A Mathematical Study of Particle Rebound for Thin-Walled Samplers

The aspiration of spherical particles which are suspended without sedimentation in an ideal fluid that flows past a thin-walled sampler is investigated. A recent experimental study showed that secondary aspiration caused by particles bouncing off the walls of the nozzle and subsequently entering the nozzle inlet is of importance in determining the aspiration efficiency of samplers in many situations. Therefore the extent of oversampling caused by particle rebound is investigated using a numerical method. Three situations are considered, namely, when the particles (1) adhere to the sampler surface on impact, (2) bounce off the surface and all those that hit within a certain area, known as the stagnation area, are eventually sampled, and (3) bounce off the surface with no loss of energy. Results for the three cases are compared and they are found to display the same trends as found in the experimental investigations.     

The entrance of airborne particles into a blunt sampling head

A study is made of the rapid aspiration of small dust particles into a bulky sampling head. It is assumed that the dust is suspended without sedimentation in air which is flowing past the sampler. Numerical results for a two dimensional cylindrical sampler in ideal flow are obtained which can easily be extended to other two dimensional and axisymmetric bodies. Also, the effects of changing the position of the point of sampling are discussed. Results are given for the two extreme cases where the dust particle can either (i) move freely or bounce on the surface of the sampler or (ii) adhere to the surface of the sampler. In the first case, which is considered in detail, a simple approximate expression is given for the variation of the aspiration coefficient with the Stokes number.

The variation of the ‘bluntness’ and the position of the stagnation points are derived in simple two and three dimensional situations.     

Classification of particles in particle-laden stream through a stainless steel fibrous filter

This investigation experimentally explores the penetration curve of particles shot onto a stainless steel fibrous filter or a flat surface. The effect of the pore size of the stainless steel fibrous filter, with or without an oil coating, on the particle penetration was examined at various flow rates, nozzle diameters and dimensionless particle diameters, Sqrt(Stk). The penetration of the flat surface by particles was also determined for comparison. Experimental results demonstrate that oleic acid particles larger than Sqrt(Stk)₅₀ are collected on the stainless steel fibrous filter with a low penetration, while smaller particles stay in the particle-laden stream with high penetration. The penetration of potassium chloride particles exceeds that of oleic acid particles, because potassium chloride particles bounce off the stainless steel fibrous filter and the flat surface. Particles bounce off the metal filter less easily than the flat surface. Coating the stainless steel fibrous filter with oil effectively reduces problems of particle bounce. The potassium chloride particles sucked the coated oil forming a small mountain on the surface. When the loaded particle mass on the coated stainless steel fibrous filter ranges between 0.4 and 2.3 mg, Sqrt(Stk)₅₀ is a constant 0.35.     

Sampling Errors in Cylindrical Nozzles

This paper reviews publications on aerosol aspiration by axisymmetric tubes, a widely used form of practical sampler. Axisymmetric tubes are widely used, as a rule, in stack sampling and sometimes in other areas of aerosol sampling as well (e.g., workplaces, ambient atmosphere). Numerous reports on aspiration coefficients for particles sampled from disperse flows contain two contradictory viewpoints on the sampling efficiency at suction velocities exceeding that of wind: although some authors claim that the sample representativeness worsens, others maintain that it is improved. Aerosol aspiration from calm or weakly turbulent air has not been investigated fully, despite the fact that the problem of determining sampling errors under such conditions is important in relation to occupational hygiene and environmental monitoring. Along with the analysis of the results published by other investigators (Davies et al., Vincent et al., etc.), this paper contains the axisymmetric sampler aspiration data obtained by us during the last 5-year period.

Experimental evidence is given for the secondary aspiration of particles after their bounce or blow-off, not only from the front face of the sampling tube but also from its external side surface. This effect is responsible for the qualitative discrepancy between the aspiration coefficient values obtained by different methods. The sampling conditions, for which aspiration distortions can be compensated for by using the inertial aspiration coefficient calculated from conventional theory, have been determined for axisymmetric samplers. The aspiration coefficient dependences on the anisokinetic coefficient, Stokes number, sampler wall thickness, and yaw angle have been analyzed for the aerosol sampling from steady-state flows. Possibilities of using these dependences to estimate errors in sampling aerosols from flows with the wind vector fluctuating in direction and magnitude are discussed. The poorly predictable secondary aspiration and flow turbulence effects observed with thick-walled samplers are shown to invariably influence the aspiration coefficient, making correction for sampling errors extremely difficult.

