Page 117 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

Chapter 4
Properties of Aerosol Particles

As a counterpart of Chap. 2, this chapter covers the basic properties and dynamics
of aerosol particles. An aerosol is a mixture of solid particles and/or liquid droplets
suspended in a gas. The gas phase can be air or other gases. In air pollution studies,
we can also call it particulate matter. In this book, particulate matter (PM) is
interchangeable with aerosol without examining their fine differences. The particles
can be either solid aerosol particles or liquid droplets with little deformation or
evaporation.
This chapter starts with classic particle dynamics followed by basic terms that
are widely used in air emission engineering including aerodynamic diameter,
equivalent diameters, Stokes number, Stokes law, adhesion and reentrainment of
particles, and diffusion of particles in the air. At the end, particle size statistics and
dynamics are introduced.

4.1 Particle Motion

It is important to analyze the behavior of particles in various force fields, which
guides the design and operation of many particulate air pollution control devices. In
most of the cases, the particle is subject of at least two forces acting along opposite
directions, one is parallel and another opposite to the direction of motion. The latter
is the resistance of the surrounding gas to the particle in motion.

4.1.1 Particle Reynolds Number

In particle dynamics, the particle Reynolds number follows similar definition except
that characteristic length is the particle diameter, d p , and that the velocity in Eq. (2.69)
is replaced with the magnitude of the velocity (speed) of gas with respect to that of the
particle, u vj.
j

© Springer Science+Business Media Singapore 2014 91
Z. Tan, Air Pollution and Greenhouse Gases, Green Energy and Technology,
DOI 10.1007/978-981-287-212-8_4