Page 131 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

4.2 Rectilinear Particle Motion 105

Z t s ffiffiffiffiffiffi r ffiffiffiffiffiffiffi
D p D p t
NðtÞ¼ n 0 dz ¼ 2n 0 ð4:34Þ
pz p
0
2
where N(t) has a unit of (#/m ).
Again, z is the dummy variable for integration. In engineering practice, there is
always limited space for containers, and therefore, this equation can only be used to
set the upper limit of losses to the walls of the container.
The preceding analyses were based on the properties of a single particle without
considering the particle size distribution. The same principles may be applied to a
group of particles with a uniform size well dispersed in air, which is referred to as
monodisperse aerosol.

4.3 Particle-Surface Interaction

The particle is subjected to adhesion force between the particle and the solid surface
and pull-off force. When the adhesion force is greater than or equal to the pull-off
force, the particle adhere to the surface, otherwise, the particle leaves the surface.
The former is called attachment and the latter reentrainment. The adhesion force is
primarily the van der Waals force between two rigid spheres [3] and the pull-off
force can be quite different including, but are not limited to, drag, shear, and
impaction from other aerosol particles.
Unlike a gas molecule, most aerosol particles, especially the submicron sized
ones, attach firmly to a surface they come in contact. When the particle comes in
contact with another particle, both particles adhere to each other, and the process is
referred to as coagulation. Various particle separation devices such as fiber filters
are designed to separate particles from the gas stream by taking advantage of the
adhesion. It is especially important for submicron particles because the adhesive
forces on submicron particles exceed other common forces by orders of magnitude.
Closely related to adhesion of particles are the processes of resuspension and
reentrainment. They are important for the buildup and removal of particles on
surfaces. For example, for fugitive dust emissions caused by vehicles on paved and
unpaved roads.
Wang et al. [27] investigated the resuspension of micron sized particles
(0.4–10 μm) from a flat surface simulating a ventilation duct. The results in Fig. 4.3
show that the resuspension rate depends on the time, air speed, and the particle size.
In-depth analyses suggest that more particles slide and roll off rather than being
lifted into the air flow.
Not all airborne particles adhere to the surfaces in contact. It depends on the
properties of both the particle and the surface, and the nature of the impact between
the particle and the surface. A large solid particle impacting a hard surface at high