The inertial aspiration coefficient values measured for low-velocity wind and calm air have been analyzed. These results point to the not-so-obvious dependence of this coefficient on the sampling conditions. Experimental data are included, which make it possible to determine aspiration distortions at the orifices of samplers used with commercial aerosol analyzers.     

Reducing Particle Bounce and Loading Effect for a Multi-Hole Impactor

Aerosol sampling is used to evaluate the health hazards associated with particles deposited in the human breathing system. Impactors, which are extensively employed as aerosol samplers, have low collection efficiency because of particle bounce. The impaction plate is typically coated with oil or grease to prevent particle bounce. However, such coating materials cannot sustain long-term heavy particle loading.

In this study, the impaction plate was recessed, forming a cavity filled with Trypticase Soy Agar (TSA) to reduce particle bounce and re-entrainment. An ultrasonic atomizing nozzle was employed to generate challenge aerosols. An Aerodynamic Particle Sizer (APS) was utilized to measure the number concentrations and the size distributions upstream and downstream of the size-selective devices. A multi-hole impactor and Personal Environmental Monitor PM _2.5 (PEM–PM _2.5 ) were used to evaluate particle bounce and heavy particle loading. Liquid type-Dioctyl phthalate (DOP), soluble solid type-potassium sodium tartrate tetrahydrate (PST) and insoluble solid type-polymethyl methacrylate (PMMA) were investigated, as were different impaction surfaces/surface combinations. The multi-hole impactor coated with silicone oil was compared with a TSA-filled plate. Laboratory results demonstrate that the solid PST particles bounced off the TSA-filled plate less than off the silicone-coated aluminum plate. This study also used a 700-μm-thick layer of silicone oil to prevent TSA dehydration. The experimental results revealed that the silicone-TSA double layer minimized PST particle bounce during the two-hour heavy sampling (mass concentration was around 7.22 mg/m ³ ). Moreover, the PEM-PM _2.5 impactor yielded consistent results when the silicone-TSA double layer method was used. These results are useful for designing bounce-free impaction substrates during heavy load sampling.     

Particle Motion in the Vicinity of a Bulky Sampling Head Operating in Calm Air

A study is made of the aspiration of particles into a bulky sampling head with a sampling orifice located at a general point on its surface. The particles are assumed to be suspended in an ideal fluid that is at rest apart from the motion caused by the action of suction into the head. The sampling head considered is a two-dimensional circular cylinder with the orifice modelled as a line sink. The work extends that by Davies and Peetz (1954) and Davies (1967), where the aspiration of particles into the top and bottom of a two-dimensional cylindrical sampling head were considered. It is found that the region around the head can be divided into four regions, namely, where particles (i) enter the sampling head, (ii) deposit on the head, (iii) pass by the head, and (iv) where no particles enter and hence forms a shadow. The size and shape of these regions is found to depend not only on the position of the sampling orifice on the surface of the head, but also on the value of a nondimensional parameter k . This parameter is a multiple of the ratio of the particle settling velocity times the sampler size to the rate of aspiration of the fluid by the sampler. For all positions of the sampling orifice, the shadow zone is found to extend to infinity for large values of k , which corresponds to small suction rates, but becomes finite as k decreases.     

A Numerical Investigation into the Sampling Performance of a Tube Sampler

In this study the fluid flow and particle trajectories for a tube sampler are investigated numerically. The flow around the sampler is assumed to be inviascid and, using a mathematical approach, flow characteristics are determined. The paths of the particles in the fluid are then traced and the aspiration efficiency of the sampler is investigated. Numerical results for the sampling efficiency are obtained over a large range of parameters and they are found to be in good agreement with related empirical work. In order to give a general guide on the sampling efficiency of a tube sampler, new empirical formulas for both the aspiration coefficient and the overall aspiration efficiency are developed based on extensive numerical calculations.     

