Fundamentals of Particle Dynamics - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 144

In the Eulerian-Eulerian approach, the influence of flow properties (mass, mo-
mentum, energy, etc.) from one phase on the other is represented by the terms χ N and
F N in the governing equations. In the Eulerian-Lagrangian method, the flow prop-
erties of the fluid phase are stored in the computational cell. The disperse particle
phase is mesh independent as the particle trajectory is tracked by solving an ordinary
differential equation (ODE). After each integration time step, the particle is moved
and its position and velocity are evaluated. At this new location, the forces acting on
the particle by the surrounding fluid are computed from the values stored in the com-
putational cell. For one-way coupling, the fluid phase properties do not change and
therefore no iteration is needed. For two-way coupling, the fluid phase properties at
the cell in which the particle is present will change due to the particle's motion; and,
thus, an iterative process is needed to compute the new flow condition and particle
position and velocity. In the next section we describe the particle dynamics and the
different forces that act on the particle.

6.3 Particle Diameters

Except for clean rooms, the air typically contains some form of solid particles or
droplets. Particle size influences its physical behaviour, affecting its motion and
dispersion in an airflow field. Therefore, it is important to characterise the particle
properties such as mean diameters, and size distributions.
A particle's characteristics vary due to its originating source, such as the dispersion
of pollen in the air from flowers and bees, diesel/exhaust fumes from cars, and the
atomization of liquid in a nasal spray device or of a powder in metered dose inhalers.
The range of diameters of common aerosol particles is between 0.01

μ m to about

10-40 m (Fig. 6.5 ). The lower limit of 10 nm roughly corresponds to the transition

from molecules to particles. Particles larger than 40

μ

μ m normally do not remain

Fig. 6.5 Typical range of particle diameters. The particles shown are scaled in size relative to each
other