DEVELOPMENT OF A DICHOTOMOUS SLIT NOZZLE VIRTUAL IMPACTOR

A high-volume slit nozzle virtual impactor has been developed to collect fine and coarse particles. The size cut-off and particle loss characteristics of the developed slit virtual impactor agree well with those of the 16.7 lmin⁻¹ commercially available dichotomous sampler. The effects of various flow and physical design parameters on the collection of both fine and coarse particles have been investigated. The results of these tests indicated that many of the theoretical principles established for round nozzle virtual impactors can be successfully applied to slit nozzle virtual impactors. However, the effects of the flow volume and Reynolds number (Re) on the cut-off behavior and particle losses are more pronounced for slit virtual impactors. The impactor's particle size cutpoint decreased as the total inlet flow and Re increased. For Re<about 7000, particle losses increased with particle size. For Re of about 7000, particle losses exhibited a maximum near the 50% cutpoint, which is typical in round nozzle virtual impactors. For Re>7000, losses of fine particles were significant, while coarse particle losses were low. Changes in the minor-to-total flow ratio and collection slit width also affected particle losses.     

Development and Evaluation of a PM 10 Impactor-Inlet for a Continuous Coarse Particle Monitor

Conventional PM 10 inlets available operate at a flow rate of 16.7 l/min. The purpose of this study was to develop and test a PM 10 inlet designed to operate at 50 l/min to be used with a recently developed continuous coarse particle monitor (Misra et al.). Laboratory tests using polystyrene latex particles established the inlet's 50% cutpoint at 9.5 w m. Further evaluation of PM 10 inlet was performed in a wind tunnel at wind speeds of 3, 8, and 24 km/h. Tests showed that the 50% efficiency cutpoint as well as the very sharp particle separation characteristics of the inlet were maintained at these wind speeds. Field evaluation of the PM 10 inlet was performed in Riverside and Rubidoux, CA. A 2.5 w m cutpoint round nozzle virtual impactor was attached downstream of the developed PM 10 inlet. The Dichotomous PM 10 Partisol Sampler, operating at a flow rate of 16.7 l/min was used as a reference sampler. The Dichotomous Partisol uses an FRM PM 10 inlet operating at 16.7 l/min to remove particles larger than 10 w m in aerodynamic diameter. Commercially available 4.7 cm Teflon filters were used in both the Partisol and the PM 10 inlet to collect particulate matter (PM). Results showed good agreement between coarse PM (2.5-10 w m) mass concentrations measured by means of the PM 10 inlet and Partisol. Chemical analyses showed excellent agreement between coarse PM concentrations of Al, K, Si, Ca, and Fe obtained by the two samplers. The agreement also persisted for nitrate and sulfate. Finally, the excellent agreement between coarse concentrations of the PM 10 inlet and Partisol persisted for wind speeds up to 19 km/h.     

Improvement of the Classification Performance of a Rectangular Jet Virtual Impactor

The effect of flow characteristics on the performance of a virtual impactor with a rectangular jet has been studied experimentally. Flow visualization revealed a three-dimensional flow structure and flow instability on the classification performance, classification experiments were carried out using the virtual impactor with an improved collection nozzle which was designed to avoid the adverse pressure gradient. The newly designed collection nozzle improved the classification performance in the high inertia region, which is larger than a cut point of the virtual impactor. However, the separation efficiency is still lower than that predicted by the theory based on the two-dimensional flow. Since the flow visualization revealed that the flow in the classification zone was partially disturbed by the new collection nozzle, the entrance part of the impactor was newly designed to introduce 2 additional clean airflows. These clean airflows prevented flow disturbance and attained the theoretical classification performance in the high inertia region.     

A Streamlined,High-Volume Particle Impactor for Trace Chemical Analysis

The design and characterization of a streamlined, high-volume particle impactor intended for use with trace chemical analysis is presented. The impactor has a single round jet and is designed to operate at a flow rate of 1000 L/min. Computational fluid dynamics (CFD) was used as a tool to optimize the aerodynamic performance of the impactor by iteratively redesigning the geometry and curvature of the internal walls. By eliminating recirculation zones within the flowfield of the impactor and using flowfield streamlines as new walls, successive designs revealed a significant reduction in the pressure drop across the impactor. Particle trajectories were simulated in the impactor and the 50% cutpoint was determined to be 1.05 μm. The impaction surface itself is easily removed from the body of the impactor assembly, potentially facilitating rapid trace chemical analysis using a variety of chemical detection techniques. A prototype impactor was fabricated with a 3D rapid prototyping printer and characterized in terms of particle cut-off diameter using test aerosols generated by an Ink Jet Aerosol Generator (IJAG) and fluorescence intensity measurements. The experimental particle cut-off diameter was not able to be measured because the smallest aerosol particles that could be tested were 1.86 μm which were collected at 100% efficiency. Particulate contamination from the high-explosive compound C4 was also collected with the impactor to demonstrate operational utility for trace explosives detection.

