The mixing and axial transport of smoke in oscillatory tube flows

To gain a better insight on the mechanisms of transport in high-frequency ventilation, we examined the transport of smoke in a straight tube, a model of the trachea. A smoke bolus was injected into a ventilator, and the movement of the smoke along the tube was monitored by a laser-photomultiplier system. With this system we studied the instantaneous movement of the smoke cycle by cycle and that over a longer period. From the latter, a transport coefficient can be calculated to characterize how fast the smoke is transported along the tube. We found that for low frequency, the flow is laminar with a transport coefficient smaller or approximately 700 cm2/s. For very high frequencies, the oscillatory flow becomes turbulent and effects a strong radial and axial mixing. The transport coefficient increases to as high as 5,000 cm2/s. Over a certain intermediate range of the frequency, the mixing can be a laminar one over several cycles of oscillation and a turbulent one for some subsequent cycles.