Investigation of Transport Phenomena in Micro/Nano/Subnano-Scale Channels Applied in Knudsen Compres

Y.-L. HanDepartment of Aerospace and Mechanical Engineering
University of Southern CaliforniaInvestigation of transport phenomena is one of the major research topics in micro/nano/subnano-scale (M/N/SN-scale) technologies. Practical applications include concentration, separation, mixing, delivering, pumping, etc. Two specific devices, Knudsen Compressors and Continuous Trace Gas Preconcentrators, have been selected to illustrate the utilization of transport phenomena in micro/nano-scale flows.
Knudsen Compressors are solid-state, micro/meso-scale gas pumps or compressors with no moving parts. Based on the rarefied flow phenomenon of thermal creep, Knudsen Compressors operate by imposing a temperature gradient across a high porosity, low thermal conductivity transpiration membrane. Knudsen Compressors with an aerogel membrane (mean channel size > 20 nm) operated by the radiant heating technique have been studied over a pressure range from about atmospheric pressure down to 10-5 atm. At low pressures, mechanically machined aerogel membranes with circular or rectangular channels have been found to be attractive candidates as Knudsen Compressor membranes. The performance of these membranes has also been found limited by rarefaction effects in the connector section such as "reverse" thermal creep flow and by membrane exit vortices. The Direct Simulation Monte Carlo (DSMC) technique was employed for further investigations of these effects in connector sections of Knudsen Compressors at low pressures. A two-dimensional simulated domain was adopted to mimic a simplified, rectangular channel, single stage Knudsen Compressor. The effects of the "reverse" thermal creep flows and membrane exit vortices have been visualized in the simulations.
The Continuous Trace Gas Preconcentrator is an innovative nano-channel flow application. The operating theory and the preliminary design of the preconcentrator are based on three separation mechanisms: mass separation, quantum separation, and size separation. The separation membranes are an array of aligned channels with nanometer to sub-nanometer size and relatively short lengths. As a consequence of mass separation, size, or quantum separation, the membranes inhibit target molecules from passing through the capillaries while allowing the carrier gas to pass more freely. With a suitable membrane, such as single-walled or multi-walled carbon nanotube membranes, the continuous trace gas preconcentrator is expected to have an excellent performance with a factor of 100 to 1000 times or more increase in the target molecule concentration. The ability to align multi-walled carbon nanotubes has been successfully demonstrated with the construction of a 1cm x 1cm multi-walled array of carbon nanotube towers, and the process of fabricating multi-walled carbon nanotube membranes is ongoing. Fabrication and characterization of a proof-of-concept continuous micro/meso-scale preconcentrator is under way.
It is expected that further research on Knudsen Compressors and Continuous Trace Gas Preconcentrators will yield significant advances in the basic understanding of M/N/SN-scale channel flows as well as efficient micro devices.

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