Slot virtual impactors provide an efficient low power method of concentrating aerosols. A circumferential slot virtual impactor (CSVI) is especially effective because it has a continuous slot, and, therefore, has no losses associated with the ends of the slot. A CSVI can also fit a longer slot in a smaller footprint than a linear slot virtual impactor. A two-stage circumferential slot virtual impactor system has been designed and tested. The CSVI units are similar in principle to that tested by Haglund and McFarland (2004). Specific geometric changes to the nozzle region were introduced based on the numerical models of Hari (2005). The greatest change to the nozzle geometry of Haglund and McFarland (2004) is the introduction of a radius on the accelerator nozzle. The radius on the accelerator section allows larger particles to make a smoother transition into the focused jet. The smoother transition reduces the amount of wall losses for larger particles. The geometric changes show a significant increase in the particle size range that the virtual impactor can effectively concentrate. The extension of the dynamic range of the improved geometry was evident in the results for both the 100 L/min first stage and the 10 L/min second stage CSVI units. The two stages were tested individually and in series where the nozzle Reynolds number was 250 for both units.The results of the experiments on the two stage CSVI system showed a peak collection efficiency of 90%. The first and second stage had a Stokes cutpoint of 1.2, corresponding to a particle size of about 2.5

The Circumferential Slot Virtual Impactor (CSVI) and The XMX/2A are two virtual impactors designed for sampling aerosol particles from a dilute environment by separating the aerosol into a fine and a coarse particle fraction. Dust particles in the ambient air may deposit within the virtual impactors and affect their performances. In this study the effect of dust loading within the CSVI on the efficiency of transmission was determined for particles from 0.49 to 9.9 mm in aerodynamic diameter (AD), and the performance of the three stage XMX/2A aerosol concentrator was characterized with 1 mm-9.9 ?m AD polystyrene latex microspheres (PSL). In the first experimental configuration, the two-stage CSVI had a first stage inflow of 100 L/min and a second stage minor flow of 1 L/min, each stage operating at an inflow/minor flow ratio of 10. An In-line Virtual Impactor (IVI) was used as a pre-separator for sampling inlets to exclude large particles. When the 100 L/min IVI with the two-stage CSVI was tested with Arizona Fine Road Dust (ARD A-2) particles, the transmission efficiency dropped to 50% when the dust entering the two-stage CSVI accumulated to about 100 mg. When it was tested with ASHRAE dust, a decrease of 43% in the efficiency was detected after more than 200mg ASHRAE dust entered the two-stage CSVI. After cleaning the CSVI unit, the transmission efficiency returned to 99%, which indicated that the dust dissemination resulted in the plugging of the CSVI unit. The transmission efficiency of CSVI dropped more quickly below 50% when tested with the ARD A-2 dust, which had smaller particle sizes. In the second configuration, XMX/2A, a three-stage aerosol concentrator designed to draw 800 L/min of air was used at a measured sampling flow rate of 742 L/min. XMX/2A equipped with an inlet was tested with 1 mm-9.9 mm AD PSL in a testing chamber. The peak transmission efficiency of XMX/2A was 39.5% for 8 mm AD PSL. By using the room air as cooling air and introducing dilution air to the flow cell, the transmission efficiency of each particle size increased. A combination of monodisperse PSL and oleic acid particles represent the performance of CSVI. In the IVI-CSVI dust test, the CSVI unit SN003 had the best performance when tested with ASHRAE dust. XMX/2A had relatively low transmission efficiency when tested with PSL particles in the chamber.

