Mixing in flowing gas

We used a mass spectrometer to analyze continuously from a flow stream after changeover of one gas to a mixture of other gases. At the end of a straight cylinder (length = 100 cm, inner diameter = 1/2 inch), light-weight gases appeared later but reached full-scale deflection earlier than heavy gases. Apparently gases with high molecular diffusivity tended to diffuse radially so that they were not carried forward axially in rapidly moving streams. This blunting of the profile of mean concentration vs distance as fresh gas move down a tube, plus spreading of a gradient region of transition between fresh gas and stale gas, are the fundamental processes of diffusion/convection interactions in the lung. Turbulence and molecular diffusion are similar in that they limit the penetration of inspired gases that could occur if laminar flow were the only process acting. However, turbulence and molecular diffusion cause true irreversible mixing of fresh and stale gases, whereas dispersion by laminar flow does not.