Turbulent stack plumes

One of the most obvious manifestations of air pollution is the visible plume formed downwind from a stationary source, A relatively. simple model for such systems Ls the continuous point source in a turbulent fluid with a mean velocity, tl(j , z). The coordinate. T is measured downwind from the source, parallel to the ground, and z is the coordinate perpendicular to the surface (Fig. 11.6). The velocity components in the y and z directions vanish, and diffusion in the a direction can be neglected compared with convection. Brownian diffusion is also neglected compared with eddy diffusion. These are the usual simplifying assumptions made in the theory of diffusion of molecular species in turbulent stack plumes, and with them (11.38) becomes... 

Figure 11,6 Schematic diagram of turbulent stack plume with coordinate system employed in the text.

Ejfective Height of an Emission The effective height of an emission rarely corresponds to the physical height of the stack. If the plume is caught in the turbulent wake of the stack or of buildings in... 

The final effective plume height H, in m, is stack height plus plume rise. Where buoyancy dominates, the horizontal distance Xf from the stack to where the final plume rise occurs is assumed to be at 3.5 , where x is the horizontal distance, in km, at which atmospheric turbulence begins to dominate entrainment. 

One simplified method for determining stack height is a geometric method described in ASHRAE. The geometric method assumes an exhaust plume shape with a lower boundary having a 1 5 slope relative to the horizontal. The stack and plume are raised until the lower plume boundary is above rooftop penthouses, separation zones, and zones of high turbulence. ASHRAE provides equations for the sizes and locations of the separation and turbulence zones. A stack height reduction credit is provided to account for the vertical exhaust momentum. 

An alternative simple model for contaminant dilution of rooftop exhaust stacks is presented in Halitsky. This model combines a jet region specification for the upward exhaust movement with a more traditional Gaussian plume region controlled by atmospheric and building-generated turbulent dilution. 

The effective height of an emission rarely corresponds to tlie physical height of tlie source or the stack. If tlie plume is caught in tlie turbulent wake of tlie stack or of buildings in the vicinity of tlie source or stack, tlie effluent will be mixed rapidl) downward toward the ground. If the plume is emitted free of these turbulent zones, a number of emission factors and meteorological factors influence tlie rise of the plume. 

Transport involves the mean wind speed and direction. Dispersion is produced primarily by atmospheric turbulence and is discussed in more detail below. Transformations of the pollutants are due to chemical reactions, deposition, and interactions with liquid water. These constitute separate problems treated elsewhere. Most air pollution arises from continuous emitting sources, such as stacks or highways. They may be groimd or elevated sources. An important parameter in the determination of concentration is the effective source height, which is the sum of physical source height and plume rise. 

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