Page 252 - Industrial Ventilation Design Guidebook

5,2 HUMAN RESPIRATORY TRACT PHYSIOLOGY 2 I 3

FIGURE 5.22 Schematic depiction of airflow pattern through the larynx. Note how eddies form
downstream as air passes through the tracheal jet created by the vocal cords. This effect varies accord-
ing to vocal chord position.

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Studies of airflow through models of bifurcating airways ' show that tur-
bulence generated in the trachea does not sufficiently decay in the largest bronchi
to produce laminar flow. Despite Re diminishing to below 1000, flow through at
least the fourth generation bronchi is believed to be turbulent at all but the lowest
Vg. Eventually, however, flow disturbances dampen along the bronchial tree and
flow becomes laminar. Entrance flow predominates because bronchi are typically
only three to four diameters long. In addition, bifurcations modify velocity pro-
files because of asymmetric shear forces along inner and outer walls possibly
caused by flow separation along the outer wall near the bifurcation (Fig. 5.23).
Consequently, disturbed laminar flow appears to exist during both inspiration
and exhalation in most bronchi.
Mean airstream velocity diminishes as inspiratory flow moves toward the
lung parenchyma because of the rapid increase in total cross-sectional area.
The largest increases in area occur in the distal bronchioles and pulmonary
airways, causing u to approach zero because

where Q = volumetric flow rate (V E ). Although flow is laminar, Poiseuille
flow does not occur despite Re < 1.0 because of the complex geometry of
these airways. Axial diffusion (also known as Taylor dispersion) accounts for
mass transport within distal bronchioles and combines convection and diffu-
sion in an oscillating fluid with a low Re such that