Deposition velocities boundary layer theory

Flow around single cylinders is the elementary model for (he fibrous filter and is the geometry of interest for deposition on pipes, wires, and other such objects in an air flow (Chapter 3). The flow patterns at low and high Reynolds numbers differ significantly, and thi.s affects impaction efficiencies. For Re > 100. the velocity distribution outside the velocity boundary layer can be approximated by inviscid flow theory. This approximates the velocity distribution best over the front end of the cylinder which controls the impaction efficiency. The components of the velocity in the direction of the mainstream flow, x, and normal to the main flow, y, are... 

Analysis of Environmental Data. Although the methodology for analyzing the data has been previously reported ( ) there are some differences that should be noted. First, a later version of the RAPS data base was used as an initial sourse. Second, time-of-wetness in this paper is defined differently, thus, a relationship to calculate relative humidity from temperature and dew point is based on data for dew points greater than 0 C. Third, deposition velocities are calculated from boundary layer theory rather than empirical relationships. 

However, we should always expect the deposition velocity derived from boundary layer theory to be somewhat larger than observed in practice (neglecting measurement errors). Since the physical chemistry limitations should be independent of size and shape of the object in question, we may use boundary layer calculations to indicate the relative characteristics of different situations. 

For SO2 in air, Vj/u o = 0.36 Cf, providing a means to estimate deposition velocities from boundary layer theory or experimental results from first principles. 

Reference (3) reanalyzed data from a number of such outdoor test sites and derived an SO2 deposition velocity of 1.55 - 1.75 cm./sec. for zinc, operable only during times of surface wetness. In that report, reference was made to SO2 deposition velocities over water surfaces (1.6 cm/sec), which are likely to be low turbulence situations. It was also noted that the deposition velocity to water is dependent upon atmospheric stability. SO2 deposition velocities for copper and aluminum were less straightforward (3), but appeared to be somewhat lower, perhaps reflecting less chemically active surfaces. The zinc result is reasonably consistent with the theoretical values developed above from boundary layer theory and tests, since it lies between these values (1.2 - 3.7 cm/sec). Incorporating actual test site wind speeds could obviously help reduce the scatter in these determinations. 

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