Page 360 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

338 11 Air Dispersion

Fig. 11.9 Plume rise versus 90
distance
80
70
60
Plume rise, m 50
40
30
20
10
0
0 100 200 300
Distance downwind x, m

11.3.3 Plume Downwash

Opposite to plume rise, a plume may drop due to the interaction between the plume
and the atmosphere near the stack. This is called plume downwash. Plume down-
wash may result in an increase of ground-level air pollutant concentrations, because
of the lower final plume height and decreased buoyancy in case of buoyant emis-
sions. Canepa [7] gave a comprehensive overview about the studies of downwash
effects in air dispersion.
As shown in Fig. 11.10, a stack downwash is a result of the wake downwind of
an emitting stack due to the stack itself. Stack downwash is not a big problem for
tall and large utility and industrial stacks, but it is important for short stacks because
of the low wind speed at lower elevation.
There are many models developed over the past 70 years for plume downwash,
and the basics are introduced as follows. Generally speaking, stack downwash

Fig. 11.10 Schematic v s
representation of stack
downwash u

Stack

Ground