A theoretical study of aerosol sampling by an idealized spherical sampler in calm air

The performance of an idealized spherical sampler operating in calm air for an inlet arbitrarily oriented relative to the gravity force is studied theoretically. Under potential flow assumption the air velocity field is obtained by using a model of a finite-size sink on a sphere. The particle motion equations are solved to find the limiting trajectory surface and to calculate the aspiration efficiency. The singular points of the motion equations as a function of settling velocity of particles and the sampler orientation angle are investigated. The connection between the pattern of typical zones of particle trajectories around the sampler and the location of the singular points is illustrated. The effects of partial sampling from zones without particles and of particle screening are discussed. The results of parametrical investigations of the dependence of the aspiration efficiency on the Stokes number and their analysis are presented. In the case of vertically upwards orientation of the sampler the proposed mathematical model gives fair agreement with experimental data from the work by Su and Vincent (Abstracts of sixth international aerosol conference, Taipei, Taiwan, 2002a, pp. 639–640).     

The aspiration efficiency of a tube sampler operating in a slow moving air stream facing vertically upwards

The aspiration efficiency of a thin-walled, cylindrical aerosol sampler facing vertically upwards in a slow moving vertical air stream is numerically investigated using both a potential flow model and a viscous flow model. In order to predict the air flow around the sampler, for the potential flow model we use a boundary element method whereas for the viscous flow model we use a control volume method. The motion of the particles is then predicted by considering both the drag and gravitational forces. We have found that both numerical models produce similar predictions for the aspiration efficiency and the predictions reveal a more complex sampling behaviour when the sampler is operated in a slow moving air environment, where the air velocity is comparable with the magnitude of the particle settling velocity, than for faster moving air flows. The comparison of the numerical predictions with the only available experimental data indicates that the aspiration efficiency is in qualitative agreement but further investigations are required in order to fully reveal all the sampling characteristics in slow air flows.     

Influence of Particle Shape on Filtration Processes

The influence of particle shape on filtration processes was investigated. Two types of particles, including spherical polystyrene latex (PSL) and iron oxide, and perfect cubes of magnesium oxide, were examined. It was found that the removal efficiency of spherical particles on fibrous filters is very similar for corresponding sizes within the range of 50–300 nm, regardless of the fact that the densities of PSL and iron oxide differ by a factor of five. On the other hand, the removal efficiency of magnesium oxide cubic particles was measured, and found to be much lower than the removal efficiency for the aerodynamically similar spheres. Such disparity was ascribed to the different nature of the motion of the spherical and cubic particles along the fiber surface, following the initial collision. After touching the fiber surface and before coming to rest, the spherical particles could either slide or roll compared to the cubic ones, which could either slide or tumble. During tumbling, the area of contact between the particle and the fiber changes significantly, thus affecting the bounce probability, whilst for the spheres, the area of contact remains the same for any point of the particle trajectory. The extra probability of particle bounce by the cubes was derived from the experimental data. The particle kinetic energy was proposed to be responsible for the difference in removal efficiency of particles with alternative shapes, if all other process parameters remain the same. The increase in kinetic energy is shown to favor the increase of the bounce probability.     

Reconsidering Adhesion and Bounce of Submicron Particles Upon High-Velocity Impact

Adhesion and bounce of liquid and solid particles upon high-velocity impact with a surface has been investigated using semi-empirical and explicit hydrodynamic simulations. Ammonium nitrate (AN) and sodium chloride (NaCl) were selected as test compounds for the liquid and solid particles, respectively, and tungsten (W) as the target surface. Changes in the shape, temperature, strain, and rebound velocity of these particles upon high-velocity impact are investigated assuming operational conditions (particle diameter, and velocity) of Aerodyne aerosol mass spectrometer (AMS). The simulations show that the AN particles adhere to the W surface, which is consistent with previous experimental studies. In the case of NaCl, the collection efficiencies depend significantly on the stress–strain characteristics of the crystal. Our results suggest that, in addition to particle phase and impact velocity, anisotropy of the elastic properties and brittleness are key factors in controlling the adhesion and bounce of solid particles.

