake.

Experimental Study on the Loading Characteristics of Needlefelt Filters with Micrometer-Sized Monodisperse Aerosols

In this work, three types of needlefelt filters, made of Polyester (PE), Ryton Sulfar (RS), and Polyaramid (PA), were tested to in- vestigate the aerosol loading characteristics of fabric filters when challenged with micrometer-sized monodisperse potassium sodium tartrate (PST) particles. A fibrous filter with packing density of 9%, thickness of 0.38 mm, and fiber diameter of 5.1 θ m was included for comparison. A vibrating orifice monodisperse aerosol generator was used to produce three different sizes (5, 10, and 20 θ m) of PST particles for aerosol loading experiment. An ultrasonic atomizing nozzle and a TSI constant output nebulizer were used to generate polydisperse PST particles for the aerosol penetration test. The aerosol penetration of submicrometer-sized particles through the filters was measured by using a Scanning Mobility Particle Sizer. An Aerodynamic Particle Sizer was used to measure the penetration fraction of aerosol particles larger than 0.8 θ m. The pressure drop across the filter was ...

The effects of particle charge on the performance of a filtering facepiece.

This study quantitatively determined the effect of electrostatic charge on the performance of an electret filtering facepiece. Monodisperse challenge corn oil aerosols with uniform charges were generated using a modified vibrating orifice monodisperse aerosol generator. The aerosol size distributions and concentrations upstream and downstream of an electret filter were measured using an aerodynamic particle sizer, an Aerosizer, and a scanning mobility particle sizer. The aerosol charge was measured by using an aerosol electrometer. The tested electret filter had a packing density of about 0.08, fiber size of 3 microns, and thickness of 0.75 mm. As expected, the primary filtration mechanisms for the micrometer-sized particles are interception and impaction, especially at high face velocities, while electrostatic attraction and diffusion are the filtration mechanisms for submicrometer-sized aerosol particles. The fiber charge density was estimated to be 1.35 x 10(-5) coulomb per square meter. After treatment with isopropanol, most of fiber charges were removed, causing the 0.3-micron aerosol penetration to increase from 36 to 68%. The air resistance of the filter increased slightly after immersion in the isopropanol, probably due to the coating of impurities in isopropanol. The aerosol penetration decreased with increasing aerosol charge. The most penetrating aerosol size became larger as the aerosol charge increased, e.g., from 0.32 to 1.3 microns when the aerosol charge increased from 0 to 500 elementary charges.

Evaluation of acoustical particle counter for the sizing of fog droplets

An acoustical particle counter (or acoustical particle sizing device) was evaluated for counting and sizing of fog droplets. Fog droplets were generated under controlled conditions in the laboratory. Fog droplet sizes were measured by an optical and an electron microscope and compared to results from the acoustical particle counter. Most of the droplets were found to be in the size range of 5-30 µm. The mean diameters estimated from the acoustical particle counter were in agreement with the microscope values of mean droplet diameter. A Rich 100 condensation nuclei monitor was also operated simultaneously during the fog droplet counting to monitor the condensation nuclei counts in the laboratory. The results indicate that condensation nuclei count is inversely correlated with the fog droplet threshold diameter. Aerosols of uniform size (35 µm) were generated by the vibrating orifice monodisperse aerosol generator and counted at three different flow rates by the acoustical particle counter. The counts/liter/minute were similar, indicating the reliability of the acoustical particle counter.

Response characteristics of the Knollenberg active scattering aerosol spectrometer to light-absorbing aerosols

Response characteristics of the Knollenberg active scattering aerosol spectrometer ASAS-300X are investigated on monodisperse lightabsorbing aerosols generated by a Berglund-Liu vibrating orifice monodisperse aerosol generator. The responses on the light-absorbing aerosols are in fairly good agreement with Mie scattering theoretical predictions. The response of the Knollenberg ASAS-300X to light-absorbing spherical aerosols is of almost monotonically increasing single-valued behavior compared to the multivalued behavior of nonabsorbing spherical aerosols, and the instrument's response to light-absorbing aerosols is closer to the manufacturer's calibration than is its response to nonabsorbing aerosols.

Low flow rate sharp cut impactors for indoor air sampling: design and calibration.