Two experimental configurations of linear slot nozzle virtual impactors were constructed and experimentally investigated for use as bioaerosol concentrators. In one configuration, the Linear Slot Virtual Impactor (LSVI), the nozzle was a straight slot having a length of 89 mm (3.5"). In the second configuration, the Circumferential Slot Virtual Impactor (CSVI), the nozzle was curvilinear following a circular path having a diameter of 152.4 mm (6.0") and the resulting total slot length was 479 mm (18.8"). Multiple prototypes of the two configurations were constructed having nozzle widths that varied from 0.508 mm (0.015") to 0.203 mm (0.008"). Optical and physical measurements were made of the nozzle dimensions in the critical region of the virtual impactor units. For the LSVI units the misalignment between the acceleration nozzle and the receiver nozzle was measured between 6 µm (0.00025") and 29 µm (0.00114"). This represented a range of 2% to 10% misalignment relative to the acceleration nozzle width. The CSVI Unit 1 and 2 misalignments were measured to be 15 µm (0.00061") and 9 µm (0.00036"), or 10% and 1.8% relative misalignment, respectively. The virtual impactors were tested with liquid and solid monodisperse aerosol particles. For operation at flow rate conditions predicted from the literature to produce a cutpoint of 0.8 µm AD, an acoustic resonance was observed, corresponding to significant nozzle wall losses of particles and an absence of normal particle separation in the virtual impactor. The onset of the resonance phenomenon was observed to begin at a nozzle Reynolds number of approximately 500 for the LSVI configuration, and 300 for the CSVI configuration. For flow rates just below the onset of resonance, normal virtual impactor behavior was observed. The value of Stk50 was 0.58 for both devices, corresponding to a particle cutpoint size of 1.1 µm AD for the LSVI configuration and 2.2 µm AD for the CSVI. The collection efficiency was greater than 72% for all particle sizes larger than twice the cutpoint up to the largest particle size tested (≈ 10 µm AD). The peak collection efficiency for both concentrators was greater than 95%.

The characteristics of liquid and solid aerosols have been intensively investigated by means of optical diagnostic techniques. Part I describes the characteristics of liquid aerosol formation formed by heat transfer fluids (HTFs) from bulk liquids. Part II investigates the characteristics of convective transport behavior of solid particles in virtual impactor (VI). The objective of part I is to establish correlations which offer predictions on atomized particle size of HTFs which are widely and commonly used in process industries. There are numerous reports stating that mist explosions formed from leakage cause disastrous accidents in process industries. For safety concerns, the characteristics of mist formation should be known in order to prevent HTFs from catching on fire or exploding. The empirical data on formation of mist are collected by the optical measurement technique, the Fraunhofer diffraction. The Buckingham-PI theorem is applied to establish a correlation between empirical data and representative physical properties of HTFs. Final results of correlations are solved by a statistical method of linear regression. The objective of part II is to investigate the characteristics of convective transport behavior in virtual impactor (VI) which is used to sort polydisperse precursor powder in the process industries of superconductor wire. VI is the device to separate polydisperse particles as a function of particle size by using the difference in inertia between different sizes of particles. To optimize VI performance, the characteristics of convective transport should be identified. This objective is achieved by visualization techniques. The applied visualization techniques are Mie-scattering and laser induced fluorescence (LIF). To investigate analytically, a local Stokes number is introduced in order to offer criteria on predicting the efficiency of VI performance and boundary effect on particle separation. The achieved results can enhance performance and eliminate defects by having knowledge of the behavior of solid particles in VI.

The characteristics of liquid and solid aerosols have been intensively investigated by means of optical diagnostic techniques. Part I describes the characteristics of liquid aerosol formation formed by heat transfer fluids (HTFs) from bulk liquids. Part II investigates the characteristics of convective transport behavior of solid particles in virtual impactor (VI). The objective of part I is to establish correlations which offer predictions on atomized particle size of HTFs which are widely and commonly used in process industries. There are numerous reports stating that mist explosions formed from leakage cause disastrous accidents in process industries. For safety concerns, the characteristics of mist formation should be known in order to prevent HTFs from catching on fire or exploding. The empirical data on formation of mist are collected by the optical measurement technique, the Fraunhofer diffraction. The Buckingham-PI theorem is applied to establish a correlation between empirical data and representative physical properties of HTFs. Final results of correlations are solved by a statistical method of linear regression. The objective of part II is to investigate the characteristics of convective transport behavior in virtual impactor (VI) which is used to sort polydisperse precursor powder in the process industries of superconductor wire. VI is the device to separate polydisperse particles as a function of particle size by using the difference in inertia between different sizes of particles. To optimize VI performance, the characteristics of convective transport should be identified. This objective is achieved by visualization techniques. The applied visualization techniques are Mie-scattering and laser induced fluorescence (LIF). To investigate analytically, a local Stokes number is introduced in order to offer criteria on predicting the efficiency of VI performance and boundary effect on particle separation. The achieved results can enhance performance and eliminate defects by having knowledge of the behavior of solid particles in VI.

