Page 361 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

11.3 Gaussian-Plume Dispersion Models 339

effect becomes important when the exit gas speed is less than 1.5 times the wind
speed, v s \1:5 u. However, low effluent speed does not necessarily cause stack
downwash. Bjorklund and Bowers [6] proposed the following procedure to cal-
culate the final plume rise of a buoyant plume with stack downwash.

0
Dh ¼ f Dh m ð11:44Þ
m
where Dh m is the final plume rise without stack downwash effect determined using
Eq. (11.40). f is the correction factor to the plume rise due to stack downwash. The
correction factor depends on the Froude number ðFrÞ of the stack emission gas and
the square of F r is

v 2
2 s T a
Fr ¼ ð11:45Þ
ð
gd s T s T a Þ
where T a is the temperature of ambient air surrounding the top of the stack. The
correction factor can be determined using Eq. (11.46).

8 2
1 for v s [ 1:5 u OR Fr \3
>
<
u 2
f ffi 31 for u\v s 1:5 u AND Fr 3 ð11:46Þ
v s
>
0 for v s u AND Fr 3
: 2
11.3.3.1 Building Downwash
A building downwash occurs when the plume is near a building and is brought
downward by the flow of air over and around the building. To understand the
building downwash and related plume drop, we have to understand some basic fluid
dynamics. As illustrated in Fig. 11.11, consider a building block attacked by a
horizontal air flow, there are aerodynamic cavity zones produced around the
building: one is the separation zone on the roof, and another cavity zone behind the
building. Sometimes they may merge into one large cavity covering both roof and
downwind the building, depending on the wind speed and the surrounding

Displacement zone boundary
Wake boundary

Cavity
Wake

Fig. 11.11 Schematic representation of building downwash