Biomedical Engineering Reference
In-Depth Information
Fig. 8.49
Total deposition in
the first two regions of the
nasal cavity at different
particle injection velocities
where
ρ
p
d
p
18
μ
g
U
D
=
τ
U
D
St
=
(8.30)
This suggests that for small Stokes numbers (i.e. St
0), the particle velocity
approaches the airflow velocity quickly. For large Stokes numbers (i.e. St
→
),
u
∗
approaches zero meaning that the particle velocity is unaffected by the gas. Thus, it
is expected that rapid changes in the particle velocity will occur within the anterior
section of the nasal cavity. The anterior region of the nasal cavity is further divided
into three regions, and particle flow paths and deposition patterns within this region
are shown in Fig.
8.49
and
8.50
a,
8.50
b, and
8.50
c.
Figure
8.49
shows the total particle deposition in the first two regions of the
nasal cavity at different particle injection velocities. Regions are defined in Fig.
8.1
.
A small influence on the impactability of a 10
→∞
m particle is observed when there
is an increase in the initial particle velocity and is seen by the slight increase in
deposition. The low Stokes number brings about a rapid decrease in velocity, and the
particle assumes the airflow velocity before the change in direction of the flow. The
influence of
u
∗
amplifies as the particle size increases, where a greater proportion
of particles deposit in the two frontal zones. The geometry of the frontal section of
the nasal cavity conforms to that of a 90
◦
bend where the particle Stokes number
is of importance. The larger particles exhibit much higher Stokes numbers which
prevent the particles from following the curved streamlines. It is observed that 50
μ
μ
m
particles entrained in the flow (
u
∗
=
1) is an example of this. Therefore an injected
m particle (
u
∗
=
50
10) exhibiting a greater amount of initial momentum will only
exacerbate the linear projectile motion of the particle.
About 70 % of 15
μ
μ
m particles are able to follow the curved streamlines when
u
∗
=
1 as particle impactablility is governed by the Stokes number only. However,
significant deposition increases when
u
∗
>
4. The Stokes number, a ratio of the par-
ticle's relaxation time to the flow characteristic time, indicates how long it takes for
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