Chuen Jinn Tsai

Elastic flattening and particle adhesion

The effect of elastic flattening on particle adhesion to solid surfaces has been investigated. The attracting and repulsing molecular potential energies and forces are obtained for a flattened sphere-surface system at first. The energy conservation principle and force balance are then used to obtain the analytical equations that describe the relationship between the elastic flattening and van der Waals adhesion force. This is Particle Technology Laboratory Report No. 754.

Particle detachment from disk surfaces of computer disk drives

Abstract The air drag force on a deposited spherical particle which is present inside the slider airbearing of a computer disk drive has been found to be very substantial. A critical moment model has been proposed for determining the critical condition to detach a smooth spherical particle from a smooth solid surface. Using the model, it has been found that the detachment of submicron particles from computer disk surface by a slider air-bearing is possible. The critical speed for particle detachment from disk surfaces has been calculated, which depends on the adhesion parameter, surface energy and particle size. This model can be experimentally validated using computer disk drives.

Novel active personal nanoparticle sampler for the exposure assessment of nanoparticles in workplaces.

A novel active personal nanoparticle sampler (PENS), which enables the collection of both respirable particulate mass (RPM) and nanoparticles (NPs) simultaneously, was developed to meet the critical demand for personal sampling of engineered nanomaterials (ENMs) in workplaces. The PENS consists of a respirable cyclone and a micro-orifice impactor with the cutoff aerodynamic diameter (d(pa50)) of 4 μm and 100 nm, respectively. The micro-orifice impactor has a fixed micro-orifice plate (137 nozzles of 55 μm in the inner diameter) and a rotating, silicone oil-coated Teflon filter substrate at 1 rpm to achieve a uniform particle deposition and avoid solid particle bounce. A final filter is used after the impactor to collect the NPs. Calibration results show that the d(pa50) of the respirable cyclone and the micro-orifice impactor are 3.92 ± 0.22 μm and 101.4 ± 0.1 nm, respectively. The d(pa50) at the loaded micro-Al(2)O(3) mass of 0.36-3.18 mg is shifted to 102.9-101.2 nm, respectively, while it is shifted to 98.9-97.8 nm at the loaded nano-TiO(2) mass of 0.92-1.78 mg, respectively. That is, the shift of d(pa50) due to solid particle loading is small if the PENS is not overloaded. Both NPs and RPM concentrations were found to agree well with those of the IOSH respirable cyclone and MOUDI. By using the present PENS, the collected samples can be further analyzed for chemical species concentrations besides gravimetric analysis to determine the actual exposure concentrations of ENMs in both RPM and NPs fractions in workplaces, which are often influenced by the background or incident pollution sources.

Capture and Rebound of Small Particles Upon Impact with Solid Surfaces

A new particle bounce model has been proposed and compared with the previously published experimental data. By considering the relationship between contact deformation mechanics and contact surface energy, it is possible to calculate both the energy required to break the contact surface and the unrecoverable energy contained in the bulk plastic deformation zone when the material yield point is exceeded. Energy spent to deform the local asperities between the contact surfaces was also found to be important when the incident kinetic energy is small. The present model considers the breaking of the contact surface, plastic deformation, and local asperity deformation, to be the major energy loss mechanisms during the impaction process. Both the elastic and plastic deformations have been treated, thus making it possible to calculate the coefficient of restitution over a wide range of particle incident speeds.

Numerical study of particle deposition in bends of a circular cross-section―laminar flow regime

Particle deposition in a 90° bend has been studied numerically using a realistic three-dimensional developing flow field. In addition to the Stokes number as the impaction parameter, both the curvature ratio and the Dean number have been found to have considerable effects on the deposition efficiency. At a fixed Stokes number, the deposition efficiency increases with an increasing Dean number and a decreasing curvature ratio. The inlet velocity profile also influences the deposition efficiency. In the case of a parabolic velocity profile, the deposition efficiency is always higher than that of a uniform profile. These increases in deposition efficiency are due to the increase of secondary flow strength and the increased skewness of the axial velocity profile toward the outside of the bend.

