Rapid local evaluation of gas dispersion in volume-cycled tube flow

We have developed a new method for the rapid measurement of local gas dispersion in volume-cycled tube flow. After injecting Argon tracer-gas into the oscillating flow, the time-averaged effective diffusion coefficient (〈Deff/Dmol〉) for axial transport is evaluated from local Argon concentration measurements taken by a mass spectrometer. Experiments were conducted in two tubes (r = 0.85, 1.0 cm) over a range of frequencies (0.42 &le f &le 8.5 Hz) and tidal volumes (7 &le VT &le 48 ml.). Two methods of evaluating 〈Deff/Dmol〉 from the concentration data are used; one uses the complete data set, while the other method uses only the local peaks of the oscillating concentration data. The experimental results show very good agreement with the theoretical predictions of transport in the absence of oscillation and during volume-cycling in the range of 4 &le α &le 11 and A 1/2 and A = VT/πr3 are the dimensionless frequency and amplitude parameters. We also show that concentration measurements taken at any radial position and any axial position within one stroke amplitude of the injection site provide similar values of 〈Deff/Dmol〉. These methods may be applied towards measuring regional gas transport properties within the bronchial tree, and thus may be used to asses the efficacy of gas transport during non-conventional modes of ventilation.