Biomedical Engineering Reference
In-Depth Information
8.3.4
Closure
The introduction of particles into the airway presents additional modelling require-
ments. In this case study, the Lagrangian modelling approach is used where an
equation representing the force balance on individual particles is applied in order to
determine the particle velocity. The particles then become a secondary dilute phase
(usually solid or liquid) present in the primary phase (usually gas or liquid).
Strategies for modelling different particle morphology such as spherical, non-
spherical, submicron, and fibrous particles were shown. These different particles are
representative of everyday particles that are inhalable through the respiratory system.
For example, the modelling of spherical particles was applied to low-density drug
particles; non-spherical particles for ragweed pollen; submicron particles for fumes,
vapour, and drug particles; and fibres for asbestos and MMVFs. In some instances
the modelling requirements were available within the commercial CFD software.
Where there were insufficient models, additional custom models need to be written
by the user.
The results showed that the effects of particle morphology on deposition patterns
and deposition efficiencies were found to be significant for the nasal airway. The
dominant mechanism of deposition is by inertial impaction, especially for particles
greater than 5
m at low flow rates. Under these conditions, the inertial parameter
is a useful tool that allows comparisons between particles that exist in this inertial
framework. Thus aerodynamic factors related to the particle morphology, such as
the shape factor, must be accounted for through the drag coefficient. The application
of spherical particle deposition was used to simulate wood dust inhalation, which
is a common problem among wood crafting and processing. The simulations were
able to show local deposition patterns where pine dust had much higher deposition
efficiency than the heavy and light oak dusts within the anterior nasal segment, as
pine dust comprised of much larger particles. The oak dusts consisted of smaller
diameter particles, which lead to its lower deposition. Compared with heavy oak
dust, the light oak dust had lower deposition efficiency, despite it having the same
particle distribution as heavy oak dust. This was due to a lower material density that
light oak dust exhibited and consequently a lower inertial parameter.
Allergenic ragweed pollen and toxic asbestos fibres were also considered. It was
found that about 20 % of 16
μ
μ
m pollen particles deposited in the main airway while for
30
m pollen particles, 66.2 % deposited. These values are smaller when compared
with a sphere having the same aerodynamic diameter which indicates that the drag
coefficient for pollen is greater than for a spherical particle. For asbestos fibres,
where the density and cross-sectional diameter are small, the fibre length becomes
insignificant for the deposition efficiency. The deposition increased from 8.7 to 9.6 %
for an increase in fibre length of 10-300
μ
m. Conversely, MMVFs with higher
density and larger cross-sectional diameter exhibit greater mass per unit length, thus
deposition increased from 14 to 50 % over the same fibre length range. It is imagined
that these results may assist in the design of new particles and guide practical clinical
tests for toxicity.
μ
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