A method of predicting the particle removal efficiency of gravitational wet scrubbers and the particle size distribution properties, that considers diffusion, interception, and impaction, is presented to study the particle removal mechanisms of gravitational wet scrubbers. This method assumes a lognormal size distribution of aerosol particles as well as three additive collection efficiencies. Thus, the overall collection efficiency is described as the sum of all three. It is represented as a U-shaped curve with a minimum in the region of around 1.0 mm in particle diameter. This allows aerosols in the diffusion-and in the impactiondominant regions to be removed at a higher rate compared with aerosol in the intermediate region. As aerosols pass through the gravitational wet scrubber, the geometric standard deviations of the size distribution of polydispersed aerosols decrease. The geometric mean diameter of aerosol in the diffusion-dominant region increases, whereas it decreases in the impaction-dominant region. The present study also shows that in optimum operation conditions such as low droplet falling velocity, small droplet size, and high liquid-to-gas flow ratio, the gravitational wet scrubber has sufficient ability to remove particles whose diameters are much smaller than 1.0 mm.

ABSTRACT. The collection of small particles by a system consisting of multiple fluid spheres such as water droplets or gas bubbles was studied analytically. Kuwabara's free vorticity model based on the solid particle system was extended to include the effects of induced internal circulation inside a liquid droplet or gas bubble system on the flow and the mass transfer rate. Using the resolved flow field, analytic solutions were obtained for the particle collection efficiencies due to diffusion and interception. The results were applied to the problem of particle capture by gas bubbles in liquid or by droplets suspended in gas. The results indicated that the particle collection efficiency by the multiple fluid sphere system is higher than that by a solid sphere system. This is due to the internal circulation which develops inside the fluid spheres. AEROSOL SCIENCE AND TECHNOLOGY 29:389-401 (1998) O 1998 American Association for Aerosol Research NOMENCLATURE a = radius of sphere b = radius of boundary c, c' = dimensional and dimensionless transferred coordinate particle concentration c, = transferred coordinate particle concentrations at the outside boundary layer d, = diameter of the particle d, = diameter of the sphere D = particle diffusion coefficient E = overall particle collection efficiency L = length of collector system m = positive number M = dimensionless diffusion rate of particle to sphere M, = maximum value of M n, n' = dimensional and dimensionless particle concentration n,, n, = particle concentrations at outside boundary layer and at r = a Pe = Peclet number (= 2ualD)r, r' = d~mensional and dimensionless radial position coordinates u = flow velocity u,, u, = radial and circumferential components of flow velocity a = volume fraction, packing density, or solidity of packed bed 13, 6' = dimensional and dimensionless radii of boundary layer K W Lee and C. H. JungAerosol Science and Technology 29:5 November 1998 q = single sphere efficiency qD, qR = single particle collection efficiencies due to diffusion, interception pO, p1 = viscosity of fluids outer and inner boundary cr = viscosity ratio of inner to outer spheres (= p7p0) w = vorticity 5 = distance between center line and particle position in region far from collection sphere 8 = circumferential position coordinate

The time evolution of particle size distribution due to Brownian and turbulent coagulation (using the kernel of Kruis and Kusters (1997)) was systematically investigated. Using a new de nition of dimensionless size distribution parameters based on the geometric mean values, self-preserving particle size distributions for turbulent coagulation were found to exist. The width of such distributions depends on the initial size distribution as well as the turbulence intensity. When starting with submicron aerosols, however, only the turbulence intensity plays a role in determining the nal selfpreserving form, whereas the initial conditions have no in uence. Typically, broad particle size distributions with ¾ g in the 1.5-1.9 range are obtained. Because of the importance of scavenging by the largest particles in the size distribution, the possibility of developing a "runaway mass" exists, for which some experimental indications in turbulent systems exist.

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A multicomponent aerosol dynamic model, that is suitable for regional/global chemical transport models, was developed by combining a thermodynamic module and a modal approach module and by using a dynamic gas/aerosol relationship. The modal approach, which represents the particle size distribution as the superposition of three log-normal size distributions without any assumption on the characteristics of the log-normal distribution, was introduced for simulating the evolution of the chemical species of aerosol particles efficiently. The dynamic approach was utilized for gas/particle transport mechanisms to simulate mass transfer between the gas phase and size-resolved aerosol particles. Both dynamic approachesthe local equilibrium method and global equilibrium method-are compared, and the importance of dynamic approach compared to the equilibrium approach is demonstrated. It is also shown that the global equilibrium method can reduce the computing time significantly compared to the conventional local equilibrium method without much loss of accuracy.