Copyright 2013 American Association for Aerosol Research     

Collection efficiency and particle loss of virtual impactors with different methods of increasing pressure drop

Three types of virtual impactor were designed and fabricated to evaluate the separation efficiencies and particle losses, with different methods of increasing pressure drop, but with the same ratio of major flow rate to minor flow rate. One (type A) was a virtual impactor, with nozzle and collection probe diameters of 1.6 and 2.3 mm, respectively. The second (type B) was a modified version of type A preceded by an orifice to reduce the pressure of the particle-laden air before it entered the nozzle. The third (type C) was also a modified version of type A with nozzle and collection probe diameters of 1.2 and 1.6 mm, respectively, to further increase the particle-laden air velocity of the acceleration nozzle.The separation efficiency of the type A impactor increased with an increase of the total flow rate and pressure drop, and had high particle loss at 50% cut-off diameter. The separation efficiency of the type C impactor was higher than those of type A and type B, but the total particle loss, which was almost totally due to the collection probe loss, was larger than that of the other two types in the small particle size range. The separation efficiency curve of the type B impactor was almost the same as that for the type C and the orifice particle loss increased with an increase in the particle size.     

In-Line Impactor Inlet for Bioaerosol Sampling

The proof of concept of a novel in-line real impactor (IRI) for preseparation of large particles in ambient inlets was demonstrated with a 1,250 L/min design. Numerical simulations predicted a cutpoint Stokes number 0.3 for a ratio of jet-to-plate spacing to jet width (S/W) of 2.0 and 0.5 for a ratio of 4.0. This variation in cutpoint Stokes number allows minor adjustments in cutpoint for a given device size. Experimental benchmark tests support the prediction of a shift in cutpoint with S/W. Inlet systems with flow rates of 100 and 400 L/min were designed by Stokes scaling of the 1,250 L/min IRI and integrating the lower flow devices with an existing inlet aspiration section and an insect screen. Experiments with the inlet system were conducted in a wind tunnel with particles from 3 to 20 μm aerodynamic diameter (AD) and wind speeds of 2, 8, and 24 km/h. A nominal cutpoint of approximately 11 μm AD was selected to accommodate bioaerosol sampling needs, and the wind tunnel results showed the average cutpoints of the 100 and 400 L/min inlet systems at the three wind speeds were 11.2 and 11.6 μm AD, respectively. Stand-alone tests with the 100 and 400 L/min IRIs were conducted where dry dusts (Arizona road dust/fine and coarse) were impacted on three types of collection surfaces (dry, grease-coated, and oil-soaked porous surfaces) to characterize solid particle carryover. The oil-soaked porous media allowed the least carryover of large solid particles.     

雾化器结构参数对循环流化床锅炉SNCR脱硝性能影响研究

还原剂雾化质量对循环流化床锅炉旋风分离器SNCR脱硝效率具有重要影响,为研究空气雾化喷嘴结构参数对雾化质量及脱硝效率的影响,采用数值模拟的方法对喷嘴的4个结构参数即撞击件长度、出口直径、混合室长度和气液入口交角,进行单因素分析和正交数值试验,结果表明影响旋风分离器烟气脱硝效率的主要因素是喷嘴出口直径,次要因素是撞击件长...     

A Circumferential Slot In-Line Virtual Impactor

An In-line Virtual Impactor is presented, which has an application as a pre-separator for sampling inlets, where the device scalps large particles from the aerosol size distribution. Numerical simulation was the principal tool employed in the design process, with physical experiments used to verify computational predictions. Performance investigations were primarily carried out for a configuration that provides a nominal cutpoint particle size of 10 μ m aerodynamic diameter at an inlet flow of 111 L/min and a major flow exhaust of 100 L/min; however, the concept is scalable in terms of both flow rates and cutpoint sizes. An inverted dual cone configuration contained within a tube provides a characteristic circumferential slot of width 2.54 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. When the system is operated at the major exhaust flow rate of 100 L/min, the pressure drop is 45 Pa. Good agreement is obtained between numerically predicted and experimentally observed performance.     