Two single round nozzle impactors have been developed for use in Harvard’s indoor air pollution health study. Both impactors operate at flow rates of 4 L/m and are nearly identical, differing only in their cut sizes of 2.5 μm and 10 μm aerodynamic diameters. Two identical cascaded stages of the same cut size are used to obtain sharp cut-off characteristics. The particles are deposited on impaction plates made of oil impregnated, porous material to reduce particle bounce and are discarded. Only the particles collected on the afterfilter are analyzed. Special care has been taken to collect the particles uniformly on the afterfilter to aid in particle analysis. The jmpactors were calibrated with a vibrating orifice monodisperse aerosol generator. However, due to the sharp cut of the impactors, doublets and triplets in the calibration aerosols, even in small quantities, gave erroneous calibration curves. Therefore, the number of doublets and triplets in the challenging aerosols were measured and appropriate c...

Investigation of airborne particles by inductively coupled plasma emission spectrometry calibrated with monodisperse aerosols

An emission spectrometric method for the investigation of airborne particles has been developed. Sample air was directly introduced into an ICP to vaporize and atomize the particles and to excite the atoms or ions. The emission signal from the ICP was observed as a flash for each particle when the particle concentration was sufficiently low. The flashes were converted into electric pulses, which were counted by a pulse height analyzer. Thus, for particles of homogeneous composition the particle concentration and size distribution were determined simultaneously for a given element. To calibrate the measuring system, monodisperse aerosols were prepared by using a vibrating orifice monodisperse aerosol generator. Sodium chloride, calcium nitrate and copper nitrate aerosols were generated using solutions of known concentrations. The concentration and size distribution of copper particles were determined in aerosols generated by a small arc at the switch of a lamp socket when a 200-W lamp was turned off. Most of the particles were less than 1μm in diameter with a peak distribution at 0.7 μm. The smallest particles measurable for calcium oxide and copper were 0.1 and 0.3 μm in diameter, respectively.

Improved liquid feed system for the Berglund-Liu vibrating orifice monodisperse aerosol generator

An improved liquid feed system for the Berglund-Liu vibrating orifice monodisperse aerosol generator (VOMAG) is described. The feed system employs a positive-displacement pump and pulse dampener to provide an accurate solution feed rate to the VOMAG. Advantages over the syringe pump feed system are reduced orifice clogging, elimination of leaks, and large solution reservoir for increased run times. Solution feed rate to the VOMAG using this feed system was reproducible to +/-2%. The system was low cost and required only a simple calibration.

NO x formation in monodisperse fuel spray combustion

Experimental results are presented for the monodisperse fuel spray combustion of isopropanol in air. A vibrating orifice monodisperse aerosol generator supplied fuel to an aerodynamically stabilized atmospheric pressure flame in a combustion tube. Measurements were made over the full range of monodisperse operating conditions encompassing droplet diameters from 14–80 μ and equivalence ratios from 0.7 to 1.4. The data include exit plane measurements of NO and NO x , temperature profiles, and supporting emissions measurements of hydrocarbons, carbon monoxide, carbon dioxide, and oxygen. The experimental results indicate that the NO x formation in the spray combustion of isopropanol is affected by the droplet diameter, equivalence ratio, fuel feed rate and the distribution of dispersion and dilution air. The most significant effect is an observed change in NO and NO x with droplet diameter; both decrease with decreasing droplet diameter, reach a minimum around 50 microns, and then increase again to a fairly constant value. At an equivalence ratio of 1.39, a 61% reduction in NO x and 87.5% reduction in NO are observed from the maximum values recorded in the two respective modes. Possible mechanism explaining these NO x results are examined and discussed. It appears that the anomalous NO x behavior is associated with droplet interactions and the transition from a diffusive type of droplet/spray burning to that of a prevaporized and premixed case. Decreasing the droplet size results in a trend of increasing droplet interactions, which suppresses temperatures and reduces NO x . This trend continues until prevaporization and mixing effects begin to dominate and the system tends towards the premixed limit.

Experimental observation of aerosol deposition in turbulent flow

The deposition rate of aerosol particles in turbulent flow has been measured in vertical pipes for nominal Reynolds numbers of 10,000 and 50,000 and for particle dia. of 1·4–21 μm using monodisperse aerosols generated by the vibrating orifice monodisperse aerosol generator. The experiments covered a τ + range of 0·21–774, where τ + is the dimensionless particle relaxation time. For small τ + values, the deposition velocity was found to increase rapidly with increasing particle size, reaching a peak at τ + = 30. Beyond τ + = 30, the dimensionless deposition velocity, V +, showed only a moderate dependence on τ +, decreasing slightly at higher τ + values. Below τ + = 10. V + was found to increase as the second power of particle relaxation time and as the fourth power of particle size. The experimental results are in good agreement with several of the existing theories in the region below τ + = 30. However, the theories are inadequate to explain the results beyond τ + = 30.