Size distribution of polydispersed aerosols during condensation in the continuum regime: Analytic approach using the lognormal moment method

An analytic solution for polydispersed aerosol condensation was obtained in the continuum regime. In this new approach, lower order moments, as compared to a previous study, were used to the log-normal aerosol size distribution, in order to obtain an analytical solution. The resultant analytic solution based on the lognormal size distribution was compared with the exact solution and proved to be in good agreement. The obtained solution was also found to be consistent to a greater degree with the exact solution was the previous analytic moment solution for the polydispersed aerosol condensation process. This is particularly true for cases in which broad size distributions exist.     

A moment model for simulating raindrop scavenging of aerosols

The dynamics of a polydispersed aerosol size distribution, scavenged by precipitation, are numerically studied. The collision efficiency formula proposed by Slinn (Precipitation Scavenging in Atmospheric Sciences and Power Production—1979, Division of Biomedical Environmental Research, US Department of Energy, Washington, DC, USA, 1983, Chapter 11) and the moment method were introduced to represent the particle removal mechanism by raindrops and the aerosol size distribution, respectively. Consequently, the dynamics of the particle size distribution were reduced to a set of ordinary differential equations using the moment approach. A generalized raindrop distribution, including two widely used distributions; the Marshall–Palmer (MP) and Krigian–Mazin (KM) raindrop distributions, was adopted.Our model results have shown that raindrops with smaller diameters, and narrower distributions, collect aerosols more efficiently. Further, it was shown, in the small particle size region that the geometric mean diameter increases, while in the large particle region it decreases. For the two size ranges, the geometric standard deviations decrease with time, and a scavenging gap, the minimum particle removal efficiency region, exists between these particle size ranges.The dynamics of the particle size distributions, the MP and KM raindrop distributions, in the small particle range, show that the effects of the overestimation in the MP distribution were not as great as expected. Also, this study ascertained that the conventional parameterization of the constant collision efficiency introduces significant errors for estimating the particle size distribution dynamics by wet scavenging.     

Wet scrubbing of polydisperse aerosols by freely falling droplets

In this study, analytical solutions for removal of a polydisperse aerosol by wet scrubbing were derived employing Brownian diffusion and inertial impaction as removal mechanisms. Size distribution of aerosol particles were assumed to be represented by a time-evolving log-normal function during the scrubbing process. Derived solutions were compared with the direct integration solution, which is not based on the log-normal assumption, showing good agreement. Error resulting from the log-normal assumption was shown to be greater in the impaction-dominant regime than in the diffusion-dominant regime due to higher size dependency of collision kernel which destructed log-normal shape of size distribution. The monodisperse model significantly overpredicted particle removal in the diffusion- and impaction-dominant size regimes due to its incapability of tracing average particle size change, while it underpredicted particle removal in the intermediate size range because of neglect of polydispersity effect. A new solution for the minimum collection efficiency particle diameter was also provided. The minimum efficiency diameter was shown not to be very sensitive to the scrubbing condition and to lie around for wide range of size and concentration of water drops.     

Removal of sulfuric acid aerosol in a wet electrostatic precipitator with single terylene or polypropylene collection electrodes

Wet electrostatic precipitators (ESPs) are good options for effective control of sulfuric acid aerosol emission. However, various problems caused by materials and non-uniform distribution of water film limited the applicability of typical wet ESPs. Research on ESP technology has tried to find more suitable and anti-corrosive methods to solve these imperfections. This research was inspired by the requirement to replace rigid collection electrode by single terylene or polypropylene fabrics. A patented system was designed, and the capillary difference between terylene and polypropylene fabrics was illustrated. Contrastive V–I curves of different collection electrodes were investigated under same conditions. The effects of several important parameters on the removal of sulfuric acid aerosol were analyzed. The results demonstrated that the variations of absorbed mass were significantly influenced by physical properties of the liquids and the structure of fabrics. The behavior of the new ESP was consistent with the typical ESP using a thimbleful of water penetrating terylene or polypropylene collection electrode via capillary flow. The collection efficiencies by terylene and polypropylene fabrics were higher than those by fiberglass reinforced plastics (FRP) under certain conditions. The collection efficiency had linear relationship with specific surface area (SCA) and mass concentration. The collection efficiency increased with increasing electric field strength, average diameter of particles and with decreasing gas temperature. As long as there was any water on the collector surface, any particle would exhibit similar collection efficiencies, whether of high resistivity or not. Experimental and theoretical investigations indicated that single terylene or polypropylene collection electrode had significant advancement which could improve wet ESP applications, such as superior performance and continuous operation ability compared with typical materials.     

