Geoscience Reference
In-Depth Information
Interception
The collision efficiency for interception,
E
I
, indicates the proportion of particles
remaining in the streamline when the distance of the streamline of fluid flow to an
approaching raindrop is smaller than their radius. Obviously, this phenomenon is
mainly of interest for the largest particles. According to Slinn (
1984
),
E
i
can be
expressed as
air
H
2
O
C
2
p
Re
d
1
4
D
p
D
d
D
p
D
d
E
i
D
C
(8.13)
where
air
is the dynamic dry viscosity of air and
H
2
O
is the water viscosity.
Inertial Impaction
The collision efficiency due to impaction,
E
imp
, concerns large particles, i.e., those
for which inertia does not allow them to follow the streamlines of the air when
a raindrop is approaching; thus, the collision occurs and the droplet catches the
particle.
A critical Stokes number St* can be defined as the maximum S
t
of the relative
flow a particle can have without impacting, when it is directly in the path of the
approaching droplet:
12
1
10
C
12
ln .1
C
Re
d
/
St
D
(8.14)
1
C
ln Re
d
According to Slinn (
1982
), for St > St*,
E
imp
can be expressed as
1
=
2
St
!
3
=
2
d
p
St
E
imp
D
(8.15)
St
C
2
3
St
When looking at collision processes, it appears that the number of collisions
between a droplet of a given size falling in an atmospheric column and the particles
contained in this column is highly dependent on the particles size. This efficiency
is minimum for particles having a diameter around 0.5 m (Greenfield
1957
)
(Fig.
8.1
). When the particle size decreases,
E
increases due to Brownian diffusion.
When the particle size increases, the collision efficiency increases due first to the
effect of interception and then to the effect of inertial impaction.
Slinn (
1982
) emphasizes the point that although the collision processes dis-
cussed above are reasonably well known, other processes such as particle growth,
thermophoresis (i.e., the effect of the force induced by the existence of a tem-
perature gradient) or electrophoresis (i.e., the effect of the force resulting from
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