Page 251 - Industrial Ventilation Design Guidebook

2 I 2 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS
such that Poiseuille flow (parabolic laminar flow) is not established and eddy
currents develop. The Reynolds number, Re, quantifies this relationship be-
tween inertial and viscous forces and is given by

where /a = fluid viscosity, D = tube diameter, u = mean fluid velocity, and
p = fluid density. The equivalent diameter, D e = 4A/P, where A ~ conduit
cross-sectional area and P = wetted perimeter, replaces D for noncircular con-
duits. In circular straight tubes, Re > 2300 typically indicates the presence of
turbulence. Convection caused by eddy currents enhances deposition of buoy-
ant airborne particles by bringing more of these particles into contact with the
mucosal surface. Airway heat and water vapor exchange is also enhanced by
turbulent airflow.
Turbulence in nasal cavity airflow is a consequence of both high air-
stream velocities, caused by small nasal cross-sectional areas, and very irreg-
ular nasal airway geometry, which induces flow distortions. Nasal turbinate
Re exceeds 2300 even during normal quiet breathing and nasal cavity air-
flow is apparently turbulent at most V E. Flow in the pharynx, larynx, and
trachea is also generally turbulent at most V E despite Re > 2300 only at
higher V E (30 L/min and greater). This airstream mixing enhances convec-
tive heat and mass transfer in extrathoracic airways and plays a major role
in airway defense mechanisms.
Humans preferentially breathe nasally, possibly because of the highly effi-
cient filtration, humidification, and warming performed on the inspiratory
airstream, but inspiratory flow passing through the convoluted passageways
incurs a substantial pressure drop. Filtration by the oral cavity is much less ef-
fective, but the pressure drop is also lower. As a result, humans normally
breathe nasally until VE reaches approximately 30 L/min, when oronasal
breathing begins. This shift in breathing pattern occurs because, at lower flow
rates, the pressure gradient between the atmosphere and pulmonary airways
generated by inspiratory negative pressure in the lungs can overcome nasal re-
sistance, but as flow rates increase, the nasal cavity pressure drop increases
proportionally with Re. Consequently, oral breathing must supplement nasal
breathing above roughly 30 L/min in order to maintain respiratory airflow.
Since flow through the oral cavity has a lower pressure drop than flow
through the nasal cavity, a greater proportion of airflow during oronasal
breathing passes through the oral cavity. It is unclear whether oronasal breath-
ing produces laminar or turbulent airflow in the oral cavity, though
Re < 2100 at V E < 30 L/min.
Pharyngeal turbulence results from the 90° bend at the nasopharynx and
irregular surfaces at the oropharynx and larynx. Vocal cords constrict the pas-
sageway and cause significant flow distortions and turbulence within the lar-
ynx. The passageway abruptly expands from the laryngeal orifice into the
trachea. Rapid expansion produces a jet in proximal tracheal airflow and tur-
bulence all along the trachea. Turbulent tracheal airflow occurs because the
abrupt expansion causes reverse flow in the boundary layer, causing flow sep-
aration in the proximal trachea (Fig. 5.22). Under these conditions, turbulence
forms at Re much less than 2300 in an abrupt expansion (often at Re ~ 300).