Case Studies in the Human Respiratory System - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 289

under consideration is highly convoluted, narrow and complex. Furthermore, nasal
cavity replicate casts with wider airways may cause less deposition due to secondary
flows (Häußermann et al. 2001).

8.4.4 Swirl Fraction, λ

The effects of swirl can be investigated by setting the swirl fraction. For this demon-
stration a flow rate of 10 L/min laminar flow is used. The swirl fraction sets the
fraction of the velocity magnitude to go into the swirling component of the flow,
and thus a higher fraction will produce a greater tangential velocity component. This
increases the time taken to travel a given axial distance as the particle is swirling
more and its residence time becomes longer. Additionally the induced drag from
the cross-flow of air helps to reduce the initial high momentum of the particle, and
the chances of particles travelling through the frontal regions of the nasal cavity
increases. Figure 8.47 shows high deposition of particles occurring in the frontal
regions (Regions 1-3).

Fig. 8.47 Particle deposition within different sections of the nasal cavity. Sections 1-3 are the
frontal region, Sects 4-7 are the middle region and Sects 8-10 are the posterior region

The amount of swirl has a different effect for the two different particle sizes. For

10 μ m particles with λ = 0 . 9, deposition in the frontal regions increases, while for

λ m particles,

an increase in the swirl fraction decreases the deposition in the frontal zones. Some
deposition in the middle zones occurs, and 2.2 % of particles escape when the
λ

= 0 . 5, a higher percentage of particles escape. Conversely, for 20 μ

= 0 . 9. The reason for these local deposition patterns can be better understood