Effects of gravity and branching on longitudinal gas mixing in glass tube models

We investigated the effects of gravity and branching on gas mixing in glass tube models. The mixed gas (5% He and 5% SF6) was made to flow through a straight tube and tubes with bifurcated branches, and the effective axial diffusion coefficient (Deff) was measured. The direction of the branches was varied from upward to downward by 90% turns, and the flow distribution to each branch was separately controlled. In the case of a straight horiontal tube, the flow velocity dependency of the Deff of He and SF6 was observed as we expected from Aris' equation. In the case of glass tubes with branches, the Deff of SF6 was greater in the downward-directed branches than it was in the upward-directed branches when gas mixing occurred at the front of the gas mixture; however, when gas mixing occurred at the tail, the opposite effects were observed. These characteristics were more marked at slower flow rates and when the flow to each branch was different. Our findings suggest that gravity has an effect on gas mixing in glass tube airway models, especially at a low flow rate and uneven flow distribution.