Page 126 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

100 4 Properties of Aerosol Particles

4.2.3 Aerodynamic Diameter

Aerodynamic diameter (d a ) is an equivalent diameter that finds many applications
in aerosol characterization and particulate emission control. It is defined as the
3
diameter of a spherical particle with a standard density of 1,000 kg/m that has the
same settling velocity as the real particle, when both of them present in the same
gravitational field.
Figure 4.2 illustrates the aerodynamic diameter of an irregular particle. Imagine
two particles, one is spherical with a standard density (ρ o ) and a diameter (d a ) and
another is nonspherical, are released in the same calm air. They have the same
aerodynamic diameter if they fall with the same settling velocity. In another word,
the aerodynamic diameter of the nonspherical particle is d a .
For the same gravitational settling velocity, it can be written in terms of aero-
dynamic diameter and equivalent geometric diameter as
2 2
q d gC c q d gC c
p e
0 a
v TS ¼ ¼ ð4:25Þ
18lS f 18l
3
where q is the standard particle density, 1,000 kg/m , which is the same as water at
0
normal condition.
The aerodynamic diameter of a particle is the key particle property for evaluating
or comparing the performances of different types of particulate air cleaners, as to be
introduced later in this book. Many particulate matter emission control devices, such
as cyclones and filters, separate particles from the gas/air stream aerodynamically.

Fig. 4.2 An irregular particle and its equivalent volume diameter and aerodynamic diameter