Continuous aerosol size spectrometry with a variable-orifice impactor

An aerosol size spectrometer based on a round-jet impactor of continuously variable geometry is tested using an iris diaphragm as the nozzle. The impactor is run by fixing the nozzle-to-plate distance L, the aerosol mass flow rate m′, and the volumetric pumping capacity Q, which results in a nearly fixed downstream pressure at variable nozzle diameter d_n. Tests are carried below 1.3 and 2.5 mm. The size spectrometer has an excellent resolution when the jet Reynolds number and nozzle-to-plate distance are kept within the ranges 175 Re 700; 0.75 L/d_n 3.33. The instrument may also be run at atmospheric pressure, with an estimated lower size range of 1.7 μm.     

A Versatile Flat-Deposit Impactor-Type Aerosol Collector Part 1: Design and Qualitative Study

Cascade impactors provide an efficient method for sampling aerosols according to their size, grossly between 0.1 mu m and a few tens of microns in diameter. We have designed such an apparatus, with rotatable substrate plates and radially aligned rectangular orifices. The nominal cutpoint diameters are, respec tively, for stages 1-7 of the collector: 10, 5, 2, 1, 0.5, 0.25, and 0.1 mu m aed for an airflow of 10/L min. The sampler has an auxiliary set of orifice plates for operation at 20/L min giving the same cutpoint characteristics. Annular deposits are obtained following complete rotation of the collecting substrates. The airflow is controlled with a critical orifice at the last stage. A calculation program was written for a rectangular orifice impactor in order to determine the cutpoints in different conditions. The constant exit pressure mode of operation appears to be the best way to minimize fluctuations of the effective cutpoints against the varying conditions of ambient temperature and pressure. The SPAL sampler has proven satisfactory with respect to manipulations required, maintenance, and results delivered.     

Development of a PM2.5 sampler with inertial impaction for sampling airborne particulate matter

A simple and low cost PM_2.5 impactor for sampling airborne particulate matter was developed, designed and evaluated. The design was an assembly of an acceleration nozzle and an impaction plate. Particles with sufficient inertia were unable to follow air streamlines and impacted on the plate. Smaller particles followed the streamlines, avoided being captured by the plate and could then be collected on a downstream filter. Analytical and numerical models were formulated to predict collection efficiency, flow fields and vectors, and particle trajectories in the impactor. The modeling suggested that an optimal operational domain exists for the PM_2.5 impactor. A prototype was then built and tested. The collected particles on the impaction plate and downstream of the PM_2.5 impactor were analyzed by using scanning electron microscopy. Experimental results agreed well with the theoretical predictions. Testing of the PM_2.5 impactor prototype showed promising results for this airborne particulate matter sampler.     

CFD Study on the Effects of the Large Particle Crossing Trajectory Phenomenon on Virtual Impactor Performance

Two-dimensional CFD simulations were performed on a full-section numerical model of an as-built slot virtual impactor prototype and its completely symmetric ideal counterpart. The simulations reproduce the trends of the experimentally observed performance including verification of a third region in the transmission efficiency curve, which is a drop-in transmission efficiency for large particle sizes. Visualization of simulated particle tracks show this decrease is attributed to a crossing trajectory phenomenon, whereby larger particles that acquire enough inertia in a chamfered acceleration nozzle, crossover the vertical mid-plane and impact on the opposite-side wall, particularly on the wall of the receiver section. Some experimental data presented in the literature for rectangular slot and round-nozzle virtual impactors with chamfered 45° half-angle acceleration nozzles (similar to the geometry tested herein), show a similar drop-in transmission efficiency that commences at a particle Stokes numbers of about 6. However, many studies do not demonstrate the drop in transmission efficiency because wall losses are not taken into account. The Reynolds number, based on the acceleration nozzle size and velocity, does not noticeably affect the onset of the phenomenon. The crossing trajectory phenomenon can severely restrict the size range over which a virtual impactor can be used as an efficient particle concentrator. Geometrical asymmetry from dimensional tolerance considerations in the construction of a virtual impactor exacerbates the impact of the crossing trajectory phenomenon.     

