An Experimental Determination of the Efficiency with Which Aerosol Particles are Collected by Water Drops in Subsaturated Air

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  • 1 Deparment of Atmospheric Sciences, University of California, Los Angeles 90024
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Abstract

Experiments have been carried out to determine the efficiency with which aerosol particles of 0.25 μm radius are collected due to Brownian diffusion, and due to hydrodynamic, phoretic and electrical effects by water drops of 150 to 2500.μm equivalent radius falling in subsaturated air. In the absence of electrical effects it was found that with increasing drop size the collection efficiency decreases to a minimum and then rises again as the collection due to phoretic forces is overcompensated by the collection due to hydrodynamic forces. With further increase in drop size the collection efficiency was found to rise to a maximum, This rise was attributed to hydrodynamic effects in the rear of the drop which increase as the stagnant eddy at the downstream end of the falling drop increases in size, but progressively decrease as the drop assumes a size, and thus a Reynolds number, large enough for turbulent eddies to be shed from the rear of the drop. The present results are qualitatively consistent with the predictions of experiments reported in the literature and quantitatively agree with the theoretical predictions made by the model of Grover et al. (1977). Electrical charges on drop and aerosol particles were found to significantly raise the collection efficiency, in good agreement with the efficiency theoretically predicted by the model of Grover et al. (1977).

Abstract

Experiments have been carried out to determine the efficiency with which aerosol particles of 0.25 μm radius are collected due to Brownian diffusion, and due to hydrodynamic, phoretic and electrical effects by water drops of 150 to 2500.μm equivalent radius falling in subsaturated air. In the absence of electrical effects it was found that with increasing drop size the collection efficiency decreases to a minimum and then rises again as the collection due to phoretic forces is overcompensated by the collection due to hydrodynamic forces. With further increase in drop size the collection efficiency was found to rise to a maximum, This rise was attributed to hydrodynamic effects in the rear of the drop which increase as the stagnant eddy at the downstream end of the falling drop increases in size, but progressively decrease as the drop assumes a size, and thus a Reynolds number, large enough for turbulent eddies to be shed from the rear of the drop. The present results are qualitatively consistent with the predictions of experiments reported in the literature and quantitatively agree with the theoretical predictions made by the model of Grover et al. (1977). Electrical charges on drop and aerosol particles were found to significantly raise the collection efficiency, in good agreement with the efficiency theoretically predicted by the model of Grover et al. (1977).

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