A circumferential slot In-line Virtual Impactor (IVI) has been designed using Computational Fluid Dynamics (CFD) simulation tools and experimentally characterized using monodispersed liquid aerosols to validate simulation results. The base design, IVI-100, has an application as a pre-separator for sampling inlets, where the device scalps large particles from the aerosol size distribution. The IVI-100 samples air in at 111 L/min and deliver the fine aerosol fraction in a 100 L/min flow and provide a cutpoint particle size of 10 µm, with a pressure drop of 45 Pa. An inverted dual cone configuration encased inside a tube provides the IVI-100 with a characteristic circumferential slot of width 0.254 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. Good agreement is obtained between numerically predicted and experimentally observed performance. An aerosol size selective inlet for bioaerosol and other air sampling applications using an upgraded prototype of IVI-100, mounted inside a BSI-100 inlet shell was tested in an aerosol wind tunnel over a speed range of 2 – 24 km/hr. The BSI-IVI-100 inlet has a cutpoint of 11 µm aerodynamic diameter and delivers the fine fraction at 100 L/m. The geometric standard deviation of the fractionation curve is 1.51 and the performance is not affected by wind speeds. An IVI-350, which is an adaptation of the IVI to be used as a powder fractionator, was designed based on computational simulations, and provides a cutpoint of 3 µm AD, while operating in a total flow rate of 350 L/min. Four Identical IVI -350 units will be operated in parallel to fractionate aerosolized powders in a 1400 L/min flow. An optimized inlet, with a contoured tear-drop shaped insert provides uniform flow to four identical IVI units and prevents powder accumulation in the system entrance.

Two linear slot virtual impactor arrangements were developed and investigated in this study. Both arrangements encompassed two-stage impaction for concentration of bioaerosols. The first arrangement consisted of eight linear slot impactors in parallel for the first stage with the designed dimensions of 87 mm (3.4”) for the throat length, 0.305 mm (.012”) for the accelerator throat width, and 0.457 mm (.018”) for the receiver throat width. The second stage contained a single unit with the designed dimensions of 71 mm (2.8”) for the throat length, 0.36 mm (0.014”) for the accelerator throat width, and 0.49mm (0.019”) for the receiver throat width. The second arrangement contained a single impactor for the first stage with a designed throat length of 87 mm (3.4”), a designed accelerator throat width of 0.43 mm (.017”), and a designed receiver throat width of 0.63mm (.025”). The second stage also contained a single impactor with a designed throat length of 8.73 mm (3.4”), a designed accelerator throat width of .43 mm (.017), and a designed receiver throat width of 0.63 mm (0.25”). To verify the tolerances of the machined impactors, optical measurements were made. Both arrangements were subjected to liquid and solid particle tests and have a theoretical concentration factor of 100X. The arrangements were tested at flow rates that ranged from 10 L/min to 1000 L/min, where the collection efficiency of the minor flow was determined. An unknown acoustical phenomenon was present during aerosol tests at elevated flow rates causing low minor flow collection efficiencies. In order to test the impactors at elevated flow rates, the acoustical generation phenomenon was systematically studied and suppressed. The cutpoint for the first arrangement was 1.3µm AD, and the cutpoint for the second arrangement was 1.0 µm AD. The average Stk50 for both arrangements was 0.71. The throat velocity through the impactors ranged from 21.8 m/s to 73 m/s, and the peak efficiency for these specific throat velocities ranged from 99% to 74%, respectively.

