31

FOcm DFC-11 (1z-61)

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Memorandum Denver, Colors 4- - - - - - - W. E. Wagner February 12, 1973 BUREAU OF R~n CI..:gJ.TrOi~ - - _ - _ ---

.HYDRAULiC LAB;}t,A'TOkY,.:

D. L. Kin8 e~c~. --_

ri = - - -- Steven Mallen R 1T _I ORROW~D R.ETURl1 FROUFTLY

Phoresis scavenging of small aerosol particles -----------

Study of the theory of diffusiophoretic and thermophoretic forceg near - - - - - - an evaporating water droplet leads to the following expression fQr the-efficiency of a droplet of radius R to collect a particle of radiLus-a---------

~ '~/ 7

GPO 832-606

E

INFORMATIONAL ROUTING HYDRAULICS BRANCH OFFICIAL FILE COPY

PAP 4

where UT(R) is the terminal velocity of a droplet of radius R and

is the combined phonetic velocity of a particle with a 0.1 Micron, V is a ventilation factor and (TA, - TD) is the temperature difference between the ambient and the droplet. At 10°C and 60 percent relative humidity the temperature difference is approximately 6-7°C.

In Figure 1 values of E are plotted versus R for a a 0.1 micron. Also shown (dashed curve) is a plot of E versus height for a droplet that completely evaporates by the time it reaches the ground.

The percentage of particles removed can be calculated from the following expression

where N is the original number of particles per CC and W is the washout coefficient which is determined from the efficiency and droplet number and size distribution.

in a typical experiment we consider a volume of air 10 m square and 100 to above the surface, Assuming we inject water at the rate of 4,` gm/sec (approximately 160 gal/min) at 10°C and 60 per-cent relative humidity it would take ,.,t/)4 seconds to remove 70 per-cent of the particulates or 26,500 gallons of water. Figure 2 shows the percentage of particles removed versus time. Such inefficiency can be explained by the height versus efficiency curve in FiLture 1. Of the entire 100 m only the last 1-2.meters are efficiently cleared, the con-tintious spraying is needed to clear the upper 98 percent of the volume where the efficiency is low.

An alternative would be to spray 30 micron_radii-droplets at 1 m intervals all along the height of the column. Calculations using the same water input rate showed no increase in removal efficiency.

In conclusion the proposed study would be highly inefficient for particulate removal.

Vote: One or two possible alternatives have not as yet been inves-tigated; however, my present knowledge of their potential for increasing efficiency does not warrant expenditure of any further work time in developing them. I will, however, investigate these areas on my own time and report any conclusions which would justify submitting a proposal for continued research and application.

J SMallen:vlh-s

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