Filtration and Sampling of Aerosols by Fibrous Filters - Aerosols: Measurement, Dosimetry, and Health Effects - page 443

Streamlines

1

Fiber

3

2

Flow
direction

FIGURE 17.2 Main mechanisms of aerosol capture by ibers: 1, diffusion; 2, interception; 3, inertia.

and in the second it is diminishing by the value of order η G ∝ G (1 + R ), where G is the ratio
of the settling velocity to low velocity U [8]. Deposition eficiency of the aerosol particles is
essentially affected by the forces of intermolecular interaction, or van der Waals forces. In some
cases neglecting this mechanism can result in 15%-20% underrating of the design eficiency of
the ilter.
The task of calculating the ilter eficiency comes down to determining the capture coeficient
depending on the parameters of the ilter, the particles and the medium:

(17.6)

η η = ( Re Kn Pe R St G F a

i

, , , , , , , , α σ ε , , , … )

where
Kn = λ/ a stands for the Knudsen number, that characterizes the ratio of the mean free-path length
of the gas molecules to the iber radius
Pe = 2 Ua/D is the diffusive Peclet number, that characterizes convective transport predominance
over the diffusive one, D is the coeficient of the particle diffusion, given by

k TC Kn
r

( )

(17.7)

B

D =

6πμ

where
C ( Kn ) is the Cunningham correction factor [30]
k B is the Boltzmann constant
T is the temperature
μ is the gas viscosity

Table 17.1 gives the values of the diffusion coeficient for particles smaller than 1 μm.
R = r p / a is the interception parameter
St

2 ρ

= 2 C Kn r U ( ) /9 μ is the Stokes number (inertia parameter) a

p