An Efficient Single-Stage Wet Electrostatic Precipitator for Fine and Nanosized Particle Control

In this study, an efficient parallel-plate single-stage wet electrostatic precipitator (wet ESP) with a width of 75 mm, effective precipitation length of 48 mm and gap of 9.0 mm was designed and tested to control fine and nanosized particles without the need of rapping. The collection plates are made of sand-blasted copper plates coated with TiO2 nanopowder instead of hydrophilic membranes. Three gold wires (diameter: 100 μ m) were used as the discharge electrodes and a pulse jet valve was used to regularly purge the wires. The design of the present wet ESP is aimed at solving the problems of traditional dry ESPs: reduction of the collection efficiency due to particle deposition on the discharge electrodes and collection electrodes, back corona, and particle re-entrainment. The collection efficiency at initially clean and heavy particle loading conditions was tested and compared to a similar dry ESP. Experimental results showed that when the wet ESP was initially clean, the particle collection efficiency ranged from 96.9–99.7% for particles ranging from 16.8 to 615 nm in electrical mobility diameter at an aerosol flow rate of 5 L/min (residence time of 0.39 s) and an applied voltage of 4.3 kV. After heavy loading with TiO2 nanopowder about 1.2 ± 0.06 g/plate, the collection efficiency of the present wet ESP for corn oil particles was shown to reduce only slightly to 94.7–99.0% for particles from 16.8 to 615 nm in diameter.

Chemical mass closure and chemical characteristics of ambient ultrafine particles and other PM fractions

Ambient ultrafine particles (UPs or PM 0.1 ), PM 2.5 and PM 10 were investigated at the roadside of Syuefu road in Hsinchu city and in the Syueshan highway tunnel in Taipei, Taiwan. A SMPS (TSI Model 3936), three Dichotomous samplers (Andersen Model SA-241), and three MOUDIs (MSP Model 110) were collocated to determine the PM number and mass concentrations simultaneously. The filter samples were further analyzed for organic carbon (OC), element carbon (EC), water-soluble ions, and trace elements. The OC artifact was studied and quantified using the quartz behind quartz (QBQ) method for all PM fractions. Taking into account the OC artifact, chemical mass closure (ratio of the reconstructed chemical mass to the gravimetrical mass) of PM 0.1 , PM 2.5 , and PM 10 was then calculated and found to be good. The chemical analysis results of UPs at both sites showed that UPs in the present tunnel was mostly contributed from the vehicle emissions while UPs at the roadside was mainly influenced by urban sources.

Particle transmission characteristics of an annular denuder ambient sampling system

Transmission measurements have been performed on University Research Glassware, Inc. model 2000–30B glass annular denuders at 10 L/min using monodisperse particles in the 0.01–1-μm diameter size range. Through control of the aerosol charge state, particle losses due to diffusion and electrostatic effects were separately measured and theoretical descriptions of both were developed. For Boltzmann charged (atmospheric) particles in the important 0.1–1-μm diameter size range total losses averaged only a few percent or less. Particle transmission measurements also confirmed the design value of 2.5 μm for the 50% cutpoint diameter of the URG 2000-30EN cyclone inlet used with these annular denuders.

The Influence of Relative Humidity on Nanoparticle Concentration and Particle Mass Distribution Measurements by the MOUDI

A humidity control system was operated upstream of two collocated MOUDIs (micro-orifice uniform deposit impactors) for sampling ambient aerosol particles. One MOUDI used silicone-grease-coated aluminum foils (ALs) as the impaction substrates and was considered as the reference impactor, while the other used uncoated ALs or uncoated Teflon filters (TFs) as the impaction substrates for quantifying the effect of different relative humidities (RHs) and impaction substrates on the PM0.1 concentrations and mass distributions of ambient PMs. Test results showed that decreasing RH in general increased particle bounce from uncoated substrates with the bounce from uncoated ALs being more severe than that from uncoated TFs. Particle bounce did not influence the overall mass distribution of ambient fine particles when RH ranged between 40% and 80%, whereas it led to undersampling of particles greater than 2.5 μm in aerodynamic diameter severely. Oversampling of PM0.1 occurred by as much as 95%–180% or 25%–55% when the MOUDI used uncoated ALs or TFs, respectively, as RH was reduced from 50% to 25%. Particle bounce was found to be negligible, and PM0.1 and PM2.5 could be sampled accurately with less than 5% error at the RH of 75%–80% or 65%–80% when uncoated ALs or TFs were used, respectively.

PERFORMANCE EVALUATION OF AN API AEROSIZERTM

Abstract A numerical method was developed to simulate the compressible flow field and particle trajectory in an API Aerosizer TM . Experimental particle time-of-flights (τ TOF ) data using monodisperse solid and liquid particles were also obtained. Theoretical particle τ TOF results were compared with the present experimental data and those of Cheng et al. (1993, J. Aerosol Sci. 26 , 501–514). Good agreement was found for solid particles. The original τ TOF calibration for the API Aerosizer TM was found to underestimate measured τ TOF mainly because of the assumption of one-dimensional flow in the nozzle, and incorrect use of the particle drag coefficient in the supersonic flow field. Effects of decreasing ambient pressure and increasing particle density on increasing τ TOF were found to be important.