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The Effect of Oscillatory Flows on the Mass Transfer and Separation of Gas Species In Wavy-Walled Tubes

Aaron M. Thomas and Gatwech Thich. Chemical Engineering, University of Idaho, PO Box 441021, Moscow, ID 83844-1021

Oscillatory flows are a novel mechanical separation mechanism in the mass transfer and separation of gas species. When a tube connects two reservoirs of different gas mixtures, dilute species present in a carrier gas exhibit a large increase in mass transfer over that due to pure molecular diffusion despite no net flow from one reservoir to the other during flow oscillations. The flow oscillations cause the species to transport at different rates and thus give rise to a separation of the dilute species in the mixture. Adding periodic recesses on the boundary (wavy-walls) increases the overall separation of the dilute species over the separations that exist in a straight-walled tube under the same conditions. This is a result of recirculation regions that exist in the expansion potions of the wavy-walled tubes. Because of the differing diffusion coefficients of the species in the system, some species become trapped in the recirculation regions as others are more likely to stay in the faster moving regions and transport more rapidly down the tube. The difference in the throughput between the species depends on the time constants of the system due to oscillation frequency, fluid response (kinematic viscosity), and difference in diffusion coefficients as well as the size of the recirculations.

This work is a combination of analytical findings and experimental results. A perturbation scheme is used to determine the flow profiles in a tube with periodic recesses. The flow profiles show the growth and decay of recirculations regions during the oscillation cycle. The size and strength of these recirculations depend on the size of the recesses on the boundary and the amplitude and frequency of the oscillations. The results of the flow profiles are then used to find the concentration profiles, mass transfer, and ultimately the separation of species by again employing a perturbation analysis. Experimental results of the separation of simple mixtures show excellent agreement with the analytical models. The amount of separation between dilute species indeed greatly increases with the introduction of periodic recesses on the boundary. Further experiments were conducted on carbon dioxide removal from air as applied to air regeneration in recycled environments such as long-manned space missions. These results show a high selectivity of air over carbon dioxide that in turn demonstrates the high potential of using oscillatory flows as a component in an air regeneration system.