Hwa-Chi Wang

Filtration efficiency of nanometer-size aerosol particles

Abstract A filter efficiency model for nanometer-size particles was developed which incorporates the effect of particle rebound from the filter surface due to their thermal velocity. The thermal rebound probability is calculated using the JKR adhesion theory [Johnson, K. L., Kendall, K. and Roberts, A. D. (1971) Proc. R. Soc. Lond. A. 324, 301] and the Boltzmann velocity distribution. In addition to the two classic filtration regimes (diffusion and impaction/interception) a third, ‘thermal rebound’, regime is identified below ≈10 nm, in which filter efficiency decreases progressively and complete penetration is predicted for inert gas molecules. The particle size below which there is a measurable deterioration of filter efficiency depends sensitively on particle-surface adhesion energy and temperature, besides other filter parameters.

On the shape of impactor efficiency curves

The effect of impactor stage geometry on impactor collection efficiency curves is explored numerically. It is found that the geometry of the impaction stage where the gas is accelerated does not affect the 50% cut size but has a strong effect on the sharpness and shape of the efficiency curve. This phenomenon is explained from the aerodynamic focusing effect of the particles in the nozzle (gas acceleration) section. The flat-plate orifice configuration, commonly used in cascade impactors, tends to focus particles closer to the centerline than the angled nozzle. When the real nozzle configuration is used, the prediction shows a characteristic S shape, in good agreement with the experimental results in the literature.

An improved constant output atomizer

An improved atomizer based on the design of Liu and Lee has been developed. The atomizer requires a solution feed rate of only 0.1 mL/min for a stable output. A monodis-perse aerosol generator can be obtained using an electrostatic classifier and the system is capable of continuous operation for extended periods of over 24 hours depending on the size of the solution reservoir used.

The modelling of particle resuspension in turbulent flow

Abstract The resuspension of small particles by a flowing gas occurs over time periods much longer than the time required to obtain fully developed flow. In this work an analysis of particle shedding is presented and compared with experimental data. The model used here is compared with other published models (e.g. Wen and Kasper, 1989 , Reeks et al. 1988 ) The effect of the adhesion and removal force distributions on the particle shedding rate and the evolution of the adhesion force distribution through time is examined. For a wide range of adhesion and removal force distributions the experimentally observed long time behavior is found.

Comparison of Experimental and Theoretical Heterogeneous Nucleation on Ultrafine Carbon Particles.

Using a modified turbulent mixing CNC, the heterogeneous nucleation of different compounds (working fluids) on nanometer sized carbon particles was examined. The working fluids were dibutyl phthalate, octadecane, octadecanol, and octadecanoic acid. Based on the particle size distributions measured with a scanning mobility particle sizer system, nucleation and consequent growth were examined with respect to different temperature and vapor pressure for each working fluid. Nucleation rates for all conditions were calculated from the fitted size distribution data by subtracting the residual nonactivated particle concentration for each condition. Experimental nucleation rates were compared to the calculated ones based on Fletchers heterogeneous nucleation theory. This theory matches well with the experiments with octadecanol and octadecanoic acid, and at high supersaturation ratios for dibutyl phthalate. However, the theory shows discrepancies with the observed phenomena at low supersaturation for dibutyl phthalate, and especially for octadecane. Several possible hypotheses for the discrepancies and observed particle growth are discussed.

A transition from heterogeneous to homogeneous nucleation in the turbulent mixing CNC

A new method for changing the supersaturation in the Turbulent Mixing CNC has been developed and used to examine the transition from heterogeneous nucleation of test particles to homogenous nucleation of working fluid: dibutylphthlate (DBP). Supersaturation was controlled by changing the DBP vapor pressure in the nozzle flow by saturating only a predetermined part of the flow, while the total flow and temperature remain constant. This approach allows for the changing of the initial DBP vapor pressure, while keeping the flow structure and temperature field unchanged. The DBP concentration in the outlet of the vapor generator was measured experimentally for different ratios of saturated and bypass flows and found to be close to estimated values. Experimental results for transitions from heterogeneous nucleation to homogeneous nucleation are presented for NaCl and WOx particles at various DBP vapor pressures. With an increasing of the DBP vapor pressure, the concentration of enlarged particles increases until it reaches a plateau. At higher initial values of DBP pressure, homogeneous nucleation prevails, and the number concentration of particles follows a curve typical for homogeneous nucleation recorded in the absence of nuclei. Nuclei with different mobility diameters were activated at different values of vapor pressure. There are significant differences in the slopes of particle activation curves for NaCl and WOx particles. The reasons for such differences are a subject for continuing research.

Note on inertial deposition of particles on an orifice disc

Abstract A recent numerical model [Ye, Y. and Pui D. Y. H. (1990) J. Aerosol Sci. 21 , 29] to predict particle deposition on an orifice disc was compared with an analytical model by Pich [Pich, J. (1964) Colln. Czech. chem. Commun. 29 , 2223]. These two models agree well on the cut-off size but disagree with regard to the shape of the predicted efficiency curves. Pichs model gives an S-shaped efficiency curve, which can be used to predict particle loss for an orifice-type pressure reduction device.

DESIGN AND PERFORMANCE CHARACTERISTICS OF A TURBULENT MIXING CONDENSATION NUCLEI COUNTER

Abstract The design and optimization of operation parameters of a Turbulent Mixing Condensation Nuclei Counter (TMCNC) are discussed as well as its performance using dibutylphthalate (DBP) as the working fluid. A detection limit of 3 nm has been achieved at a flow rate of 2.8 l min −1 (0.1 cfm). In addition, the effect of saturation temperature on particle growth in the TMCNC was investigated to identify the temperature range where particles could grow to a detectable size regardless of their initial sizes and materials. Size distributions of particles after condensation growth were measured as a function of saturation temperature for three types of nuclei: atmospheric aerosol particles, monodisperse NaCl particles, and liquid DBP particles generated by homogeneous nucleation. The size distribution after condensation growth can be described by a log-normal distribution with σ g =1.2–1.3. The modal diameter of the size distribution can be predicted by the classical equations for particle growth in supersaturated media. The final particle size distribution is a function of the initial particle diameter unless the saturator temperature is above 125°C.

Minimizing the effect of density in determination of particle aerodynamic diameter using a time of flight instrument

Abstract Particle aerodynamic diameter measurement using an aerosizer (a time-of-flight (TOF) particle size measurement instrument) requires assuming the density of particle being measured. In this paper, a relationship between TOF of spherical particles with different densities through three laser beams, and the lumped parameter, Log[ d ae C D −1/2 ], is found. This allows the effect of density in particle aerodynamic diameter measurement to be minimized.

Particle Release from Surfaces by Mechanical Shocks

The influence of mechanical shocks on a gas cylinder, one of the critical components in a gas distribution system, was evaluated with respect to particle generation. An experimental technique was developed for applying impact shocks and for simultaneous measurements of the shock intensity, the corresponding particle concentration and size distribution. It was found that a significant amount of submicron particles could be generated by impact shocks on the order of 103 g. The particle size distribution based on number concentration indicated a peak around 0.1 μm. It is further shown that data reported here might correspond to ppb-ppm resuspension. It implies that allowable shock intensity for today’s high-purity gas distribution systems should be defined based on an extremely low level of resuspension, whereas classical adhesion theory and experiments are based on 50% removal.