Environmental Engineering Reference
In-Depth Information
likelihood of that iber touching an airway surface increases.
125,126
Fiber orientation also plays a
critical factor in deposition by interception. While ibers oriented with the low streamlines deposit
similarly to particles of their equivalent diameter, ibers oriented away from the streamline, or ibers
that are “tumbling,” have an increased likelihood of deposition by interception.
127,128
Interception
may also be important for non-ibrous particles having low density and large geometric diameters,
because the larger geometric diameter increases the likelihood of a particle coming into contact
with an airway surface.
2
It has been suggested that such “porous” particles may have important
applications to aerosol therapy.
127,128
However, recent indings have indicated that certain issues
remain unresolved concerning the use of porous particles as vehicles for drug delivery.
129
Several studies have experimentally characterized iber movement and deposition in human
lung casts or bifurcated tubes.
130,131
Other studies have modeled iber movement and deposition in
the human lung,
126,132,133,159
while still others have modeled interception of both spherical particles
and ibers.
134
The motion of particles, including ibers, in airway bifurcations has been systemati-
cally studied by Balásházy et al.
135-137
Harris and Fraser
132
generated simpliied equations to model
the deposition of ibers by interception under laminar and turbulent conditions. They determined
that deposition by interception increases as iber length increases and airway diameter decreases.
Because of the potential importance of ibers in inhalation toxicology and aerosol therapy, their
expressions for interception deposition eficiencies are presented in the following.
3.7.2.1.1 Laminar Conditions
Harris and Fraser formulated expressions for interception of ibers in laminar parabolic lows. For
ibers that are tumbling in the low, the deposition eficiency may be given by
132
⎡
⎤
⎛
⎜
2
⎞
⎟
+
4
f
r
θ
L
L
f
(
)
−
(
)
−
(
)
=
(
)
t
s
f
P
Int
r
2
θ
L
2
+
L
2
L
4
−
L
4
L
2
tan
θ
r
2
+
24
tan
3
θ
(3.35)
⎢
⎢
⎥
⎥
a
s
f
f
f
f
f
s
a
s
4
π
8
8
⎣
⎦
a
where the fraction of ibers tumbling is given by
2
⎛
⎜
tan
ϕ
⎞
⎟
=
⎛
⎞
⎟
f
t
tan
−1
(3.36)
⎜
π
β
e
where φ has a value of 80°, and
⎛
⎜
⎞
⎟
π
L
L
f
θ
=
−
sin
1
−
(3.37)
s
2
f
⎡
⎢
⎤
⎥
⎛
⎜
0.68
L
⎞
⎟
a
L
=
L
cos tan
−
1
0.34
β
e
+
(3.38)
f
f
r
a
and
126
0.087
⎛
⎜
⎞
⎟
L
d
f
β
e
=
1.07
(3.39)
f
Harris and Fraser also present an equation for interception of ibers that are oriented with the
laminar low streamlines.
132
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