Numerical Simulation of Scavenging of Small Particles by Charged Droplets

The trajectories of fine aerosol particles in the vicinity of a free falling collector droplet and their deposition on it were investigated numerically by solving the equations of motion of the particle and the droplet in quiescent air. The droplet was assumed to be charged to one half of the Rayleigh limit. The Coulomb, image, Stokes, inertial, and gravitational forces acting upon the particle near the droplet were taken into consideration in the equations of motion. The equations of the droplet motion were also incorporated into the set of equations including the Coulomb and image forces on the droplet due to the particle charge. The flow field in the vicinity of the droplet was determined by numerical solution of the Navier-Stokes equations. The equations of particle motion were solved in threedimensional (3-D) space by the Runge-Kutta method of the fourth order. The collection efficiency of the particles on the droplet was determined by searching the limiting trajectory within the entire space. The results for particles charged to 10 elementary charges of the same and opposite polarity as the droplet, as well as the electrically neutral ones, were compared. The assumption on the charge of the particle was rather arbitrary. It was assumed that particles are not intentionally charged but only possess a charge generated by tribocharging due to random contacts and were independent of the particle size. Charging the collector causes the Coulomb forces between these 2 species to improve particle deposition on the droplet and in this way the aerosol is removed from the gas. For the aerosol particles charged to the same polarity as the collector, the collection efficiency is still higher than for uncharged particles due to the action of the image forces. In this case, the collection efficiency increases for smaller droplets and for particles with increasing diameter.     

Technical Note: Particle Extinction Coefficient for Polydispersed Aerosol Using a Harmonic Mean Type General Approximated Solution

In the present study, an approximated expression for the single particle extinction efficiency (Q _ext ) has been obtained, which was valid for a wide range of particle sizes. The approximate single particle extinction efficiency was obtained using the harmonic mean type approximation between the particle extinction efficiencies from the Rayleigh scattering and geometric scattering regions. The results were compared with the numerically calculated Mie's results, which showed that both the approximated and original Mie's results were comparable.

The approximated polydispersity of the overall extinction coefficient (b _ext ) obtained using the moment method was then tested and compared with the numerically integrated results. The approximated solution obtained showed how the b _ext effectively varied with the size distribution and greatly reduced computational time.

Consequently, a simple and straightforward method for estimating the extinction coefficient for polydispersed aerosols, which can be widely applied in atmospheric aerosol models, has been developed in this study.     

A Wet Electrostatic Precipitator (WESP) for Soft Nanoparticle Collection

Many nanoparticle collection devices have limitations related to retention of particle integrity from bounce, shattering, or aggregation. Suspensions of soft nanoparticles (e.g., proteins, lipids) are required for drug delivery and therapy. To enable direct collection of soft nanoparticles into liquid media, a wet electrostatic precipitator (WESP) was designed and evaluated in this work. Different sections were used for ion generation and particle charging, for minimal contact between the corona wire and particles, which were charged using positive nitrogen ions. WESP dimensions and operating parameters were optimized using charge distribution modeling. The prototype WESP was designed for operation with a continuous flow of liquid over the collection plate, to allow continuous particle collection from the exit stream of an aerosol reactor. The collection efficiency of the WESP, in dry and wet modes, was measured using aerosols of monodisperse polystyrene latex (PSL), polydisperse sucrose, and stearic acid (soft lipid) particles, through SMPS measurements, corrected for diffusional losses, at the entry and exit of the device. Measured collection efficiency was 70%–90% for particles of sizes 80–600 nm diameter in reasonable agreement with theoretical estimates. However, for small particles (20–80 nm diameter) measured collection efficiency ranged 40%–70%, significantly lower than theoretical estimates, possibly from incomplete neutralization of negative charges attained during air-jet atomization. Transmission electron microscopy (TEM) images and dynamic light scattering (DLS) measurements confirm that wet collection produces a suspension of free, unaggregated nanoparticles with sizes similar to their measured mean mobility diameter.

