The flow theory and air flow structures in symmetric double-bifurcation airway models
assuming steady laminar, incompressible flow, unaffected by the presence of aerosols,
has been described in a companion paper (Part 1). The validated computer simulation
results showed highly vortical flow fields, especially around the second bifurcations,
indicating potentially complex particle distributions and deposition patterns. In this
paper (Part 2), assuming spherical non-interacting aerosols that stick to the wall
when touching the surface, the history of depositing particles is described. Specifically,
the finite-volume code CFX (AEA Technology) with user-enhanced FORTRAN
programs were validated with experimental data of particle deposition efficiencies
as a function of the Stokes number for planar single and double bifurcations. The
resulting deposition patterns, particle distributions, trajectories and time evolution
were analysed in the light of the air flow structures for relatively low
(ReD1 = 500) and high (ReD1 = 2000)
Reynolds numbers and representative Stokes numbers, i.e. StD1 = 0.04 and
StD1 = 0.12. Particle deposition patterns and surface concentrations
are largely a function of the local Stokes number, but they also depend on the
fluid–particle inlet conditions as well as airway geometry factors. While particles
introduced at low inlet Reynolds numbers (e.g. ReD1 = 500)
follow the axial air flow, secondary and vortical flows become important at higher Reynolds numbers, causing
the formation of particle-free zones near the tube centres and subsequently elevated
particle concentrations near the walls. Sharp or mildly rounded carinal ridges have
little effect on the deposition efficiencies but may influence local deposition patterns.
In contrast, more drastic geometric changes to the basic double-bifurcation model,
e.g. the 90°-non-planar configuration, alter both the aerosol wall distributions and
surface concentrations considerably.
Vortical Flow Structures Induced by Red Blood Cells in Capillaries