<p> A virtual impactor type dust flow concentrator was developed and an experimental investigation was performed to characterize the hydraulic and particulate matter (PM) separation performance of the device. In particular, the pressure drop characteristics, the ratio of the flow through the two branches, and the PM concentration in the minor and main branch of the flow concentrator were evaluated using experiments on a diesel exhaust rig and an air rig. Tests were performed to examine the effect of the inlet flow Reynolds number, the inlet tube lip position relative to the concentration probe, and the flow distribution between the minor and main branches. Numerical simulations were also performed for a simplified concentrator geometry to examine the flow streamlines and pressure drop. The results showed that the ratio of the concentration in the minor and main flow branches changed as the flow rate in these branches changed, reaching a maximum at a minor flow ratio that depended on the lip position. It was observed the difference of the particulate matter concentration in the minor and main branch was greater for higher inlet Reynolds number. For example, the concentration ratio increased 50% as the Reynolds number increased from 2,200 to 25,700. A similar result was observed when the inlet tube lip was moved further into the concentration body. The pressure losses seem to be mainly caused by the changes in flow directions and the change in the cross sectional areas.</p> / Thesis / Master of Applied Science (MASc)

Transport of solid particles in a slit virtual impactor has been analyzed using visualization techniques. Particle trajectories were observed using laser-induced fluorescence of monodisperse particles seeded in the virtual impactor flow. It was observed from these trajectories that for smaller inertia particles essentially followed the flow streamlines, whereas higher inertia particles tend to deflect from their initial streamlines. These transport characteristics were used to determine particle collection efficiency curves, and the percentage of defect particle transmission, particles transmitted to the major flow that are well beyond the experimentally determined 50% cutoff. Defect percentages were found to be in good agreement with those based on a local stokes number approach, an analytical model using a converging flow velocity profile. It was hypothesized that these defects occur by virtue of larger particles passing through the near wall flow region and consequently transported to the major flow. The trajectories of such defect occurrences clearly show that these particles originated in the near wall region. Performance at higher mass loadings was evaluated using a background dust matrix generated by a turntable aerosol generator. At high mass loadings, clogging of the slit led to the deterioration of the impactor's performance. The time taken to clog the silt was estimated by modeling the slit edge as a single filter fiber of rectangular cross section with the primary mechanism of filtration being interception and was found to be in good agreement with the experimental data. Elimination of defect transmission and clogging would be possible by the provision of a sheath airflow, which ensures that the near wall regions are free of particles.

Transport of solid particles in a slit virtual impactor has been analyzed using visualization techniques. Particle trajectories were observed using laser-induced fluorescence of monodisperse particles seeded in the virtual impactor flow. It was observed from these trajectories that for smaller inertia particles essentially followed the flow streamlines, whereas higher inertia particles tend to deflect from their initial streamlines. These transport characteristics were used to determine particle collection efficiency curves, and the percentage of defect particle transmission, particles transmitted to the major flow that are well beyond the experimentally determined 50% cutoff. Defect percentages were found to be in good agreement with those based on a local stokes number approach, an analytical model using a converging flow velocity profile. It was hypothesized that these defects occur by virtue of larger particles passing through the near wall flow region and consequently transported to the major flow. The trajectories of such defect occurrences clearly show that these particles originated in the near wall region. Performance at higher mass loadings was evaluated using a background dust matrix generated by a turntable aerosol generator. At high mass loadings, clogging of the slit led to the deterioration of the impactor's performance. The time taken to clog the silt was estimated by modeling the slit edge as a single filter fiber of rectangular cross section with the primary mechanism of filtration being interception and was found to be in good agreement with the experimental data. Elimination of defect transmission and clogging would be possible by the provision of a sheath airflow, which ensures that the near wall regions are free of particles.