State of equilibrium between dust pile and jet flow in slit nozzle impactor

The shape of the dust pile equilibrated on a collection plate with a two-dimensional impinging jet flow is studied experimentally and analytically in relation to re-entrainment in a cascade impactor. Consequently, it becomes clear that the shape of the equilibrated dust pile may be expressed as a function of the dimensionless number B, which represents the ratio of the weight of piled dust per characteristic volume of the nozzle to the momentum of fluid jet per unit time, and of the effective angle of internal friction of the dust. A maximum weight and volume of the dust pile for an impactor can be estimated by use of the value of B, calculated from the operating conditions. When an impactor is used for the separation of small quantity of a precious dust, the maximum weight and volume of the dust pile are the critical value.     

Bacterial aerosol neutralization by aerodynamic shocks using a novel impactor system: Design and computation

Neutralization of bacterial aerosol releases is critical in countering bioterrorism. As a possible bacterial aerosol neutralization method that avoids the use of chemicals, we investigate the mechanical instabilities of the bacterial cell envelope in air as the bacteria pass through aerodynamic shocks. To carry out this fundamental investigation, a novel experimental impactor system is designed and built to simultaneously create a controlled and measured shock and to collect the bacteria after they pass through the shock. In the impactor system the aerosol flows through a converging nozzle, perpendicular to a collection surface that has an orifice through which the shocked bacteria enter the deceleration tube. Both experimental measurements of the pressure in the impactor system at multiple points and computational fluid dynamics simulations are used to quantitatively characterize the shocks created in the impactor. Specifically, the developed computational model describes the evolution of both the gas and the particle velocity and temperature in the impactor system to determine the forces exerted on the bacterial aerosol as they pass through the shock. The results indicate that the developed computational model predictions compare well with the experimental pressure measurements. The computational model is also used to predict the magnitude of the acceleration needed to neutralize various bacterial aerosols and guide on-going experimental work.     

Effect of an Orifice on Collection Efficiency and Wall Loss of a Slit Virtual Impactor

The effect of an orifice on the collection efficiency and wall loss of a slit virtual impactor was investigated both numerically and experimentally. The ratios of the collection nozzle width (W_c ), distance between acceleration nozzle and collection nozzle (S), length of acceleration nozzle (T), inlet width (D), and nozzle span (l) to the acceleration nozzle width (W_a ) were fixed at W_c /W_a = 1.4, S/W_a = 1.5, T/W_a = 1.1, D/W_a = 6, and l/W_a = 10, respectively. The minor-to-total flow ratio was set to 0.1 in laminar flow regime. The collection efficiency and wall loss of the slit virtual impactor were found to be characterized by the square root of the Stokes number. For the slit virtual impactor without an orifice, the square root of the Stokes number corresponding to the cut-off diameter was determined to be (Stk₅₀)^1/2 = 0.77 and the maximum wall loss at the collection nozzle reached 30% or 40%. When an orifice having the same width as the acceleration nozzle was placed upstream of the acceleration nozzle at a distance of 20W_a , the value of (Stk₅₀)^1/2 decreased to 0.68 and the wall loss at the collection nozzle decreased below 5%.

Copyright 2014 American Association for Aerosol Research     

Computational investigation of airflow,shock wave and nano-particle separation in supersonic and hypersonic impactors

In this paper, the airflow field and particle transport in supersonic/hypersonic impactors were studied and the corresponding collection efficiencies were analyzed. A series of simulations were performed using the coupled Navier–Stokes and energy equations. A new correlation for the distance of Mach disk to impactor plate in supersonic/hypersonic impactors was obtained. A computer model for evaluating the Lagrangian particle trajectories including all the relevant forces was developed. Particular attention was given to accurate analysis of Brownian motions of nano-particles in supersonic and hypersonic flows. The importance of the accurate modeling of the Brownian motion of nano-particles was further emphasized. The simulation results of present model for collection efficiency were found to be in better agreement with the experimental data compared to the earlier models. In particular, the model accurately predicted the loss of the nano-particles in the upstream nozzle due to their Brownian motion.     

A Novel Particle Trap Impactor for Use with the Gas-Quenching Probe Sampling System

A novel particle trap impactor has been developed for use with the gas-quenching probe in order to exclude solid particles from entering into the probe during sampling of gaseous metallic species in fluidized bed combustion conditions. The impactor must be small in size (Ø_impactor ≤ Ø_probe = 45 mm) but capable of collecting a relatively large amount of particles at elevated temperatures. As the first step, the impactor was designed, constructed, and tested at room temperature for KCI aerosol particles. The impactor with a nozzle of 0.95 mm in diameter, in combination with the orifice-to-jet diameter ratio of 1.5 and the ratio of the jet-to-plate spacing to jet diameter at 1.4 yielded a sharp cutoff curve with a maximum collection efficiency of about 0.9 and a √Stk₅₀ value of about 0.22. In addition, the collection efficiency of the impactor was compared with the particle removal efficiency of a filter of the same type as the filter previously used with the gas-quenching probe. The difference from the comparison is very small, indicating that the impactor can be used to replace the filter to prevent fly ash particles from entering the gas-quenching probe in fluidized bed combustion conditions.     