Copyright 2012 American Association for Aerosol Research     

Experimental Study of Particle Deposition on Semiconductor Wafers

A sensitive method for detecting particle deposition on semiconductor wafers has been developed. The method consisted of generating a monodisperse fluorescent aerosol, depositing the known-size monodisperse aerosol on a wafer in a laminar flow chamber, and analyzing the deposited particles using a fluorometric technique. For aerosol particles in the size range of 0.1–1.0 μm, the mobility classification-inertial impaction technique developed by Romay-Novas and Pui (1988) was used to generate the monodisperse test aerosols. Above a particle diameter of 1.0 μm, monodisperse uranine-tagged oleic acid aerosols were generated by a vibrating-orifice generator. The test wafer was a 3.8-cm diameter silicon wafer placed horizontally in a vertical laminar flow chamber which was maintained at a free stream velocity of 20 cm/s. A condensation nucleus counter and an optical particle counter were used to obtain the particle concentration profile in the test cross section and to monitor the stability of aerosol concentration during the experiment. The results show that the measured particle deposition velocities on the wafers agree well with the theory of Liu and Ahn (1987) in the particle size range between 0.15 and 8.0 μm. The deposition velocity shows a minimum around 0.25 μm in particle diameter and increases with both smaller and larger particle size owing to diffusional deposition and gravitational settling, respectively.     

Influence of fiber orientation distribution on performance of aerosol filtration media

This work is conducted to better our understanding of the influence of fibers’ in-plane and through-plane orientations on pressure drop and collection efficiency of fibrous media. The Stokes flow equations are numerically solved in virtual, 3-D, fibrous geometries with varying in-plane and/or through-plane orientations. Pressure drop and aerosol collection efficiency characteristics of such media are calculated and compared with available studies from the literature. Our results indicate that pressure drop and submicron particle capture efficiency of common fibrous filters with a fiber diameter of about 10 μm are independent of the in-plane orientation of the fibers, but decrease with increasing the fibers’ through-plane orientation. More interestingly, it was found that filters with higher through-plane fiber orientations have a higher figure of merit if challenged with nanoparticles. The figure of merit of these media, however, decreases as the particle size increases, reversing the effect of fibers’ through-plane orientation. It was also shown that when the diameter of the particles is comparable to that of the fibers, collection efficiency increases with decreasing the fibers’ in-plane orientation, while the pressure drop remains almost unchanged. This indicates that decreasing the fibers’ in-plane orientation increased the figure of merit of media made of nanofibers.     

Analytic solutions for filtration of polydisperse aerosols in fibrous filter

In this study, analytical solutions for penetration efficiency of a polydisperse aerosol in fibrous filter were derived employing Brownian diffusion and inertial impaction as removal mechanisms. Size distribution of aerosol particles was assumed to be represented by a log-normal function during the filtration. Derived solutions were compared with the exact solution, which is not based on the log-normal assumption, showing good agreement. Error resulting from the log-normal assumption was shown to be greater in the impaction-dominant regime than in the diffusion-dominant regime due to higher size dependency of collision kernel which destructed log-normal shape of size distribution. The penetration efficiency of the analytic solution initially decreases faster and then decreases slower than that of the exact solution in the diffusion-and intermediate dominant size regimes due to its polydispersity of particle distribution, while it overpredicted the particle removal in the impaction size range because of neglect of polidispersity effect. A new solution for the most penetrating particle diameter was also provided showing the dependence on filtration velocity, fiber volume fraction, and fiber size.     

Growth of aerosol particles by adiabatic expansion at the throat of a venturi scrubber

The condensational growth of aerosol particles in the throat section of a venturi scrubber and the contribution of it to the dust collection efficiency were discussed.The condensable water vapor produced in the throat of venturi was quantitatively obtained for various conditions assuming the change in pressure and temperature to be adiabatic process. In terms of this condensable water vapor and the particle number concentration, the diameter of grown particles was determined, and the degree of contribution due to particle growth to the particle collection by venturi scrubber was evaluated as the change in particle collection efficiency involved in the particle growth.Experimental verification of the above results was qualitatively made by changing the humidity of the inlet gas, which is the most important property dominating the particle growth, in the particle collection by venturi scrubber.     

Prediction for particle removal efficiency of a reverse jet scrubber

Numerical computation was conducted to predict the collection performance of a reverse jet scrubber for polydisperse particles. The particle size distribution of polydisperse particles was represented by a lognormal function, and the continuous evolution of the particle size distribution in a reverse jet scrubber is taken into account with the first three moment equations. Numerical results were compared with the analytic results using average relative velocity in all zones and experimental results.