Copyright 2013 American Association for Aerosol Research     

The Effects of Bluntness and Orientation on Two-Dimensional Samplers in Calm Air

This article uses numerical simulation to investigate the effect of sampler bluntness, particle size, and sampler orientation on aspiration efficiency in calm air. The procedure is to first numerically solve the velocity field around the sampler in calm air and then to trace the particle trajectories and calculate the the aspiration efficiency. Two samplers are studied: a two-dimensional parallel plate and a two-dimensional blunt cylinder. The variation of aspiration efficiency with particle size shows two minima between two asymptotic values. When the samplers are facing upward, the asymptotic values are 1 for very small particles and the ratio of particle settling velocity to suction velocity for very large particles. At other orientations, the horizontal-facing and the downward-facing, the asymptotic value for large particles is 0. The sampler bluntness has an important effect in the region of particle size where there is competition between the particle inertia and the fluid drag force (i.e., 5 μm < d > 100 μm in our case). A blunt sampler always has higher aspiration efficiency than does a sharp-edged sampler in this region of particle sizes. For very small particles and very large particles, the aspiration efficiencies approach asymptotic values and the sampler bluntness has little effect. The results also show that the sampler orientation affects the predicted aspiration efficiency     

Experimental measurements of aspiration efficiency for idealized spherical aerosol samplers in calm air

In a recent previous paper (Su & Vincent, J. Aerosol Sci. 33 (2002) 103) we described a new method by which to investigate the relationships between aspiration efficiency, particle inertia, gravitational effect and sampling orientation for aerosol sampling in perfectly calm air. All previous experimental work to elucidate the basic nature of aerosol sampling in calm air described has been carried out for thin-walled tubes, and none has yet been reported in relation to blunt samplers. To begin to fill this important gap, the present paper describes the application of our new method towards acquiring new measurements of aspiration efficiency for simple blunt samplers.Experiments were carried out to determine the aspiration efficiencies of simple, idealized, spherical blunt samplers for a range of sampling scenarios, for two sizes of blunt samplers and different sampling inlet diameters, and for upwards and downwards sampling scenarios (and hence a range of governing dimensionless physical quantities). It was shown that aspiration efficiency decreased both with increasing inertia (as represented by the Stokes’ number) and with increasing gravitational effect (as represented by the ratio of particle settling velocity to the air velocity at the sampler inlet). The results enabled qualitative physical explanation of the difference between what was observed for upwards and downwards-facing sampling, respectively, in terms of (a) the role of the sampler body in deflecting the air flow in the region close to the body of the sampler (in turn influencing the performance of the sampler), and (b) the interception of particles in the downwards-facing scenario falling within the ‘shadow’ projected upwards by the sampler body.The significant contribution of this work has been the acquisition of a definitive set of new experimental data. These data will be valuable in the future development of understanding of the physics underlying aerosol sampler performance. Such knowledge will be of practical value because (a) blunt samplers are generally the most representative of the types of instruments used in practical occupational, and (b) calm or slowly moving air is characteristic of many indoor situations.     

A numerical study of the sampling efficiency of a tube sampler operating in calm air facing both vertically upwards and downwards

In this paper the performance of a thin-walled sampling tube operating in calm air, pointing both vertically upwards and downwards, is investigated. The problem is considered mathematically and the sampler is modelled as a long circular cylinder. Two mathematical models have been developed to determine the flow field around the sampler, one model assumes potential flow and the other takes into account the viscous effects of the flow. From detailed knowledge of the airflow, the paths of suspended particles in the fluid flow were calculated and the aspiration efficiency of the sampler determined. The dependence of the efficiency upon the particle size and suction rate are considered for various operating conditions and a comparison made with available experimental data. The results obtained using the two different flow models are compared and hence the effects of assuming potential flow, an assumption often made when studying aerosol sampling, are investigated. It is found that neglecting the viscous effects of the fluid results in a higher predicted value of the aspiration efficiency, A. However, in all the cases considered, the difference in the values of A predicted is never large and the general behaviour of A is always the same.     