Inertial Separation of Ultrafine Particles Using a Condensational Growth/Virtual Impaction System

ABSTRACT

A system for the separation of ultrafine particles (i.e., particles smaller than 0.1 μm) has been developed and evaluated. Ultrafine particles are first grown by means of supersaturation to a size that can be easily separated in a virtual impactor. Thus, inertial separation of ultrafine particles occurs without subjecting them to a high vacuum. The condensational growth/virtual impaction system has been evaluated using monodisperse 0.05 and 0.1 μm fluorescent PSL particles, as well as polydisperse ultrafine ammonium sulfate and potassium nitrate aerosols. The generated aerosols were first drawn over a pool of warm water (50°C) where they became saturated. Subsequently, the saturated aerosol was drawn through a cooling tube (8°C) where particles grew due to supersaturation to sizes in the range 1.0–4.0 μm. By placing a virtual impactor with a theoretical 50% cutpoint of 1.4 μm downstream of the condenser, ultrafine particles were separated from the majority (i.e., 90%) of the surrounding gas. The sampling flow rate of the virtual impactor was 8 L/min and its minor-to-total flow ratio was 0.1. For these operating conditions, the particle collection efficiency of the virtual impactor averaged to about 0.9 for particle concentrations in the range 7 × 10⁴-5 × 10⁵ particles/cm³. Particle losses through the system were found less than 5%. Increasing the particle concentration to levels in the range 106–107 particles/cm³ resulted in a decrease in the collection efficiency of the virtual impactor to about 50–70%, presumably due to the smaller final droplet size to which the ultrafine particles grew for the available supersaturation.     

喷嘴结构对射流鼓泡反应器混合和传质性能的影响

喷嘴结构对射流鼓泡反应器的混合和传质性能具有重要的影响。以空气-水作为模拟介质,使用双探头电导探针、电解质示踪法和动态溶氧法,对比研究了缩径式圆形喷嘴和旋扭三角形喷嘴对射流鼓泡反应器中气泡尺寸分布、平均气含率、液相混合时间和气液传质系数的影响规律。实验发现,随着气速或液体射流Reynolds数的增大,两种喷嘴对应的平均气含率、液相混合时间和气液传质系数具有相同的变化规律;与缩径式圆形喷嘴相比,采用旋扭三角形喷嘴的射流鼓泡反应器中气泡尺寸更小,平均气含率更高,宏观混合时间更短;当气体输入功占总输入功比例超过20%时,喷嘴结构对气液传质系数的影响较小,当气体输入功占总输入功比例小于20%时,旋扭三角形喷嘴的气液传质性能优于缩径式圆形喷嘴。研究结果可为工业射流鼓泡反应器喷嘴结构的优化提供理论指导。     

Enhancement of collection efficiency of inertial impactors using elliptical concave impaction plates

In this study, elliptical concave impaction plate was suggested for lowering cut-off size and therefore enhancing collection efficiency of the inertial impactor. Statistical Lagrangian Particle Tracking (SLPT) model was employed for calculating impactor collection efficiency and validated by comparing with the experimental data of Tsai, C.J., Cheng, Y.H. ((1995). Solid particle collection characteristics on impaction surfaces of different designs. Aerosol Science Technology, 23, 96–106), for three different shapes of impaction plates. Then, the effect of the ratio of major axis length (A) to minor axis length (B) for determining the curvature of elliptical concave impaction plate, on impactor collection efficiency was numerically investigated using the SLPT model, with nozzle Reynolds numbers ranging from 1440 to 2600. It was found that there existed an optimum range of the A/B ratio for minimizing the cut-off size, i.e. the A/B ratio ranged between 3.2 and 4.2 for the PM10 inertial impactor, or between 3.2 and 3.5 for the PM2.5 inertial impactor. When the elliptical concave impaction plates with the A/B ratio of 4.0 and 3.5 were applied to the MST indoor air sampling impactor having PM10 and PM2.5 stages, the cut-off size was predicted to decrease from 10 to 6.5 μm and from 2.5 to 1.6 μm, respectively, while the impactor collection efficiency curves became less steep.