In a reverse jet scrubber, the impaction is the main particle collection mechanism because of high relative velocity and short collection time. The particle collection by impaction increases with an increase in particle size, and geometric mean diameter and geometric standard deviation decrease as time goes on. High droplet velocity and gas velocity increase the particle collection efficiency, and the small droplet size also increases the collection efficiency because smaller droplet size provides broader surface area. The packing density is a factor affecting particle collection efficiency in a scrubbing process. The dense packing density also provides large surface area and leads to high collection efficiency.     

Ultrafine Particle Sampling with the UNC Passive Aerosol Sampler

A model is presented to describe the collection of ultrafine particles by the UNC passive aerosol sampler. In this model, particle deposition velocity is calculated as a function of particle size, shape and other properties, as well as a function of sampler geometry. To validate the model, deposition velocities were measured for ultrafine particles between 15 and 90 nm in diameter. Passive aerosol samplers were placed in a 1 m ³ test chamber and exposed to an ultrafine aerosol of ammonium fluorescein. SEM images of particles collected by the samplers were taken at 125 kX magnification. Experimental values of deposition velocity were then determined using data from these images and from concurrent measurements of particle concentration and size distribution taken with an SMPS. Deposition velocities from the model and from the experiments were compared and found to agree well. These results suggest that the deposition velocity model presented here can be used to extend the use of the UNC passive aerosol sampler into the ultrafine particle size region.     

Measurements of the total and regional deposition of inhaled particles in the human respiratory tract

The total and regional deposition of monodisperse aerosols in the human respiratory tract has been measured in 12 healthy subjects breathing through the mouth. Radioactively labelled polystyrene particles in the aerodynamic diameter range 3.5–10.0 μm were employed. The total deposition results are similar to those reported by Stahlhofen et al. (1980), showing only a slight progressive increase with particle size, from a mean fraction of 0.79 of the inhaled aerosol at 3.5 μm, to 0.88 for 10 μm particles. The extrathoracic airways show a very marked deposition at all sizes, predominantly in the throat. The throat values rise rapidly from a mean of 0.09 at 3.5 μm to 0.36 at 10 μm particle diameter. Two intrathoracic fractions were also obtained by the widely accepted method of measuring the relative amounts of activity cleared from the thorax as a function of time. Alveolar deposition was apparently still some 0.06 of the inhaled aerosol at 10 μm particle diameter. Tracheo-bronchial deposition showed little change at any particle size except at 3.5 μm, when it was 0.24 of the inhaled aerosol.     

A New Cross-Flow Tube Bundle Heat Exchanger with Staggered Hot and Cold Tubes for Thermophoretic Deposition of Submicron Aerosol Particles

A new tube cross-flow bundle heat exchanger has been designed and tested for thermophoretic deposition of submicron aerosol particles. The present design has five columns of hot and cold square tubes, respectively, arranged in a staggered manner to maintain a nearly constant temperature gradient in the direction of the aerosol flow. Each column has four tubes of 4 mm × 4 mm in cross section and the gap between the tube surfaces is 0.5 mm. The precipitator was tested experimentally using monodisperse NaCl test particles ranging from 38 to 397 nm in diameter at the aerosol flow rate of 0.6 and 1.2 L/min, respectively, at different temperature gradients. Results showed that the thermophoretic deposition efficiency increased with decreasing aerosol flow rate and increasing temperature gradient with the maximum thermophoretic deposition efficiency occurred at the aerosol flow rate of 0.6 L/min. The effect of inlet temperature of the aerosol flow on the efficiency was also tested and showed increasing inlet temperature increased the deposition efficiency. Numerical simulation was further conducted to validate the experimental data and good agreement was obtained. An empirical equation was also validated to facilitate the design and scale-up of the precipitator.     

亲水改性碳钢极板用于PM2.5脱除

火电厂大气污染物排放标准日趋严格,湿式静电除尘器作为终端治理设备逐渐得到广泛应用。以亲水改性刚性极板为研究对象,建立了卧式湿式静电除尘器中试实验台,开展了PM_2.5脱除特性的实验研究,研究了改性极板表面水膜增强颗粒物脱除效率的机制,考察了气体温度、停留时间、工作电压、初始浓度、冲洗水流量等主要运行参数对颗粒物脱除效率的影响规律。结果表明：改性刚性极板表面的纤维层可以减少反冲气流,减少颗粒的电迁移阻力;表面在小水量情况下亦可维持均匀稳定的水膜,水膜的存在抑制了反电晕和二次扬尘的发生,使得电晕电流高且水膜蒸发使烟气湿度提高,颗粒荷电量和电迁移速度提高,这两方面均提高了颗粒脱除效率。停留时间延长、工作电压提高均会引起颗粒脱除效率的增加,但颗粒物入口浓度、冲洗水流量对颗粒脱除效率影响不大。使用改性刚性极板的湿式静电除尘器可减少阳极冲洗水量,对粒径0.04～0.48 μm的颗粒有较高脱除效率,可在低电压下达到较高的颗粒物总脱除效率,具有较好的应用前景。     