Broad range observation of particle deposition on greased and non-greased impaction surfaces using a line-sensing optical microscope

A computer-automated optical microscope combined with a line-sensing camera was used to capture the entire range of a particle deposit downstream of an individual acceleration nozzle in a hi-volume Andersen sampler. To investigate the particle bounce and reentrainment, particles collected on collocated greased and non-greased Teflon plates on the inlet stage (d_a>7 μm) were observed by an automated particle counting, locating and sizing method. The result confirmed reproducible collection characteristics among nozzles even though application of the grease increased collection efficiency and altered the size distribution of collected particles to the larger side. In these experiments, assuming spherical particles with uniform density, approximately 65% of particle mass and 50% by number were lost from non-greased plates at 54% RH, while 45% by mass and 25% by number were underestimated at 84% RH. The spatial investigation showed that particles were densely deposited around the center of deposition on greased plates while on non-greased plates they were dispersedly distributed. Particle dispersions on the smooth impaction plate were due to bounce and/or reentrainment of small particles especially with d_PA<10 μm.     

PARTICLE TRANSMISSION EFFICIENCY THROUGH THE NOZZLE OF THE API AEROSIZERTM

This study has investigated the particle transmission efficiency through the nozzle of the API Aerosizer^TM numerically. Two-dimensional flow field in the nozzle was first simulated. Particle trajectories for both liquid and solid particles were then calculated to obtain the particle transmission efficiency under various conditions. This study shows that particle aerodynamic diameter, particle materials, particle density and laser beam diameter influence the transmission efficiency. The transmission efficiency is found to increase with increasing particle diameter when the particle aerodynamic diameter is less than several micrometers. The efficiency for liquid particles drops significantly when particle aerodynamic diameter increases from several micrometers because of particle impaction loss in the nozzle. For solid particles, the relationship of the efficiency with particle diameter is found to be more complicated. For particles less than several micrometers in aerodynamic diameter, solid particles behave similarly to the liquid particles. However, as particles are greater than several micrometers, the effect of solid particle bounce is to increase the transmission efficiency with increasing aerodynamic diameter until particles become large enough so that plastic deformation occurs in the particles. Then the transmission efficiency will decrease with increasing particle aerodynamic diameter.     

Improved Aerosol Collection by Combined Impaction and Centrifugal Motion

ABSTRACT

A new principle for collecting airborne particles, including microorganisms, has been introduced by injecting the particles into a swirling airflow from where they are removed onto a collection surface. A dry surface, a surface coated with an adhesive substance or a surface wetted by a liquid swirled onto the collection surface from a reservoir below can be used in the new collection method. The swirling air motion and aerosol injection into it are achieved by drawing the airborne particles through nozzles that are directed at an angle toward the collection surface. This principle has been incorporated into a new sampler that has been named “Swirling Aerosol Collector” (SAC; commercially available as the “BioSampler” from SKC Inc., Eighty Four, PA). The physical performance of the SAC has been evaluated against the widely used AGI-30 impinger by measuring the particle concentrations upstream and downstream of each sampler with an aerodynamic particle sizer. Tests with monodisperse polystyrene latex (PSL) particles ranging from 0.3 to 2.0 μm have shown that the SAC has better collection efficiency than the AGI-30 when the same collection liquid is used. A conventional impinger maintains constant collection efficiency for a relatively short sampling period, as the liquid evaporates quickly due to the violent bubbling of the liquid. In contrast to conventional impingers, the SAC can be used with nonevaporating liquids that are considerably more viscous than the liquids used in the impingers. Thus, the SAC can sample over any period of time. The new aerosol sampler produces minimal or no reaerosolization of particles collected in the liquid in contrast to significant reaerosolization in a conventional impinger. Since the SAC projects the aerosol particles toward the collection surface where they are removed from the swirling flow, it avoids or significantly reduces particle bounce from the collection surface even when the surface is dry.     

Axial dispersion and enterainment of particles in wakes of bubbles

The entrainment and dispersion of solid particles in bubble columns was investigated experimentally and heoretically. A mechanistic model for the dispersion caused by entrainment in wakes of large solitary bubbles was developed. The dispersion coefficient was found to be dependent on the bubble size, bubble frequency, particle settling velocity and column surface area. Experimental tests were conducted in a rectangular bubble column. The system consisted of air, water and copper powder. Spherical cap bubbles were produced by a single nozzle. Significant entrainment of particles in wakes of rising bubbles observed in the lower region of the column, whereas, turbulence seemed to dominate the dispersion in the upper region of the tank. Calculated particle distributions were found to be in good agreement with experimental data.