Numerical Simulation of Scavenging of Small Particles by Charged Droplets

Abstract

The trajectories of ne 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 ow eld 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 ef ciency 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 ef ciency is still higher than for uncharged particles due to the action of the image forces. In this case, the INTRODUCTIONThe problem of determination of the particle trajectories near a spherical collector occurs in electrostatic scrubbers, in which ne dust particles are removed by charged droplets, due to the attractive Coulomb forces. Such a technique eliminates many shortcomings inherent in conventional electrostatic precipitators and inertial scrubbers that do not allow for the effective control of the dust particles in the submicrometer size range. In conventional inertial scrubbers the inertial force of the particles smaller than a few ¹m in diameter is too low to deposit it effectively on the collector. In the case of conventional electrostatic precipitators, a low precipitation ef ciency of the dust particles in this size range is caused by the dif culty in charging them to levels higher than a few elementary charges, which results in a low particle mobility. In the electrostatic scrubbers the Coulomb force drives the particle toward the charged droplet.Deposition of charged airborne particles on an oppositely charged spherical collector has been investigated by many authors. However, the solutions have been obtained on numerous restrictive assumptions, one of which is the approximate model for the ow eld around the collector. Also, a xed, instead of a...

Uncertainty assessment of current size-resolved parameterizations for below-cloud particle scavenging by rain

“…A number of studies have suggested that thermophoresis, diffusiophoresis, and electric charges may increase E(d p ,D p ) for particles in the 0.01−1 µm diameter range (e.g., Slinn and Hales, 1971;Grover et al, 1977;Wang et al, 1978;McGann and Jennings, 1991;Byrne and Jennings, 1993;Pranesha and Kamra, 1997;Tripathi and Harrison, 2001;Tinsley et al, 2000;Jaworek et al, 2002;Andronache, 2004;Chate, 2005;Andronache et al, 2006). Thermophoresis, which is caused by uneven heating of particles in ambient temperature gradients, drives particles towards evaporating and sublimating hydrometeors.…”

Ultrafine particle scavenging coefficients calculated from 6 years field measurements

“…2), Schumann (1989) and McGann and Jennings (1991). One reason for discrepancy can be the electrical interaction between charged rain droplets and aerosol particles (Jaworek et al, 2002). An extensive analysis of the possible reasons for this discrepancy is given by Volken and Schumann (1993).…”

“…This applies to particles with diameter more than about 1 mm: For particles smaller than 1 mm Brownian diffusion is the main removal mechanism. The effect of electrical forces is often (McGann and Jennings, 1991;Byrne and Jennings, 1993;Jaworek et al, 2002) assumed to be one of the major sources of uncertainty in the calculations of scavenging coefficients.…”

Below-thunderstorm rain scavenging of urban aerosols in the health hazardous modes

“…Elapsed-time evolutions of urban aerosols in the health hazardous mode (0.02 lm B D p B 0.1 lm) after 30 and (Table 1) 60 min thunderstorm rain in Figs. 2 and 3 support the notion that electrical and phoretic effects are most active in scavenging these particles (Jaworek et al 2002;Andronache 2004;Chate and Pranesha 2004;Chate 2005). Model study of Garcia et al (1994) illustrates that when compared to initial volume of respirable dust (before rain), it remains *60% after 12 h of drizzle (0.5 mm h -1 ) or 1 h of heavy rain (25 mm h -1 ).…”

“…The capture of aerosol number by falling raindrop takes place with Brownian diffusion, inertial impaction, diffusion-thermo-phoresis and electrical effects (Wang et al 2010). Beside strong electrical forces, on aerosols and raindrops during thunderstorm, rapid evaporation and condensation cause phoretic effects (Jaworek et al 2002;Andronache 2004;Chate 2005). Scavenging coefficients from measured aerosol size distribution changes by rain in different environments have been reported elsewhere (Davenport and Peters 1978;Laakso et al 2003;Chate and Pranesha 2004;Maria and Russell 2005;Andronache et al 2006).…”