Simulation of Particle Size-Selective Air Samplers Placed in a Polydisperse Aerosol

The results of a numerical simulation of several air sampling instruments are presented. They are assumed to sample the same aerosol, with a log-normal particle-size distribution. Four instruments were studied: the 10-mm nylon cyclone, the MRE 113A gravimetric sampler, the CPM 3, and the CIP 10. The experimental data of particle collection efficiency were reduced by a model for each instrument. The model used combines two cumulative log-normal distribution functions, in order to have a good degree of flexibility necessary for representing the data of some devices that exhibit a maximum in efficiency (CPM 3, CIP 10). The concentrations “measured” by several air samplers were compared with each other; the differences were analyzed as functions of the aerosol parameters: mass median aerodynamic diameter and σ_g. The results that were obtained and those calculated from standard collection efficiencies, defining the conventional alveolar fraction of the aerosol, were also taken into account. This simulation method can be extended to any type of instrument and aerosol, and enables the prediction of the maximal deviations that could be observed between different instruments, or between one instrument and some reference standards.     

沿同心环面层流流动的气体中颗粒的传热沉积物的预测:发展中的流动和已充分发展的流动的比较

Thermophoresis is an important mechanism of micro-particle transport due to temperature gradients in the surrounding medium. It has numerous applications, especially in the field of aerosol technology. This study has numerically investigated the thermophoretic deposition efficiency of particles in a laminar gas flow in a concentric annulus using the critical trajectory method. The governing equations are the momentum and energy equations for the gas and the particle equations of motion. The effects of the annulus size, particle diameter, the ratio of inner to outer radius of tube and wall temperature on the deposition efficiency were studied for both developing and fully-developed flows. Simulation results suggest that thermophoretic deposition increases by increasing thermal gradient, deposition distance, and the ratio of inner to outer radius, but decreases with increasing particle size. It has been found that by taking into account the effect of developing flow at the entrance region, higher deposition efficiency was obtained, than fully developed flow.     

Simulation of particle deposition on filter fiber in an external electric field

Particle deposition onto a filter fiber was numerically simulated when a uniform external electric field was applied. The effects of electric field strength, particle inertia, and electrical conductivity of particles on particle deposition characteristics such as particle loading patterns and collection efficiency were qualitatively investigated. As a result, the electrostatic forces between a newly introduced particle and the already captured particles on the fiber were found to have a great influence on the particle deposition patterns compared with the results where the electrostatic forces were neglected. Conductive particles and filter fibers lead to higher collection efficiency and more linear structure of particle deposits than those of dielectrics, and the particle inertia could also be more important to the collection efficiency of a fibrous filter when electric fields are present. The simulated particle deposits obtained from this work agreed well with the existing experimental results, in which the photographs of particle loaded fibers, within an external electric field, were reported.     

Simulation of the effect of local obstructions and blockage on airflow and aerosol deposition in central human airways

Investigation of the effect of sidewall and carinal tumours, airway constrictions and airway blockage on the inspiratory airflow and particle deposition in the large central human airways was the primary objective of this study. A computational fluid and particle dynamics model was implemented, validated and applied in order to simulate the air and particle transport and to quantify the aerosol deposition in double airway bifurcation models. Our investigations revealed that surface abnormalities and tubular constrictions can significantly alter the airstreams and the related local aerosol deposition distributions. Sidewall tumours have lead to an enhanced deposition of large particles and caused lower deposition efficiency values of nano-particles compared to the deposition efficiency in healthy airways. Central tumours multiplied the deposition efficiency of large particles but hardly affected the deposition efficiency of nano-particles. Airway blockage caused a significant redistribution of particle deposition sites. The deposition efficiency of the inhaled aerosols in constricted airways was much higher than the same deposition efficiency in healthy airways. Current results might help in the understanding of the adverse health effects of the inhaled air-pollutants in patients with lung disease and might be integrated into future aerosol therapy protocols.